BLM Survey Mapping Terms

Go to Cadastral Terms

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

A [Back to Top]

1927 NAD - The North American Datum, 1927.

ABC - Airborne Control.

ACIC - Aeronautical Chart and Information Center.

ADP - Automatic Data Processing.

AEI - Aerial Exposure Index.

AMS - Above Mean Sea Level.

AMT - Above Mean Terrain.

A.S.A. - American Standards Association. See U.S.A.S.I.

ABRASION - Mechanical scratching, scraping, removal of emulsion or film base moving through cameras or processing equipment.

ABSOLUTE ORIENTATION - The scaling, leveling and attitude correction of photographs in a photogrammetric instrument to fit ground control or the mathematically equivalent process in analytical photogrammetry.  See RELATIVE ORIENTATION.

ABSOLUTE STEREOGRAPHIC PARALLAX - With a pair of truly vertical photographs of equal principal distances taken from equal flight heights, or a pair of rectified photographs, the absolute stereoscopic parallax of a point is the algebraic difference, parallel to the air base, of the distances of the two images from their respective principal points. The term “parallax” is generally used to denote absolute stereographic parallax and also to denote similar measurements when the previous theoretical conditions are not strictly attained, as, for example, in measuring parallaxes on unrectified serial photographs. Linear parallax, x parallax, and horizontal parallax are synonymous with the preferred term, absolute stereoscopic parallax.

ACCIDENTAL ERROR - Random errors that occur without regard to any known physical law or pattern. Their occurrence is assumed to fall within a distribution pattern in accord with the theory of probability. See RANDOM ERROR.

ACCRETION - The act of growing to a thing; usually applied to the gradual and imperceptible accumulation of land by natural causes, as out of the sea or a river. Accretion of land is of two kinds: By alluvion, i.e., by the washing up of sand or soil, so as to form firm ground; or by reliction, as when the sea shrinks below the usual water-mark. The term “alluvion” is applied to deposit itself, while “accretion” denotes the act. However, the terms are frequently used synonymously. Land uncovered by gradual subsidence of water is not an “accretion” but a “reliction.” See ACCRETION* and AVULSION*.

ACCURACY (USC & GS SP, PUBL. 242) - Degree of conformity with a standard or accepted value. Accuracy relates to the quality of a result, and is distinguished from precision which relates to the quality of the operation by which the result is obtained.

ACCURACY CHECKING (USGS) - The procurement of presumptive evidence of a map’s compliance with specified vertical accuracy standards.

ACCURACY TESTING (USGS) - The procurement of evidence, on a sampling basis, of a map’s compliance with specified horizontal accuracy standards. Accuracy testing is designed to determine the absolute accuracy of map features.

ACHROMATIC - Devoid of hue, or transmitting light without showing its constituent colors.

ACHROMATIC LENS - A lens that has been partly corrected for chromatic aberration, usually for two wavelengths of light. Such a lens is customarily made to bring green and red light rays to approximately the same point  focus. Also called achromat. Achromatic lenses are not sufficiently corrected for color photography or color separation.

ACRE - A measure of land, 43,560 square feet, in whatever shape. See ACRE*.

ACTINIC - Pertaining to electromagnetic radiation capable of initiating photochemical reactions, as in photography or the fading of pigments.

ACTIVE - Denotes a source of radiation external to the surface or object. See PASSIVE SYSTEM.

ACTIVE SYSTEMS - 1) A system having its own source of electromagnetic radiation. 2) A system that measures electromagnetic radiation that is reflected from a surface or object, and not produced (emitted) by the surface or object.

ACUTANCE (USGS) - An objective measure of the ability of a photographic system to show a sharp edge between contiguous areas of low and high illuminance.

ADDITIVE COLOR THEORY - Theory of production of color images by addition of colored light rather than by subtraction of unwanted colors from white light. Additive color methods are used to create composite color images from photographs taken in different parts of the spectrum. Three primary colors can be projected to produce every hue in varying degrees of saturation.

ADD TAPE - A survey tape on which minor graduations are placed ahead of the zero point. See CUT TAPE.

ADJUSTED TO DATUM - The value of a position or elevation after proper adjustment between previously adjusted stations or bench marks.

ADJUSTMENT - 1) A process designed to remove inconsistencies in measured or computed quantities by applying derived corrections to compensate for random, or accidental errors, such errors not being subject to systematic corrections. 2) Bringing the movable parts of an instrument or device more nearly into proper relation and fit. 3) Positioning the public-land lines on the topographic map to indicate their true, theoretical, or approximate location relative to the adjacent terrain, and culture, by reconciling the information shown on BLM plats and field records with the ground evidence of the location of the lines. See ADJUSTMENT*.

ADMINISTRATIVE INDEX MAP (USGS) - A special State map printed for administrative use. Quadrangle locations (by latitude and longitude), names, dates of survey, and authorship are shown by black print. Provisional quadrangle names and areas of 7½- and 15-minute quadrangles are also shown by overprint. Areas of quadrangles (in square miles) are shown in the left and right margins. Areas of partial quadrangles or portions of quadrangles falling within a particular State are shown within the individual quadrangle outlines.

ADMINISTRATIVE MAP - A planimetric map of a BLM administrative unit.

ADVANCE COMPLETION - See preferred ADVANCED FIELD COMPLETION.

ADVANCE EDITION - See preferred PRELIMINARY EDITION.

ADVANCE FIELD COMPLETION - (USGS) Field completion carried out prior to stereocompilation. It includes complete photoidentification and field interpretation; road, drainage, and woodland classifications; obtaining name and boundary information; obtaining data for accuracy checks; and assembling all data needed to compile and publish a map without additional fieldwork.

ADVANCE INTERPRETATION - See preferred FIELD PHOTOINTERPRETATION and ADVANCE FIELD COMPLETION.

ADVANCE MATERIAL INDEX MAP (USGS) - A letter size State map showing by symbols the advance information available for quadrangles work.

AERIAL - Relating to the air or atmosphere, being applicable in a descriptive sense to anything in space above the ground and within the atmosphere.

AERIAL EXPOSURE INDEX - The reciprocal of twice the exposure, expressed in meter candle seconds, at the point on the toe of the characteristic curve where the slope equals 0.6 gamma when recommended processing conditions are used.

AERIAL PHOTOGRAPH - A photograph of a part of the earth’s surface taken by an aircraft supported camera.

AERIAL RECONNAISSANCE - The securing of information by aerial photography or by visual observation from the air.

AERONAUTICAL CHART - A map of the earth’s surface designed for aviation use.

AEROTRIANGULATION - The process for extension of survey control whereby overlapping aerial photographs are related using perspective principles.

AFFINE - A geometrical condition in which the scale along one axis or reference plane is different from the scale along the other axis or plane.

AFFINE TRANSFORMATION - 1) A multiple-stage rectification technique made by photographic alternations of map dimensions which include differential magnification (transformation of one rectangle into another of different proportions) and shear (transformation of a rectangle into a parallelogram). 2) A transformation in which straight lines remain straight and parallel lines remain parallel. Angles, however, may undergo changes and differential scale changes may be introduced.

AGONIC LINE - The locus of all points on the earth’s surface at which the magnetic north coincides with astronomic north. Also denotes its representation on a map.

AIR BASE - The line joining two exposure stations, or the length of this line; also, the distance (at the scale of the stereoscopic model) between adjacent perspective centers as reconstructed in the plotting instrument.

AIRBORNE CONTROL SURVEY SYSTEM (ABC) - A system for establishing a supplemental, vertical, and horizontal control for mapping and cadastral surveying where access is difficult. Horizontal and vertical angles as well as EDM distances are measured from known ground stations to a helicopter equipped with a Hoversight. See AIRBORNE CONTROL SURVEY*.

AIRBORNE MAGNETOMETER - An instrument used to measure variations in the magnetic field of the earth while being transported by an aircraft.

AIRPORT - Any body of land or water which is used, or intended for use, for the landing and take-off of aircraft, and any appurtenant areas which are used, or intended for use, for airport buildings or other airport facilities or rights-of-way, together with all airport buildings and facilities located thereon.

AIR SPEED - The velocity of an aircraft relative to the surrounding atmosphere.

AIR STATION - The point in space occupied by the camera lens at time of exposure. See EXPOSURE STATION.

ALBERS CONICAL EQUAL-AREA PROJECTION - An equal-area projection of the conical type, on which the meridians are straight lines that meet in a common point beyond the limits of the map and the parallels are concentric circles whose center is at the point of intersection of the meridians. Meridians and parallels intersect at right angles, and the arcs of longitude along any given parallel are of equal length. The parallels are spaced to retain the condition of equal area. On two selected parallels, the arcs of longitude are represented in their true length. Between the selected parallels the scale along the meridians is a trifle too large, and beyond them, too small.

ALIDADE - The part of a surveying instrument which consists of a sighting device, with index, and reading or recording accessories. The alidade of a theodolite or surveyor’s transit is the upper part of the instrument: the telescope, micrometer microscopes, or verniers, and accessories, mounted on what is termed “upper motion” of the instrument, and used in observing a direction or angle on the graduated circle, which is mounted on the “lower motion.”  The alidade used in topographic surveying consists of a straightedge ruler carrying a telescope, or other sighting device, and used in the recording a direction on the plane-table sheet.

ALINEMENT (ALIGNMENT) - 1) Formation or position in line, or, more properly, in a common vertical plane. 2) In railway or highway surveying: The ground plan, showing the alinement or direction, and components of the center line, as distinguished from a profile, which shows the vertical element.

ALKALI FLAT - A level lakelike plain formed in low depressions where accumulated water evaporates depositing fine sediment and dissolved minerals which form a hard surface if mechanical sediments prevail or a crumbly powdered surface if efflorescent salts are abundant.

ALLUVIAL FAN or ALLUVIAL CONE - The deposit of sediment laid down by a swift-flowing stream as it enters a plain or an open valley, so called on account of its shape.

ALLUVIUM - Gravel, sand, silt, and clay sediments deposited by streams as a result of markedly decreased current velocity, e.g., on the stream bottoms, along the banks, or where streams enter a body of standing water.

ALTIMETER - An instrument that indicates directly the height above a reference surface. See RADAR ALTIMETER, BAROMETER, and BAROMETRIC LEVELING.

ALTIMETRY - The science of determining the difference of elevation by use of altimeters.

ALTITUDE - The vertical angle between the plane of the horizon and the line to the object which is observed or defined. In astronomy and surveying, the altitude is positive if the object is above the horizon, and negative if below it. In surveying, a positive altitude is also termed an angle of elevation; a negative altitude, an angle of depression. In photogrammetric surveying and airport zoning, altitude applies to the elevation above a datum of points in space (as opposed to points on the ground surface) above a datum, usually mean sea level.

ANAGLYPH - A stereogram in which the two views are printed or projected superimposed in complementary colors, red and blue or red and green. By viewing through spectacles of corresponding complementary colors, a stereoscopic image is formed.

ANALYTICAL AEROTRIANGULATION - A process of interior and absolute orientation of aerial photographs obtained by computational methods from plate coordinates and mathematical analysis of the photogrammetric system. See AEROTRIANGULATION and ANALYTICAL PHOTOTRIANGULATION*.

ANASTIGMATIC LENS - A lens that has been corrected for astigmatism and curvature of field.

ANGLE, DIP - 1) The vertical angle of the observation point between the plane of the true horizon and a sight line to the apparent horizon. 2) The vertical angle, at the air station, between the true and the apparent horizon, which is due to flight height, earth curvature, and refraction.

ANGLE OF CONVERGENCE - See ANGULAR  PARALLAX.

ANGLE OF COVERAGE - The apex angle of the cone of rays passing through the front nodal point of a lens.

ANGLE OF FIELD - See ANGLE OF COVERAGE.

ANGLE POINT - A point in a survey where the alinement deflects from a straight line.

ANGSTROM - Unit of wavelength, used chiefly in expressing short wavelengths; mathematically, it equals 10-¹⁰ meters or 10-⁸ centimeters.

ANGULAR DISTORTION - Distortion in shape of a map area caused by non-conformality of the map projection. See CONFORMALITY.

ANGULAR ERROR OF CLOSURE - The discrepancy between the summation of a series of field angles and the theoretical sum.

ANGULAR PARALLAX - The angle subtended by the eye base of the observer at the object viewed.

ANNOTED PHOTOGRAPH - A photograph on which planimetric, hypsographic, geologic, cultural, hydrographic, or vegetation information has been added to identify, classify, outline, clarify, or describe features that would not otherwise be apparent in examination of an unmarked photograph.

ANTIHALATION - 1) The reduction of blurring or fogging that takes place around the image of a brightly lighted antihalation coating. 2) A light-absorbing coating applied to the back side of the support of a film or plate (or between the emulsion and the support) to suppress halation.

ANTIVIGNETTING FILTER - See VIGNETTING.

APERTURE STOP - The physical element (such as a stop, diaphragm, or lens periphery) of an optical system which limits the size of any pencil of rays traversing the system. The adjustment of the size of the aperture stop of a given system regulates the brightness of the image and depth of field without having any effect upon the size of the area covered.

APOCHROMATIC LENS - A lens which is corrected for chromatic aberration for three wavelengths of light rather than two as in the achromatic lens.

APPARENT HORIZON - The visible junction of earth and sky viewed from a specific point.

APPROXIMATE CONTOUR (USGS) - A contour line of doubtful accuracy used in stereocomplilation to indicate uncertainty of location.

ARCTIC CIRCLE - The geographic parallel having a north latitude of 90° minus the sun’s declination at summer solstice. Although the value changes slightly every year it is shown on most maps as 66° 33’.

ARC TRIANGULATION - A system of triangulation of limited width designed to connect widely separated control or for nationwide geodetic surveying.

ARC (USGS) - 1) A part of a mathematically defined curve. 2) A triangulation arc is a system of triangulation of limited width designed to progress in a single general direction to connect other arcs or to extend control from an established datum.

AREA COVERAGE - Complete photographic coverage of an area by conventional photography having parallel flight lines and stereoscopic overlap between exposures in the line of flight.

AREA TRIANGULATION - Surveys to provide horizontal control over an area such as a county or for filling in between arcs of triangulation.

ARM OF THE SEA - A comparatively narrow extension of a larger body of water.

ARTIFICIAL HORIZON - A device consisting of a plane reflecting surface which can be adjusted to coincide with the plane of the horizon.

ASPHERICAL LENS - A lens in which one or more surfaces depart from a true spherical shape.

ASSUMED ELEVATION - An arbitrary value assigned to a bench mark to which other work is referenced. Usually an even value such as 1000.00 feet, is chosen in absence of a connection to sea level datum.

ASSUMED GROUND ELEVATION (USGS) - The elevation assumed to prevail in a local area covered by a particular photograph or group of photographs. Used especially to denote the elevation assumed to prevail in the vicinity of a critical point, such as a peak or other feature having abrupt local relief.

ASTRONOMIC - Of or pertaining to astronomy. In surveying, azimuths or geographic positions determined by direct observation on the sun or a star (or stars) are designated astromic azimuths or astronomic positions.

ASTRONOMIC AZIMUTH - The direction of a line relative to the meridian, as determined from astronomic observations. Usually measured clockwise from the south.

ASTRONOMIC LATITUDE - The angle between the plane of the earth’s equator and a normal to the geoid as determined by celestial observations.

ASTRONOMIC LONGITUDE - The dihederal angle between the Greenwich Meridian and the observer’s meridian as determined by astronomic observations and time signals.

ASTRONOMIC NORTH - The local direction of the earth’s axis of rotation as determined from celestial observations.

ASTRONOMIC POSITION - See GEOGRAPHIC POSITION, ASTRONOMIC LATITUDE and ASTRONOMIC LONGITUDE.

ATMOSPHERIC REFRACTION - The bending of the path of light rays as they pass through air of varying density.

ATTENUATION - Reduction in intensity of radiation with distance from its source due only to absorption and/or scattering. 

ATTITUDE - 1) The angular orientation of a camera, or of the photograph taken with that camera, with respect to some external reference system. Usually expressed as tilt, swing, and azimuth; or roll, pitch, and yaw. 2) The angular orientation of an aerial or space vehicle with respect to a reference system.

AUTOMATIC LEVEL - A leveling instrument in which the line of sight is automatically maintained horizontal by means of a built-in compensator acting through the force of gravity.

AUTOPOSITIVE - A term applied to a type of photographic film or paper which yields positive copies from positive originals or negative copies from negative originals without an intermediate copy.

AUTO SURVEYOR - A trade name for an inertial positioning system.

AVERAGE ERROR - The mean of all errors taken without regard to sign. It is the quotient of the sum of the absolute magnitudes of the errors divided by the number of errors included. See ERROR OF THE MEAN.

AVULSION - (USGS) The sudden removal of land from one segment and joining it to another as a result of erosion caused by inundation or current. Also, a sudden change in the course of a river, by which a portion of land is cut off, as where a river changes course to form an oxbow. See AVULSION*.    

AVULSIVE CUTOFF - (ASCE) A river’s action when avulsion takes place; also, describes the area of land enclosed by the old and the new channels.

AXES - The plural of axis.

AXIS OF HOMOLOGY - 1) The intersection of two projectively related planes. 2) The intersection of the plane of the photograph with the horizontal plane of the map or the plane of reference of the ground. Corresponding lines in the photograph and map planes intersect on the axis of homology.

AXIS OF TILT - A line through the perspective center of a photograph, perpendicular to the principal plane. Also, the line of zero tilt displacement on a tilted photograph. This line lies at right angles to the principal plane and passes through the isocenter.

AXIS, X AXIS - The line in the photograph plane through the perspective center and parallel to the line joining fiducial marks which are in the general direction of the flight line. In an aerial coordinate system the X axis is along the forward air base.

AXIS, Y AXIS - The line in the photograph plane through the perspective center and perpendicular to the X axis. In an aerial coordinate system, the Y axis is horizontal and is perpendicular to the forward air base. The origin is at the rear exposure station.

AXIS, Z AXIS - The line through the perspective center and perpendicular to the plane of the photograph. The positive direction is upward. In an aerial coordinate system, the Z axis is in a vertical plane and perpendicular to the forward air base.

AZIMUTH - The angle measured on the horizon between the meridian and the plane of the vertical circle through a celestial body or other object. Most government agencies reckon with azimuth clockwise from south, 0° to 360°.

AZIMUTH LINE - (USGS) A radial line from the principal point, isocenter, or nadir point of a photograph, representing the direction to a corresponding point on an adjacent photograph in the same line of flight.

AZIMUTH MARK - A mark set at a significant distance from a triangulation or traverse station to mark the end of a line for which the azimuth has been determined and to serve as a starting or reference azimuth for later use. Azimuth marks are set to be visible from the station without a tower.

B [Back to Top]

B/H - Base to height ratio.

BM - Bench mark.

bx - The component of the airbase which lies along the line of flight.

by - The transverse component of the air base.

bz - The vertical component of the air base. See BZ CURVE.

BZ CURVE - A graphic representation of the vertical errors along a line parallel to the flight line of a stereotriangulated strip.

BACK AZIMUTH - The azimuth of a geodetic line at the end opposite the reference end. It differs from the forward azimuth (at the reference end) by 180° plus the amount of the convergence of meridians between the two points. See AZIMUTH and GEODETIC AZIMUTH*.

BACK FOCAL LENGTH - The distance measured along the lens axis from the rear vertex of the lens to the plane of best average definition. Also called back focal distance.

BACKGROUND - Any effect in a sensor or other apparatus or system above which the phenomenon of interest must manifest itself before it can be observed. See NOISE.

BACKSHORE - That part of a beach which is usually dry, being reached only by the highest tides, and, by extension, a narrow strip of relatively flat coast bordering the sea. See BEACH* and BACKSHORE*.

BACKSIGHT - 1) A sight on a previously established survey point, other than a closing or check point. 2) In leveling, a reading on a rod held on a point with a previously determined elevation to determine the height of the instrument (HI).

BALANCING A SURVEY - Distributing corrections through a traverse to eliminate the errors of closure according to some approved method so that the resulting survey figure will be geometrically and mathematically consistent. The removal of errors by the method of least squares is termed an adjustment.

BALDWIN SOLAR CHART - A chart designed by D. H. Baldwin, of the Geological Survey, for orienting a planetable by means of the sun’s shadow.

BAND - 1) A selection of wavelengths. 2) Frequency band. 3) Absorption band. 4) A group of tracks on a magnetic drum. 5) A range of radar frequencies.

BANK OF STREAM - The continuous margin along a river or stream where all upland vegetation ceases. The right bank of a stream is the bank on the right-hand side, and the left bank, the one on the left-hand side, as one proceeds downstream.

BAR - 1) A mass of sand, gravel, or alluvium deposited on the bed of a stream, sea, or lake, or at the mouth of a stream forming an obstruction to water navigation. 2) A term used in a generic sense to include various types of submerged or emergent embankments of sand and gravel built on the sea floor by waves and currents. 3) An offshore ridge or mound of sand, gravel, or other unconsolidated material submerged at least at high tide, especially at the mouth of a river or estuary, or lying a short distance from and usually parallel to the beach.

BAROMETER - An instrument for measuring atmospheric pressure.

BAROMETRIC LEVELING - Determining elevations by use of altimeters or barometers.

BARREL DISTORTION - A type of geometric distortion found in scanning imagery in which elements crossing the flight direction are distorted by a combination of scanner-mirror rotation and forward motion of the aircraft. Straight lines (roads, etc.) cut obliquely appear as sigmoid curve in the resultant imagery. Some photographic lenses also produce barrel distortion.

BARRIER BEACH - Offshore bar. This term refers to a single elongate sand ridge rising slightly above the high-tide level and extending generally parallel with the coast, but separated from it by a lagoon. The term should apply to islands and spits.

BARRIER FLAT - The relatively flat area, often occupied by pools of water, separating the exposed or seaward edge of a barrier and the lagoon behind the barrier.

BAR SCALE - A line on a map subdivided and marked with a distance which each of its parts represents on the earth.

BASAL ORIENTATION - The establishment of the position of both ends of an air base with respect to a ground system of coordinates. In all, six elements are required. These are essentially the three-dimensional coordinates of each end of the base. In practice, however, it is also convenient to express these elements in one of two alternative ways: 1) the ground rectangular coordinates of one end of the base and the difference between these and the ground rectangular coordinates of the other end of the base; 2) the ground rectangular coordinates of one end of the base, the length of the base, and the two elements of direction (such as base direction and base tilt).

BASE-HEIGHT RATIO (B/H) - In aerial photography, the ratio of the air base to the flight height. See X  FACTOR.

BASE IN - In normal position of viewing the stereo model where the Zeiss Parallelogram is employed. See BASE OUT.

BASE IN-BASE OUT MACHINE - A colloquial term for a universal plotter having the Zeiss Parallelogram feature.

BASE LINE - A monumented line of precisely measured length tied to a triangulation system by a base net to control scale errors. See BASE LINE*.

BASE LINE PHOTOGRAPHY - Aerial photographs flown for the particular purpose of provision of an initial physical status for later comparison.

BASE MAP - A map which contains the fundamental framework necessary for adequate geographic reference and positioning of information that may be drawn or overprinted on it. It contains only the common essentials that are generally found on a wide variety of maps of comparable scales.

BASE OUT - The position of the Zeiss Parallelogram where viewing the stereo model but leaving one photograph oriented in its previous position and orienting the third photograph to the second. See BASE IN.

BASE SHEET - A sheet of paper or other material containing (or keyed to) a map projection as well as basic data in varying stages of completeness for map compilation or revision. See CONTROL BASE.

BASE-WIDTH RATIO - Ratio of airbase to width of flight coverage.

BASE CONTROL - Coordinated and correlated position data, horizontal or vertical, forming a framework of marked or monumented points for the support and control of subsequent surveys.

BATHYMETRY - The art or science of determining ocean depths.

BAYOU - A minor, sluggish waterway or estuarial creek, generally tidal or with slow or imperceptible current, and with its course generally through lowlands or swamps, tributary to or connecting with other bodies of water. Various specific meanings have been implied in different parts of the southern United States. Sometimes called slough.

BEACH - The belt or zone along the shore usually with a gentle slope toward the water, occupied by unconsolidated material, moving sand, or shore drift. The zone from the waterline to the place where there is a marked change in material or physiographic form, or to the line of permanent vegetation (usually the effective limit of normal storm waves). See SHORE and BEACH*.

BEAMAN ARC - A specially graduated arc attached to the vertical circle of an alidade or transit to simplify computing elevation difference for inclined stadia sights. It was designed by W. M. Beaman, of the Geological Survey.

BEAM COMPASS - Drafting instrument for drawing circles with a long radius. The point and the pen, or pencil tip, are separate units, mounted to slide and clamp on a long bar or “beam” so that the distance between them is equal to the desired radius.

BEARING - The direction of a line expressed as an angle less than 90°, east or west of the reference meridian, generally in the form S.50°W. It may be grid, magnetic, or true bearing, according to the nature of the reference meridian.

BED OF STREAM - The area within the high-water lines of a stream or river.  It is the area which is kept practically bare of vegetation by the wash of the waters of the stream from year to year. See BEDS OF NAVIGABLE LAKES AND STREAMS*.

BENCH - 1) A strip of relatively level earth or rock, raised and narrow. A small terrace or comparatively level platform breaking the continuity of a declivity. 2) A level or gently sloping erosion plane inclined seaward.

BENCH MARK - A relatively permanent material object, natural or artificial, bearing a marked point of known elevation above or below an adopted datum.

BENCH MARK, MONUMENTED - (USGS) A described point, whose elevation has been determined by surveys of third-order accuracy or better, marked by a metal tablet bearing identifying and other appropriate information.

BENCH MARK, NONMONUMENTED - (USGS) A described point, whose elevation has been determined by surveys of third-order accuracy or better, which is recoverable but not marked with a tablet. It may be marked with a chiseled square or cross, a nail and washer in the root of a tree, etc.

BEND - A curve in a river channel whose lateral changes involve a decrease in radius. Bends generally grow into meanders.

BIOME - A classification of terrain according to its associated plants and animals. Standard biomes for BLM are Tundra, Boreal Forest, Coniferous Forest, Woodland-Brush Land, Grassland, and Desert.

BLOCK ADJUSTMENT - The adjustment by combining of strips, sections, models, or photographs into blocks, or of small blocks into larger blocks. The adjustments may be horizontal or vertical or both; it may be done by instrumental, analog, or mathematical processes, or combinations of these. See STRIP ADJUSTMENT.

BLUE-LINE BOARD - (USGS) A blue-line guide on a rigid or semirigid material.

BLUE-LINE GUIDE - A drafting surface on which a nonphotographic blue-line image has been processed. See BLUE LINE BOARD.

BLUE-LINE PRINT - A positive copy of a map or other data printed in blue.

BLUFF - 1) Any high headland, or bank presenting a precipitous front. 2) The high vertical banks of certain rivers. 3) A high steep bank or cliff.

BLUNDER - A mistake. A blunder is not an error, though a small blunder may remain undetected in a series of observations and have the effect of an error in determining a result. Examples of blunders are reading a horizontal circle incorrectly by an even degree, neglecting to record a tape length in a measured transverse, and reversing numerals in recording an observation.

BOARD ON GEOGRAPHIC NAMES - An agency within the U. S. Department of the Interior established by Public Law 242, 80th Congress, approved July 25, 1947. This Board has jurisdiction for the review of names of  places, mountains, streams, lakes, and other natural features which may be confused by the use of local names, historical names, or writings relating to an area. The Board has authority to announce an accepted term or name for use on maps, plats, charts, reports, and historical statements prepared by or in behalf of the Federal Government.

BOAT CHART - A large scale working chart on which hydrographic data (soundings, bottom characteristics, etc.) are plotted by survey parties.

BRAIDED STREAM - A watercourse not filled by the normal stream flow, which subdivides into an interlaced pattern of channels.

BRANCH OF THE SEA - Same as arm of the sea.

BRIDGE - A structure over a traffic carrying way or water course having a span of more than 10 feet.

BRIDGING - The extension and adjustment of photogrammetric surveys between bands of ground control. See STEREOTRIANGULATION.

BUILDING GRAVER - An instrument for scribing solid rectangular map features.

BULKHEAD LINE - A line in a harbor or navigable waters defining the channelward limit of solid fills or bulkheads. Such lines are usually established by the Secretary of the Defense Department; however, state or municipal authorities under their police powers may establish bulkhead lines shoreward of those established by the Secretary of Defense.

BULL’S EYE LEVEL - See CIRCULAR LEVEL

BUNDLE SOLUTION - A computation method in analytical aerotriangulation where all the parameters are considered in a large simultaneous solution.

BUTTE - A flat-topped hill, produced when hard strata of rock overlie weaker layers, and protect them from being worn down. The butte is similar to but smaller than a Mesa, and is often produced from a mesa when the latter is reduced in size.

C [Back to Top]

CRT - Cathode ray tube.

CALIBRATED FOCAL LENGTH (CFL) - An adjusted value of the equivalent focal length so computed as to distribute the effect of lens distortion in a desired manner over the entire field used in a camera.

CALIBRATION - The act or process of determining certain specific measurements in a camera or other instrument or device for comparison with a standard or for purpose of record.

CALIBRATION, CAMERA - The determination of 1) the calibrated focal length; 2) the location of the principal point with respect to the fiducial marks; 3) the point of symmetry; 4) the resolution of the lens; 5) the degree of the flatness of the focal plane; and 6) the effective lens distortion in the focal plane of the camera which is referred to the particular calibrated focal length. In a multiple-lens camera, the calibration also includes the determination of the angles between the component perspective units.

CALIBRATION CONSTANTS - The results obtained by calibration which give the relationship of the principal point to the fiducial marks of a camera and the calibrated focal length of the lens-camera unit.

CAMERA, AERIAL - A camera specifically designed for use in aircraft. The prefix aerial is not essential where the context clearly indicates an aerial camera rather than a ground camera.

CAMERA AXIS - A line perpendicular to the focal plane of the camera and passing through the interior perspective center or emergent nodal point of the lens system.

CAMERA STATION - The point in space occupied by the camera lens at exposure.

CANAL - 1) An artificial watercourse cut through a land area for navigation, irrigation. 2) A long narrow arm of the sea extending far inland.

CANTILEVER EXTENSION - (USGS) Phototriangulation from a controlled area to an area of no control. The procedure may be progressive, starting from the control, or it may be a simultaneous mathematical procedure.

CANYON or CAΡON - A gorge, relatively narrow but of considerable size, bounded by steep slopes. It has been formed by a river cutting through the soft rocks of an arid region.

CAPE - 1) A point of land extending into the sea or a lake; a headland. 2) A relatively extensive land area jutting seaward from a continent or large island which prominently marks a change in, or interrupts notably, the coastal trend.

CARRYING CONTOUR - (USGS) A single contour line representing two or more contours; used to show vertical or near vertical topographic features, such as cliffs, cuts and fills.

CARTOGRAPHIC LICENSE - (USGS) The freedom to modify manuscript information in order to improve the clarity of the map.

CARTOGRAPHY - The art, science and technology of expressing graphically, by maps, charts, three-dimensional models and globes, the known physical features of the earth or any heavenly body, at any scale. In particular, cartography is concerned with all phases of map finishing, including the designing of format and symbolization; drafting, editing, and proofing of map content; and specifying requirements for reproduction.

CASED ROAD - A double line symbol for a road on a map-usually major highways.

CATHODE-RAY TUBE - A vacuum tube that generates a focused beam of electrons which can be deflected by electric and/or magnetic fields. The assembly contains an electron gun arranged to direct a beam upon a fluorescent screen. Scanning by the beam can produce light at all points.

CENTERING DEVICE, DIAPOSITIVE - See DIAPOSITIVE CENTERING DEVICE.

CENTER OF RADIATION - The selected point on a photograph from which radials are drawn or measured.

CENTER, PHOTOGRAPH - The center of a photograph as indicated by the images of the fiducial marks of a camera. In a perfectly adjusted camera the photograph center and the principal point are identical.

CENTRAL MERIDIAN - The meridian, usually passing near the center of the area, used as the Y or N axis in a plane-coordinate or grid system. To avoid negative coordinates in the system, this meridian is usually assigned a large X or E value.

CENTRAL PERSPECTIVE RAY - The ray from the external perspective center to the object point whose image is located at the principal point.

C FACTOR - An empirical value which expresses the contour plotting capability of a given stereoscopic system, generally defined as the ratio of flight height to the smallest contour interval accurately plottable. Not a constant but dependent on conditions present, it is used in determination of flying height or as a rough comparison of photogrammetric systems.  

CHART - (USGS) A special-purpose map generally designed for navigation. The term “chart” is applied chiefly to maps made primarily for nautical and aeronautical navigation and to maps of the heavens, though the term is sometimes used to describe other special-purpose maps.

CHECKED ELEVATION - An elevation determined by two or more independent sets of measurements, or by a closed loop, in which the results agree within a specified limit. Results are printed in black on USGS maps.

CHECK PROFILE - A profile plotted by use of field survey measurements to check a profile plotted using measurements of position and elevation from a topographic map.

CHICAGO ROD - A two piece leveling rod with a telescopic joint.

CIRCUIT - A series of connected survey lines that form a closed loop; also, a line or series of lines connecting two fixed points.

CIRCUIT CLOSURE - The discrepancy between the algebraic sum of the differences in elevation between junction points in a leveling circuit and the theoretical value of zero.

CIRCULAR LEVEL - A spirit level with the inside surface of its upper part ground spherical to form a circular bubble, and with graduations consisting of concentric circles, for application where high precision is not required.

CLASSICAL POINTS - Six symmetrically selected image points located within the neat model at which Y parallax is removed during relative orientation of the photographs. Four of the points are near the model corners; the other two are near the X-X axis near the model boundary. See PASS POINTS.

CLOSED TRAVERSE - A traverse which starts and ends upon the same station or upon stations of known position.

CLOSING THE HORIZON - Measuring the last angle of a series of angles around a station.

CLOSURE - 1) The process of measurement in a closed figure for a check on horizontal or vertical precision. 2) (USGS) A misclosure. See JUNCTION CLOSURE, HORIZON CLOSURE and TRIANGLE CLOSURE.   

COASTAL PLAIN - A plain which borders the sea coast, and extends from the sea to the nearest elevated land. It is sometimes formed through denudation by the sea, the beach being later raised by earth movement to form a plain, frequently known as a Raised Beach, or by deposition of solid matter at their mouths by rivers.

COASTLINE - The line that separates the land surface and the water surface of the sea or ocean. See BEACH and BEACH*.

COATED LENS - A lens coated to reduce light reflection from its surface and for selectively absorbing light of a particular wavelength (color) so as to prevent it from passing through the lens.

COINCIDENCE - 1) The alinement if diametrically opposed index marks to form a continuous line under the reading microscope of a theodolite. 2) The alinement of the images of opposite ends of a level bubble.

COLLIMATE - 1) To render parallel to a certain line or direction, to render parallel, as rays of light; to adjust the line of sight or lens axis of an optical instrument so that it is in its proper position relative to other parts of the instrument. 2) To adjust the fiducial marks of a photogrammetric camera so that they define the principal point. Also called adjustment for collimation.

COLLIMATION - The act of making a collimation adjustment.

COLLIMATION ADJUSTMENT - The process of bringing the line of collimation of a telescope into close agreement with the collimation axis. Also termed adjustment for collimation.

COLLIMATION AXIS - The line through the second nodal point of the objective (object glass) perpendicular to the horizontal axis of a transit or perpendicular to the vertical axis of a level.

COLLIMATION, ERROR OF - The angle between the line of collimation (line of sight) of a telescope and its collimation axis.

COLLIMATION, LINE OF - The line through the second nodal point of the objective (object glass) of a telescope and the center of the reticle.

COLLIMATION PLANE - The plane described by the collimation axis of a telescope of a transit when rotated around its horizontal axis.

COLLIMATOR - A fixed telescope with crosshairs in its focus, used to adjust a second telescope by looking through it in a reverse direction with the latter so that images of the crosshairs are formed in the focus of the second telescope, as if they originated in a distant point.

COLOR COMPOSITE IMAGE - In landsat imagery, a color negative, transparency, or print produced from bulk or precision black-and-white triplet sets.

COLOR ENHANCEMENT - Utilization of contrasting colors (rather than differences in grey values) to indicate subtle changes in film density. Can be applied in single or multi-emulsion situations.

COLOR PROOF - A multicolor print made by whirling or rubbing color proof sensitizer on a proof sheet and making consecutive exposures through the scribed features. Each color requires separate sensitization which is applied over the previous color print to form the composite multicolor proof.

COLOR SEPARATION - The preparation of separate drawings for each type of map data to be reproduced in a separate color.

COLOR SEPARATION DRAWING - Separate manuscripts prepared for each color of the completed map.

COLOR-SEPARATION GUIDE - A print of a manuscript that provides copy for final drafting, scribing, or stickup of selected data.

COMBINATION GRAVER - A swivel graver which may be converted to a rigid graver for scribing.

COMPARATOR - 1) An instrument or apparatus for measuring a dimension in terms of a standard. 2) An instrument for comparing standards of length; for subdividing such standards; or for determining a standard length of a measuring device. 3) An optical instrument, usually precise, for measuring rectangular or polar coordinates of points on any plane surface, such as a photographic plate.

COMPASS SURVEY - A transverse survey which relies on the magnetic needle for orienting the sequence as a whole or for determining the bearings of lines individually.

COMPENSATING ERROR - An error that tends to offset a companion error and thus obscure or reduce the effect of each.

COMPENSATION PLATE - See CORRECTION PLATE.

COMPILATION - 1) The production of a new or improved map (or portion of a map) from existing maps, aerial photographs, surveys, new data, and other sources. 2) The production of a map (or portion of a map) from aerial photographs and geodetic control data, by means of photogrammetric instruments.

COMPILED MAP - A map incorporating information collected from various sources, not developed by surveys made for the map in question. Most small-scale maps of large areas are compiled maps.

COMPLETION - Obtaining field information needed to edit and publish a map from a compiled manuscript. See FIELD COMPLETION and COMPLETION SURVEY*.

COMPOSITE - A print containing the reproducible information from two or more color separation drawings surprinted in register.

COMPUTER MAPPING - See DIGITAL MAPPING.

CONCLUDED ANGLE - An interior angle between adjacent sides of a polygon which is obtained by subtracting the sum of all the other interior angles of the figure from the theoretical value of the sum of all interior angles.

CONFLUENCE - A junction or flowing together of streams; the place where streams meet.

CONFORMAL - Having the map scale at any one point the same in both directions. See CONFORMALITY.

CONFORMALITY - The unique property of conformal map projections, in which all small or elementary figures on the surface of a sphere retain their original shapes on the map. See ANGULAR DISTORTION.

CONFORMAL MAP PROJECTION - See CONFORMALITY.

CONGRUENCING - Process by which two images or two digital images of a multi-image set are transformed so that the size and shape of any object on one photograph is the same as the size and shape of that object on the other photograph.

CONJUGATE IMAGE POINTS - Unique, selected objects appearing in each of two or more photographs. See the preferred term CORRESPONDING IMAGE POINTS.

CONSTANT ERROR - A systemic error which is the same in both magnitude and sign throughout a given series of observations, such as an index error of an instrument.

CONSTRUCTION SURVEY - The survey measurements made for design information, stakeout for construction or for surveys during progress of construction as well as following completion of the work for quantity determination.

CONTACT PRINT - A photographic print produced by the exposure of a sensitized emulsion in contact with the emulsion of a negative or positive transparency.

CONTACT PRINTER  (USGS) - 1) Any machine which reproduces a photographic image using emulsion contact. 2) A diapositive printer in which the sensitized surface of the diapositive plate is placed in direct contact with that of the aerial negative to produce an exact positive copy of the negative.

CONTINENT - Large land mass rising more or less abruptly above the deep ocean floor; includes marginal areas that are shallowly submerged.

CONTINENTAL APRON - The gentle incline at the base of the continental slope leading to the deep oceanic basins.

CONTINENTAL MARGIN - The zone separating the emergent continents from the deep sea bottom; generally consists of continental shelf, continental slope and continental rise.

CONTINENTAL SHELF - The submerged portion of a continent which slopes gently seaward from the low-water line to a point where a substantial break in grade occurs, at which point the bottom slopes seaward at a considerable increase in slope until the great ocean depths are reached. The point of break defines the “edge” of the shelf, and the steeper sloping bottom the “continental slope.”  Conventionally, the edge is taken at 100 fathoms (or 200 meters) but instances are known where the increase in slope occurs at more than 200 or less than 65 fathoms. See OUTER CONTINENTAL SHELF.

CONTINENTAL SLOPE - Continuously sloping portion of the continental margin with gradient of more than 1 to 40, beginning at the outer edge of the continental shelf and bounded on the outside by a rather abrupt decrease in slope where the continental rise begins at depths ranging from about 4,500 to 10,000 feet; formerly considered to extend to abyssal plains.

CONTINENTAL TALUS - The steep slope from the lower edge of the continental shelf into the ocean depths. It is characterized by a marked change in gradient and usually begins at a depth of approximately 100 fathoms.

CONTINUOUS-STRIP PHOTOGRAPHY - Photography of a strip of terrain in which the image remains unbroken throughout its length along the line of flight. See SONNE CAMERA.

CONTOUR - An imaginary line on the ground all points of which are at the same elevation above or below a specified datum surface. See APPROXIMATE CONTOUR, CARRYING CONTOUR, DEPRESSION CONTOUR, INDEX CONTOUR, INTERMEDIATE CONTOUR, SUPPLEMENTAL CONTOUR and UNDERWATER CONTOUR.

CONTOUR INTERVAL - The difference in elevation between adjacent contours.

CONTOUR MAP - See TOPOGRAPHIC MAP.

CONTRAST - Difference between tones on a photograph; the variation in brightness of different parts of a negative or positive.

CONTRAST IMPROVEMENT - Image alternation which increases the grey-tone intensity differences.

CONTROL INDEX MAP (USGS) - A map of an area showing by suitable symbols the location of all control lines or stations of a designated category, and indicating the file location of the related control data. See GEODETIC CONTROL DIAGRAMS.

CONTROLLED MOSAIC - A mosaic oriented and scaled to horizontal control; usually assembled from rectified photographs.

CONTROL BASE (USGS) - A surface upon which the map projection and ground control are plotted and upon which pass points resulting from aerotriangulation have been plotted or marked. See BASE SHEET.

CONTROL,  PHOTO - Any station in a horizontal and vertical control system that is identified on a photograph and used for correlating the data shown on that photograph; also termed photocontrol point, picture control point, and ground control point.

CONTROL POINTS - Any station (in a horizontal and/or vertical-control system) that is identified on a photograph and used to aid in fixing the attitude and/or position of a photograph or group of photographs.

CONTROL STRIP - 1) A strip of aerial photographs taken to aid planning and accomplishing later aerial photography, or to serve as control in assembling other strips. 2) A strip of film used for control of exposure, development, or both.

CONTROL SURVEY - A survey which provides horizontal or vertical position data for subordinate surveys or mapping.

CONTROL SURVEY CLASSIFICATION - A series of designations to classify control surveys according to their precision and accuracy. The highest prescribed order of control surveys is designated first order; the next lower prescribed classification, second order; the lowest, third order. Specifications issued by the Bureau of the Budget in 1958 establish three main orders of geodetic control, with additional sub-divisions of classes in some of them. Surveys which fail to meet one of the three preceding specifications are called fourth order although they meet no prescribed standards.

CONVERGENCE OF EVIDENCE - Bringing together several kinds of evidence so that a conclusion may be drawn from all available data.

CONVERGENCE OF MERIDIANS - The drawing together of the geographic meridians in passing from the Equator to the Pole.

CONVERGENT PHOTOGRAPHY - Aerial photography with the camera (or cameras) intentionally tilted so that the central perspective ray of one exposure is inclined to that of an adjacent station.

COORDINATES - A set of numbers used in specifying the location of a point.

COORDINATE SYSTEM - A mathematically defined method for specifying the locations of points. Distances or angles from suitable references located the points within the system. See GEODETIC COORDINATES, GEOGRAPHIC COORDINATES, PLANE COORDINATES, SPHERICAL COORDINATES, STATE COORDINATE SYSTEMS, STATE PLANE COORDINATES and UNIVERSAL TRANSVERSAL MERCATOR.

COORDINATE TRANSFORMATION - Mathematical conversion of coordinate values from one system to another.

COORDINATOGRAPH - An instrument by which the X and Y coordinates of points and/or lines are plotted on a plane, such as in compiling maps or in plotting control points.

COPLANER - Lying in the same plane.

COPYING CAMERA - A precision camera used in the laboratory for reproduction purposes. Also called process camera.

CORNER, FOUND - A term adopted by the U. S. Geologic Survey to designate an existent corner of the public-land surveys which has been recovered by field investigation. See CORNER*.

CORRECTION - A quantity which is applied to an observation or function thereof to diminish or eliminate the effects of errors and obtain an improved value of the observation or function. It is also applied to reduce an observation to some arbitrary standard.

CORRECTION PLATE - A glass plate having a surface ground to a predetermined shape for insertion into an optical system to compensate for known systemic errors in the photogrammetric system.

CORRESPONDENCE - The condition that exists when corresponding images on a pair of photographs lie in the same epipolar plane; the absence of y parallax. See Y PARALLAX.

CORRESPONDING IMAGE POINTS - Unique objects appearing in each of two or more photographs selected to correlate the photograph.

COULEE - 1) A short, blocky, steep-sided lava flow, generally of glassy rhyolite or obsidian, issuing from the flank of a volcanic dome or from the summit crater of a volcano. 2) The term coulee is generally applied throughout the northern tier of states to any steep-sided gulch or water channel and at times even to a stream valley of considerable length.

COURSE - 1) The bearing or, azimuth and length of a line, considered together. 2) The azimuth or bearing of a line along which a ship or aircraft is to travel or does travel, without change of direction; the line drawn on a chart or map as the intended track. The direction of a course is always measured in degrees from the true meridian, and the true course is always meant unless it is otherwise qualified; e.g., as a magnetic or compass course. 3) A route on the earth along which a river flows; the river itself.

COVE - A small bay or open harbor.

COVERAGE - 1) Inclusion of an area within overlapping aerial photos. 2) A measure of the amount of area covered in an aerial photograph, usually expressed as square miles per frame.

CRAB - A photographic condition caused by failure to orient the camera to the flight line resulting in photo edges not parallel to the air base.

CREEK - 1) A steam of less volume than a river. 2) A small steam of intermittent flow.

CRESCENT BEACHES - Crescent-shaped beaches concave toward the sea which form at the heads of bays and at the mouths of streams entering these bays along hilly and mountainous coasts.

CREST - The summit land of any eminence; the highest natural projection which crowns a hill or mountain, from which the surface dips downward in opposite directions.

CRITICAL POINT - A peak or high ground with abrupt local relief requiring investigation in planning aerial photography to avoid “hidden ground.”

CRITICAL SURFACE - A valley terrain of cylindrical form with the air base lying on or near the surface of the cylinder as extended into the air. Orientation becomes indeterminate.

CROSS SECTIONING, DIGITAL - The determination of a vertical section of terrain in which the measurements are stored as numerical values. See DIGITIZING.

CUESTA - A ridge, or belt of hilly land, formed on gently dipping rock strata (e.g. on a coastal plain) from the more durable layers, which resist denudation better than the weaker layers, and are thus left behind as uplands; it has a gentle dip slope on one side, and a relatively steep scarp slope on the other.

CULTURE - Those features of the terrain that have been constructed by man, such as roads, trails, buildings and boundary lines.

CULVERT - A structure carrying traffic over a watercourse having a 10 feet or less clear span. Usually a pipe section.

CURVES - Curved rulers, termed irregular curves, or French curves, used for drawing curved lines. The patterns for these curves are laid out in parts of ellipses and spirals or other mathematical curves in various combinations. Curves for drawing circular curves are highway curves.

CUT - 1) Depth to which material is to be excavated (cut) to bring the surface to a predetermined grade. Therefore, the cut is the difference in elevation of a surface point and a point on the proposed subgrade vertically below it. 2) The name applied to the excavated area itself. 3) A graphic ray from a known plane table position to an unknown position. See CUT*.

CUT TAPE - A survey tape on which the minor graduation are placed only between zero and one unit. See ADD TAPE.

D [Back to Top]

DEP.  - Departure (of a traverse)

DI-10 - A brand name of electronic distance measurement equipment.

DATUM - A position or element in relation to which others are determined; a reference system. In surveying, two principal types of datums are used - horizontal and vertical. See SEA LEVEL DATUM, NORTH AMERICAN DATUM OF 1927, 1927 NAD, HORIZONTAL GEODETIC DATUM and VERTICAL GEODETIC DATUM.

DATUM PLANE - A surface used as a reference from which to reckon heights or depths. The datum in most general use is based upon mean sea level and this is used as the reference for the first-order level net extending over the whole country.

DATUM SHIFT - A graphical correction required where source map data is on a different datum than the map under revision.

DEFLECTION ANGLE (USGS) - The angle, measured in the vertical plane containing the flight direction, between the datum of a model in a stereotriangulated strip and the datum of the preceding model. See DEFLECTION ANGLE*.

DELINEATION - The visual selection and distinguishing of mapworthy features on the surface of various possible source materials by outlining the features on the source material, or on a map manuscript (as when operating a stereoscopic plotting instrument); also, a preliminary step in compilation. See COMPILATION and PHOTO DELINEATION.

DELTA - The fan-shaped alluvial tract formed at the mouth of a river, when it deposits more solid material there than can be removed by tidal or other currents.

DENSITOMETER  - Device used to measure the average grey-tone density of images on a piece of film. The measurement may be a meter reading or an electronic signal.  When the observed area is smaller than a few hundred microns, the instrument is called a microdensitometer.

DENSITY - 1) The comparative amount of silver (or dye) in a given area of a photograph resulting from exposure to light and development. 2) Descriptive of the number of survey control points in an area.

DEPARTURE - The east-west component of a traverse course.

DEPRESSION ANGLE - The vertical angle measured at the perspective center between the true horizon and the photograph perpendicular. The complement of an angle of tilt.

DEPRESSION CONTOUR - A contour forming a closed loop around lower ground.

DEPTH CURVE - A line connecting points of equal depth, referenced to a water surface. Intervals between depth curves may be variable.

DEPTH OF FIELD - The range of object distance (lens to object) within which images formed by a lens, on a given surface, have acceptable sharpness.

DEPTH OF FOCUS - The range of image distances (lens to object) within which the images formed by a lens, on a given surface, have acceptable sharpness. In photogrammetry it defines the range through which the projection distance of a photogrammetric projector can be varied while preserving satisfactory image detail in the stereoscopic model.

DESCRIPTION - The formal published data describing each triangulation, trilateration, traverse station, bench mark or other mark preserving horizontal position and/or elevation. The description contains information regarding location, type of mark and augmenting data which will help find the mark and identify it.

DESERT - An almost barren tract of land in which the precipitation is so scanty or so sporadic that it will not adequately support vegetation.

DETAILS - The small items or particulars of information shown on a map by lines, symbols, and letting. The greater the omission of details the more generalized the map.

DIAPHRAGM - The device for controlling the size of the opening of a lens. The size of the lens opening governs the amount of light reaching the film.

DIAPOSITIVE - A positive photographic print on a transparent medium, generally glass, for use in a plotting instrument.

DIAPOSITIVE CENTERING DEVICE - (USGS) A special device used to position the principal point of a diapositive with respect to the plateholder.

DIAPOSITIVE - A device for making positive copies of negatives in proper format for photogrammetry.

DICHROMATE PROCESS - A photographic rub-on color process used to transfer from one drawing to another.

DIFFERENTIAL LEVELING - The process of measuring distances in elevation by spirit leveling.

DIFFERENTIAL SHRINKAGE - The difference in unit contraction along the grain structure of material as compared to the unit contraction across the grain structure; frequently applied to photographic film and papers and to mapping papers in general.

DIGITAL IMAGE (OR DIGITIZED IMAGE) - An image composed of small, uniformly shaped regions each of which have assigned coordinates and a grey tone. See RESOLUTION CELL.

DIGITAL MAPPING - The process of generating a map using computer-processed data in numerical form.

DIGITIZING - The conversion of a space relationship or measurement to numerical values in which mapping or cross-section data is converted to punch card form by a transducer and digitizer coupled to a recording system.

DIP ANGLE - The vertical angle of the observation point between the plane of the true horizon and a sight line to the apparent horizon. In photogrammetry, this angle is measured in the principal plane of the photograph at the exposure station.

DIRECT ANGLE - An angle measured directly between two lines, as distinguished in transit traverse from a deflection angle.

DIRECT COPY - See AUTOPOSITIVE.

DIRECT MEASUREMENT - Determination of a distance by physical comparison or accumulation of distance using a device calibrated in some unit of measure. See INDIRECT MEASUREMENT.

DIRECTION - The angle between a line and an arbitrarily chosen reference line. When the reference line is north or south and the angle is measured east or west, the direction is called a bearing. When the reference line is south and the angle is clockwise, the direction is called an azimuth.

  DIRECTION THEODOLITE - An instrument which has a horizontal circle which remains in one position during one set of observations.

DIRECT POSITIVE - A film. See AUTOPOSITIVE.

DIRECT RADIAL PLOT - See RADIAL TRIANGULATION.

DIRECT RADIAL TRIANGULATION - See RADIAL TRIANGULATION.

DIRECT TIE - See DIRECT TIE*.

DISCREPANCY - The difference between two results of measurement or computation.

DISPLACEMENT - 1) Any shift in image position which does not affect the perspective characteristics of the photograph. 2) Horizontal shift of plotted position of a topographic feature caused by adherence to prescribed line weights and symbol sizes. See TILT DISPLACEMENT  and   RELIEF DISPLACEMENT.

DISSECTED PLATEAU - A Plateau into which a number of valleys have been carved by erosion; its origin as a plateau is patent, however, when the tops of the mountains and ridges are seen to be level against the skyline, it shows that they once formed part of a continuous surface.

DISTANCE ANGLE - The angle in a triangle under a law-of-sines computation which is opposite the known distance.

DISTORTION - Any shift in the position of an image on a photograph which alters the perspective characteristics of the photograph. Causes of image distortion include lens aberration, differential shrinkage of film or paper, and motion of the film or camera. See FILM DISTORTION and LENS DISTORTION.

DISTORTION CURVE - A curve representing the distortion characteristics of a lens; it is plotted with image radial distances from the lens axis as abscissas and image radial displacements as ordinates. Image displacements away from the lens axis are considered as positive.

DIURAL - Having a period of, or occurring in, or related to, a day.

DIVIDE - A ridge or area of elevated land between two basins or valleys.

DIVIDED HIGHWAY - A highway with separated roadways for traffic in opposite directions.

DODGING - The process of holding back light from certain areas of sensitized material to avoid overexposure of these areas.

DOMESTIC MAP - A mapped area of the U.S.A.

DOPPLER EFFECT (Also DOPPLER SHIFT) - Apparent change in frequency of radiant energy (sound waves, or electrical waves) when the distance between the source and the observer or receiver is constantly changing.

DOT GRAVER - A device for drilling a small circular hole for scribing maps.

DOT GRID - Film positive with regularly spaced dots used as an overlay over a photo or map in determining areas.

DOUBLE CENTERING - A method of prolonging a line from a fixed point whereby the backsight is taken with the telescope in the direct position. The telescope is placed in the indirect position and the foresight is made. The point at which the vertical cross-hair intersects the hub is then marked. The transit is then rotated 180° to take a backsight with the telescope in the indirect position, and a second projected point with the telescope in the direct position is marked on the hub. A point midway between the two marked points is the true point on the prolonged line.

DOUBLE-RODDED LINE - A line of leveling wherein two sets of turning points are used to give independent sets of measurements from the same instrument setups.

DRAFTING GUIDE - See COLOR-SEPARATION GUIDE.

DRAINAGE - All map features associated with water runoff.

DRAINAGE AREA - The area of a drainage basin.

DRIFT - 1) The horizontal displacement of an aircraft, caused by the force of wind, from the track it would have followed in still air. 2) A special condition of crab wherein the photographer has continued to make exposures oriented to the predetermined line of flight while the airplane has drifted from that line.

DUMPY LEVEL - A leveling instrument with its telescope permanently attached to the vertical spindle or leveling base as a single unit.

DUNE - A hill or ridge of sand formed by transportation by the wind.

DUPLICATING FILM - Sensitized emulsion on transparent base manufactured for the special purpose of duplicating photographs, particularly color photographs.

E [Back to Top]

EC STATION - 1) Extended control station.  2) Electronic control station.

ER-55 - See later in this section.

ESNA - Electrical Survey Net Adjuster.

ECCENTRICITY - 1) The amount of deviation from a center.  2) Lack of coincidence of the several centers of rotation, circles, and indexes of an instrument. 3) Horizontal displacement of the instrument or signal from the station mark at the time an observation is made.  4) A measure of the relative shape of an ellipse or an ellipsoid.

ECCENTRICITY CORRECTION - The correction that must be applied to an observation made from an eccentric setup (reduction to center) or to an eccentric signal to compensate for eccentricity. See REDUCTION TO CENTER and SWING.

ECCENTRIC SIGNAL - A signal (target) which is not in the same vertical line with the station which it represents.

EDGE ENHANCEMENT - Image alternation which intensifies the changes between adjacent areas.

EDITING - Checking a map in its stages of preparation to insure correct interpretation of the sources used and precise reproduction.

EFFECTIVE FOCAL LENGTH (USGS) - The adjusted value of the focal length of a lens which best fulfills the geometric conditions of a given photogrammetric operation. In making diapositive plates, the settings of the printer are based on the effective focal length derived from the calibrated focal length and further adjusted to allow for film distortion. In photoalidade operation, the effective focal length is the setting of the focal-length scale which best brings a specific photograph into proper geometric perspective. The preferred expression for this value is “principal distance.”

ELECTRICAL SURVEY-NET ADJUSTER - A panel containing a number of adjustable resistances and d-c power sources for adjustment of survey data.

ELECTROMAGNETIC RADIATION - Energy propagated through space or through a material medium as waves or variations of electric and magnetic fields; known as radio waves, heat waves, light waves, etc., depending upon frequency. Also called electromagnetic energy.

ELECTROMAGNETIC SPECTRUM - Ordered array of known electromagnetic radiations, extended from the shortest cosmic rays, through gamma rays, X-rays, ultraviolet radiation, visible radiation, infrared radiation, and including microwave and all other wavelengths of radio energy.

ELECTRONIC COLOR COMBINER - An instrument which produces false color image by linearly combining a few black-and-white films of the same scenes. The films are usually obtained from multiband and time-sequential photography. The films are put in synchronized flying spot scanners, the resulting video signals are linearly combined through a matrix multiplier circuit, and the three linearly combined signals then drive the color gun of a color TV tube. An electronic color combiner usually has greater versatility for congruencing or registering.

ELECTRONIC DISTANCE MEASUREMENT (EDM) - Measurement made with devices that compare the phase difference between transmitted and returned (i.e., reflected or retransmitted) electromagnetic waves, of known frequency and speed, or the round-trip  transit time of a pulsed signal, from which distance is computed.

ELECTRONIC TRAVERSE - A traverse in which the angles are measured with a direction theodolite and distances with an electronic distance-measuring instrument.

ELEVATION - The vertical distance from a datum, generally mean sea level, to a point or object on the earth’s surface. The terms “elevation” and “altitude” have sometimes been used synonymously, but in modern surveying practice the term “elevation” is preferred to indicate heights on the earth’s surface whereas “altitude” is used to indicate the heights of points in space above the earth’s surface. See FIELD ELEVATION, SPOT ELEVATION and CHECKED ELEVATION.

ELEVATION METER - A mechanical or electromechanical device on wheels that measures slope and distance and automatically and continuously integrates their product into difference elevation.

ELEVATION TINT - The layer of color between selected contours, according to altitude. This is a method of obtaining a mental picture of the terrain more readily.

ELEVATION TONES - A method of accentuating the elevation expressed by contours through the use of colors or different tones of the same color.

ELLIPSOID - The surface generated by rotation of an ellipse about one of its axes.

ELLIPSOIDAL REFLECTOR - A mirror surface which conforms to a portion of an ellipsoid of revolution. See ER-55 PLOTTER.

EMISSION - With respect to electromagnetic radiation, the process by which a body emits electromagnetic radiation as a consequence of its temperature only.

EMULSION - A suspension of a light-sensitive material in gelatin, used for coating photographic film, plates, or papers.

ENCODER - A mechanism usually attached to the output of a measuring device (or photogrammetric instrument) which converts movements into digital information. See DIGITIZING.

ENDLAP - Progressive forward overlap of aerial photos along the line of flight. 

ENGINEERING MAP - A map showing information that is essential for planning an engineering project or development. An engineering map is generally a large-scale map of a comparatively small area or of a route. It may be entirely the product of an engineering survey, or reliable information may be collected from various sources and delineated on a base map.

ENGINEERING SURVEY - The process of collecting and recording information for planning an engineering project.

ENGINEER’S LEVEL - A precision leveling instrument for establishing horizontal line of sight, used to determine differences of elevation.

ENHANCEMENT  - Various processes and techniques designed to render optical densities of imagery more susceptible to interpretation.

EPHEMERIS (PLURAL EPHEMERIDES) - A tabulation of positions and related data for a celestial body for given dates at uniform time intervals. Also, a publication containing such data for a number of celestial bodies. See EMPHEMERIS*.

EPOLAR PLANE - Any plane containing the air base.

EQUALITY - Equality or equation in station numbers arises when a single point on a route alignment has two values. Thus station 123 + 45.6 ahead = 123 + 54.3 back.

EQUATION - 1) A statement of equality of the same point on a route survey which has two values. 2) One of a set of simultaneously solved equality statements which adjust for fixed conditions of length, angles, azimuth or position in least squares adjustment. See EQUALITY.

EQUIVALENT FOCAL LENGTH - The distance measured along the lens axis from the rear nodal point to the plane of best average definition over the entire field used in the aerial camera.

ER-55 PLOTTER - A stereoscopic plotting instrument of the direct-viewing, double projection type, characterized by the use of reduced size diapositives and an ellipsoidal-reflector illumination system in which the light source is at one focus of the ellipsoidal surface and the projector lens is at the other, producing optimum illumination of the entire image area of the diapositive. The principal distance of the projectors is 55 mm. The commercially manufactured version of this plotter is the Balplex.

ERROR - A class of small inaccuracies due to imperfections in equipment or techniques, surrounding conditions, or human limitations; not to be confused with blunders or mistakes. See ACCIDENTAL ERROR, AVERAGE ERROR, CONSTANT ERROR, COMPENSATING ERROR, INDEX ERROR, INSTRUMENTAL ERROR, PERSONAL ERROR, PROBABLE ERROR, MEAN ERROR, RANDOM ERROR, ROOT MEAN SQUARE ERROR, STANDARD ERROR,  and SYSTEMIC ERROR.

ERROR OF CLOSURE - The amount by which a value of quantity obtained by surveying operations fails to agree with a fixed or theoretical value of the same quantity.

ERROR OF THE MEAN – The resultant error of the mean or average of a number of quantities. It is the quotient of the algebraic sum of the errors divided by the number of errors included.

ESTUARINE - Of, or pertaining to, or formed in a estuary.

ESTUARY - Drainage channel adjacent to the sea in which the tide ebbs and flows. Some estuaries are the lower courses of rivers or smaller streams, others are no more than drainage ways that lead seawater into and out of coastal swamps.

ETCHED DRAWING - A color-separation negative produced by a photomechanical process. See PHOTOMECHANICAL ETCHING.

ETCHING - See PHOTOMECHANICAL ETCHING.

EXPOSURE - 1) A photograph. 2) The control of light in making a photograph. Exposure-data refers to camera shutter and aperture settings, together with light intensity measurements, filter factors, and all such controls of light reaching the film.

EXPOSURE INTERVAL - The time interval between taking successive photographs.

EXPOSURE STATION - The point in space occupied by the camera lens at the time of taking the picture.

EXTENDED CONTROL STATION - A described and monumented point established as a basis of subsidiary surveys, usually for Airborne Control System use. Triangle figures or electronic traverses are used to establish position based on higher order surveys in nearby areas.

EXTENSION OF CONTROL - Surveys executed to establish additional control from existing control.

EXTERIOR NODE - See NODAL POINTS.

EXTERIOR PERSPECTIVE CENTER - See NODAL POINTS.

EXTRAPOLATE - To calculate the value of a function lying beyond an interval from values of the function within that interval. In topographic surveys extrapolation is used in estimating the elevation of a point so situated that the elevation cannot be interpolated between two contour lines..

EYE BASE - The distance between the pupils of the eyes of an individual; also called interpupillary distance or interocular distance.

EYEPIECE - The lens or combination of lenses at the observing end of one optical distance.

EYOTT - A small island arising in a river.

F [Back to Top]

f  STOP - A point on the scale of graduations of lens aperture size. The function of focal length divided by aperture of a lens.

FALSE COLOR - Reproduction that shows objects in colors other than their true color. Usually refers to color infrared.

FALSE ORIGIN - An arbitrary zero point to the south and west of a grid zone which is assigned to avoid negative coordinate values.

FATHOM - 1) A unit of distance equivalent to 6 feet, used primarily in marine depth measurements. 2) To find the depth of something; to sound.

FEATHERING - 1) The technique of progressively dropping contours, to avoid congestion on steep slopes. 2) The thinning of overlapping edges of photographs before assembly into a mosaic in order to make match lines less noticeable.

FERTSCH EFFECT - A phenomenon of physiological optics that occurs with rapid scanning of a stereomodel in a direction parallel to the eye base and that appears as a vertical displacement of the  floating mark which reverses with the direction of scanning. The effect is directly related to the relative brightness of the two exposures in the immediate vicinity of observation, and to the fact that the human eye tends to detect the movement of the floating mark more quickly in a bright background than in a dark one.

FIDUCIAL AXES - The imaginary lines defined by opposite fiducial marks on a photograph. The  x axis is considered to be the axis nearly parallel with the flight lines.

FIDUCIAL MARKS - Those marks, which define the axes whose intersection fixes the principal point of the photograph. See FIDUCIAL AXES.

FIELD BOARD - Slang for planetable sheet. See preferred FIELD SHEET.

FIELD CHECK (USGS) - An on-the-site comparison of the features shown on a map compilation with the ground features.

FIELD COMPLETION (USGS) - Obtaining in the field additional information needed to edit and publish a topographic quadrangle map from a compiled manuscript. It includes a comprehensive examination of the compilation for completeness, quality, and topographic expression; the addition, deletion, or correction of map features; the classification of buildings, roads, drainage, and woodland; the mapping of public-land subdivision lines and civil boundaries; obtaining name information; and checking the map for compliance with vertical-accuracy standards.

FIELD CONTOURING - Placing contours on a topographic map by planetable surveys on a prepared base.

FIELD COPY - See preferred FIELD SHEET.

FIELD EDIT - See  preferred FIELD COMPLETION.

FIELD ELEVATION - Any elevation determined by field methods.

FIELD INSPECTION - The process of comparing aerial photographs with conditions as they exist on the ground and of obtaining information to supplement of clarify that which is not readily discernible on the photographs themselves.

FIELD PHOTOINTERPRETATION - The operation of annoting aerial photographs in the field to clarify uncertain photoimages, to add details not discernible on the photographs, and to delete features appearing on photographs not mapworthy or no longer present on the ground. See ADVANCE FIELD COMPLETION.

FIELD POSITION (USGS) – 1) A position determined by field work. 2) A position computed while field work is in progress.

FIELD SHEET (USGS) - A sheet of stable material generally used on a planetable board, either blank or with compiled data, on which fieldwork is plotted. Also called a planetable sheet. In field completion surveys, it contains the compilation to be completed.

FIELD SKETCHING (USGS) - The art of drawing or filling in map detail based on selected positions and elevations plotted on a planetable sheet.

FILL - Use of material to equalize or to raise topography to a certain grade; to build up with fill; to fill low ground with sand, gravel or earth; the material used to artificially raise topography.

FILM BASE - A thin flexible transparent sheet of material used as a support for photographic emulsion.

FILM DISTORTION - The nonuniform dimensional changes which occur in photographic film with changes in humidity or temperature, or from aging, handling, or other causes. (Note: The failure of film to be flat in the camera at the instant of exposure is sometimes referred to, incorrectly, as film distortion. Although lack of flatness may result in errors on the photograph similar to those caused by film distortion, the cause of these errors is incomplete functioning of the pressure or vacuum system of the camera rather than dimensional instability of the film.)

FILM MOSAIC  (USGS) - An assembly of film negatives or positives of a map manuscript adjusted and fitted to a map projection constructed on a transparent base.

FILTER - 1) An electronic device for eliminating or reducing certain waves or frequencies while leaving others relatively unchanged. 2) Any material which modifies radiation transmitted through an optical system by absorption, reflectance, polarizing or scattering.

FIORD - A long, narrow inlet into the sea-coast, with more or less steep sides.

FIRST ORDER - The designation given survey work or equipment which conforms to the highest standards of precision and accuracy. See CONTROL SURVEY CLASSIFICATION.

FIRST ORDER MAGNITUDE RELIEF FEATURE - A continent or an ocean basin.

FIRST ORDER PLOTTER - An obsolescent term originally designating a stereo instrument capable of performing aerotriangulation.

FIXED - A descriptive term used with a bench mark, point, position, station, etc., to designate a point for which geodetic coordinates (latitude and longitude, or elevation) have been established by a previous adjustment or by a more precise survey and are to be held without disturbance in a newer survey or adjustment. See FIXED BOUNDARY*.

FIXING - The process of rendering a developed photographic image permanent by chemically removing the unaffected light-sensitive material.

FLAT - 1) A level surface, without elevation, relief, or prominences; a plain; a level tract along the banks of a river. 2) A level tract lying at little depth below the surface of the water, or alternately covered and left bare by the tide. See OPTICAL FLAT.

FLATNESS OF FIELD - The quality of a lens which affords sharpness of image both in the center and at the edges of a negative.

FLICKER METHOD - The alternative projection of corresponding photographic images onto a tracing-table platen or projection screen, or into the optical train of a photogrammetric instrument. See STEREO IMAGE ALTERNATOR.

FLIGHT ALTITUDE - The vertical distance above sea level of an aircraft in flight. In aerial navigation practice the altitude of an aircraft is always stated in relation to sea level. See FLIGHT HEIGHT.

FLIGHT HEIGHT (OR FLYING HEIGHT) - The vertical distance between an aircraft in flight and the mean ground level of the area being photographed.

FLIGHT HEIGHT TO CONTOUR INTERVAL RATIO - See C FACTOR.

FLIGHT LINE - A line drawn on a map or chart to represent the planned or actual track of an aircraft.

FLIGHT MAP - A map on which are indicated the desired flight lines and (or) the positions of exposure stations before aerial photographs are taken, or a map on which are plotted, after photography, selected air stations and the tracks between them.

FLIGHT SPACING - The distance between adjacent tracks in a series of parallel aircraft flights.

FLIGHT STRIP - A succession of overlapping aerial photographs taken along a single course.

FLOATING MARK - A mark seen as occupying a position in the three-dimensional space formed by the stereoscopic fusion of a pair of photographs and used as a reference mark in examining or measuring the stereoscopic model. The mark may be formed (1) by a real mark lying in the projected object space, (2) by two real marks lying in the projected or virtually projected object spaces of the two photographs, (3)  by two real marks lying in the planes of the photographs themselves, and  (4) by two virtual marks lying in the image planes of the binocular viewing apparatus.

FLOOD-PLAIN - A plain, bordering a river, which has been formed from deposits of sediment carried down by the river. When a river rises and overflows its banks, the water spreads over the flood-plain; a layer of sediment is deposited at each flood, so that the flood-plain gradually rises.

FLY LEVELING - Spirit leveling in which some of the restrictions of precise leveling, such as limiting lengths of sights and balancing backsight and foresight distances, are relaxed to obtain elevations of moderate accuracy more rapidly.

FOCAL LENGTH - A general term for the distance between the center, vertex, or rear node of a lens (or the vertex of a mirror) and the point at which the image of an infinitely distant object comes into critical focus. The term must be preceded by an adjective such as ”equivalent” or “calibrated” to have a precise meaning. See BACK FOCAL LENGTH, CALIBRATED FOCAL LENGTH, EFFECTIVE FOCAL LENGTH, EQUIVALENT FOCAL LENGTH and NORMAL FOCAL LENGTH.

FOCAL PLANE  - The plane perpendicular to the optical axis of the lens in which images of points in the object field are focused.

FOCUS - The point toward which rays of light converge to form an image after passing through a lens or reflecting from a mirror. Also defined as the condition of sharpest imagery. See DEPTH OF FOCUS.

FOOT-METER ROD (USGS) - A stadia rod graduated in feet on one side and in meters on the other.

FOOT PLATE - A metal plate with a knob used as turning point in leveling across sand.

FORESIGHT - 1) A sight on a new survey point in connection with a survey, or on a previously established point to close a circuit. 2) In leveling, a reading on a rod held on a point to determine its elevation. See SIDE SHOT and FORESIGHT*.

FORMAT - 1) The dimensions of the negative area within the focal-plane frame of the camera. 2) The dimensions of a map.

FORMAT CENTER - The point near the center of a photograph at the intersection of the lines between fiducial marks.

FORM LINES - Lines resembling contour lines, drawn to represent the shape of the terrain but without regard to true vertical datum or regular spacing.

FOUND CORNER - A term adopted by the U.S. Geologic Survey for an existent corner of the Public Lands Surveys which has been recovered by field investigation. See CORNER*.

FRACTIONAL SCALE - A map scale expressed as a fraction, as 1/24,000. See REPRESENTATIVE  FRACTION.

FREQUENCY - Number of repetitions of a periodic process per unit time.

FUSION - That metal process which combines the two perspective images on the retinas of eyes in such a manner as to give a mental impression of three dimensional model.

G [Back to Top]

GAT  - Greenwich Apparent Time.

GCT - Greenwich Civil Time.

GHA - Greenwich Hour Angle.

GMT - Greenwich Mean Time.

GAP - Any space where aerial photographs fail to meet minimum coverage requirements.

GENERALIZATION - Modification of contours on a source map preparatory to reduction and conversion to larger contour interval to show terrain without clutter.

GENERAL PURPOSE MAP (USGS) - A map designed to provide a large amount of general information for widespread public use.

GEODESIC - The shortest line connecting two given points on the surface of an ellipsoid.

GEODESY - The science which treats mathematically the shape and size of the earth; also, the branch of surveying in which measurements are made for determining the shape of the earth including precise horizontal and vertical positions on its surface. One branch of geodesy includes gravity forces.

GEODETIC - Referred to or based on considerations of geodesy.

GEODETIC AZIMUTH - The horizontal angle at station A measured from a north-south plane (perpendicular to the reference ellipsoid) clockwise to an ellipsoidal normal section passing through station B. Geodetic azimuth is determined by applying a correction to astronomic azimuth or by computations on the referenced ellipsoid. The azimuth from A toward B is the forward azimuth while the azimuth from B toward A is the back azimuth of station B. See GEODETIC.

GEODETIC CONTROL - A system of horizontal or vertical survey stations that have been established and adjusted by geodetic methods.

GEODETIC CONTROL DIAGRAMS - A series of index maps which show the location of precise surveys of  U.S.C. & G.S., U.S.G.S. and other agencies.

GEODETIC COORDINATES - Quantities which define a horizontal position on an ellipsoid of reference with respect to a geodetic datum. See GEOGRAPHIC COORDINATES.

GEODETIC LATITUDE - 1) the angle between the plane of the equator of the referenced ellipsoid and the normal to the ellipsoid surface. 2) The latitude of a point determined by geodetic methods.

GEODETIC LEVELING - Spirit leveling of a high order of accuracy, generally extended over large areas, with application of orthometric corrections, to furnish accurate vertical control for surveying and mapping operations.

GEODETIC LINE - See GEODESIC.

GEODETIC LONGITUDE - 1) The dihedral angle between an arbitrary meridian and the meridian of an ellipsoidal normal. 2) A longitude determined by geodetic methods.

GEODETIC NORTH - The direction of the pole of the earth ellipsoid of reference.

GEODETIC POSITION - Geographic coordinates of a point determined by geodetic methods.

GEODETIC SATELLITE - An earth orbiting satellite equipped to make it useful for geodetic observations.

GEODETIC SURVEY - A precise survey of considerable extent which takes into account the shape of the earth.

GEODIMETER - Trade name for an electronic distance measuring system.

GEOGRAPHIC - Signifying basic relationship to the earth considered as a globe-shaped body. The term “geographic” is applied alike to data based on the ellipsoid (geodetically determined) and on the geoid (astronomically determined).

GEOGRAPHIC COORDINATES - (U.S.C. & G.S. Sp. Pub 242) An inclusive term, used to designate both geodetic coordinates and astronomic coordinates.

GEOGRAPHIC POSITION - Coordinates usually expressed as latitude and longitude of a point which are usually determined by geodetic methods but occasionally by astronomic observations.

GEOID -  The figure of the earth considered as a sea level surface extended continuously through the continents. It is a theoretically continuous surface that is perpendicular at every point to the direction of gravity (the plumbline). It is the surface of reference for astronomic observations and for geodetic leveling.

GEOLOGIC SURVEY - A survey or investigation of the character and structure of the earth.

GEOLOGY MAP - A map showing contours and various strata of soils and underlying rocks composing the surface of the mapped area.

GORE (USGS) – 1) An irregularly shaped tract of land, generally triangular, left between two adjoining surveyed tracts, because of inaccuracies in the boundary surveys or as a remnant of a systematic survey. 2) A lune shaped map used in making a globe. 3) A fillet of paved surface between two merging highway lanes. See HIATUS.

GORGE - A valley which is more than usually deep and narrow, with steep walls; there is no sharp distinction between a gorge and a canyon, though the latter is generally of much greater size. The sides of a small gorge are sometimes nearly vertical.

GRAD - A European angular unit of measure equal to 1/400th of a full circle. Also spelled GRADE

GRAIN - 1) One of the discrete silver particles resulting from the development of an exposed light-sensitive material. 2) The fibers of a paper, such as that used for photographic prints. 3) The predominant direction in which fibers run.

GRAPHIC RECTIFICATION - A technique for determination of photograph rectification by graphical means.

GRAPHIC SCALE - See BAR SCALE.

GRATICULE - 1) A network of lines. 2) The network of parallels of latitude and meridians of longitude plotted on a map, or chart according to a projection.

GRAVER - A scribing instrument for cutting through the plastic coating without disturbing the base material. See BUILDING GRAVER, COMBINATION GRAVER, DOT GRAVER, PEN-TYPE GRAVER, RIGID GRAVER, and SWIVEL GRAVER.

GREAT CIRCLE - The line of intersection of the surface of a sphere with any plane which passes through the center of the sphere.

GREENWICH MERIDIAN - The meridian passing through the original site of the Royal Observatory, Greenwich, England; uses as zero degrees longitude.

GRID - A network composed of two sets of lines, each set drawn according to a definite pattern and intersecting the other in a specific geometric arrangement. The most common form of grid consists of uniformly spaced parallel lines intersecting at right angles. The term is frequently used to designate a plane-rectangular coordinate system superimposed on a map projection, in which case it generally carries the name  of the projection; that is, Lambert grid, transverse Mercator grid, Universal transverse Mercator grid.

GRID AZIMUTH - Azimuth referred to the Y (or N) axis of a grid system.

GRID DISTANCE - See GRID LENGTH.

GRID LENGTH - The distance between two points obtained by inverse computation from grid coordinates of the points. It differs from the geodetic length by the amount of small correction based on the scale factor for the line.

GRID METHOD - A method of plotting detail from oblique photographs by superimposing a perspective of a grid on a photograph, thereby facilitating the transfer of detail from the photograph to a base.

GRID NORTH - The direction of the earth’s polar axis as plotted ( or computed) on a map projection.

GRID PLATE - A glass plate on which a grid is accurately ruled, used principally for the calibration of photogrammetric instruments. See RESEAU.

GRID TICK - A small mark placed at the edge of a map or drawing to indicate a measurement.

GROUND CONTROL POINT - Any point that has a known location on the earth’s surface which can be identified in ERTS imagery. See CONTROL, PHOTO.

GROUND DISTANCE - A measured distance not yet reduced to sea level (geodetic) distance.

GROUND RESOLUTION - The minimum distance between two objects on the ground (or size of object on the ground) that can be detected.

GROUND SPEED - The velocity of an aircraft along a track with relation to the ground; the resultant of the heading and airspeed of an aircraft and the direction and velocity of the wind.

GROUND SURVEY - A survey made by measurement on the surface of the earth as distinguished from aerial survey.

GROUND SWING - An error condition in microwave distance measurement caused by reflected waves from water, pavement or other smooth surface. The reflected wave combines with the direct wave causing error which can be eliminated by frequency changes.

GROUND TRUTH - Term coined for information obtained on surface or subsurface features to aid in interpretation of remotely sensed data. A vague, misleading term suggesting that the truth may be found on the ground. Ground data and ground information are preferred terms.

GULCH - A small ravine; a small, shallow canyon with inclined slopes and steep sides.

GULF - A portion of the sea partially enclosed by a more or less extensive sweep of the coast. The distinction between gulf and bay is not always clearly marked, but in general a bay is wider in proportion to its amount of recession than a gulf; the latter term is applied to long landlocked portions of sea opening through a strait, which are never called bays.

GUYOT - See TABLEMOUNT.

GYROCOMPASS - A north seeking device consisting of a gimballed rotating wheel which alines itself to the earth’s rotation. Some aircraft magnetic compasses are gyroscopically stabilized and are also called gyrocompasses.

GYRO THEODOLITE - A theodolite with a gyrocompass attached.

H [Back to Top]

HI - Height of instrument. See HI*.

HACHURES - A series of lines used on a map to indicate the general direction and steepness of slopes. The lines are short, heavy, and close together for steep slopes; longer, lighter, and more widely spaced for gentle slopes.

HALATION - Reflection during exposure from a film base from a bright area to an adjacent area of the emulsion giving the appearance of a halo upon development.

HANGING VALLEY - The valley of a tributary which enters a main river valley from a considerable height above the bed of the latter, and so forms rapids or waterfalls down the slope.

HARBOR - A stretch of water on the coast which affords shelter to seagoing vessels; it may have been formed naturally, or artificially, or by artificial improvement of a natural feature.

HAZE - Interference with visibility or photographic resolution caused by particles in the atmosphere scattering or reflecting light.

HEAD - A comparatively high promontory with either a cliff or steep face. It extends into a large body of water, such as a sea or lake. An unnamed head is usually called a headland.

HEADLAND - A steep crag or cliff jutting out into the sea.

HEAVY PLOTTER - An obsolescent term indicating a precise photogrammetric instrument such as the Stereoplanigraph or Wild A7.

HEIGHT DISPLACEMENT - Displacement of images radially inward or outward with respect to the photograph nadir, according as the ground objects are, respectfully, below or above the elevation of the ground nadir.

HEIGHT OF INSTRUMENT (HI) - 1) The height of the center of the telescope (horizontal axis) above the ground or station mark. 2) The height of the line of sight of the leveling instrument above the adopted datum.

HERTZ (Hz) - Unit of frequency, cycles per second, in the International System.

HIATUS - A gap in aerial photo coverage. See GAP and HIATUS*.

HIDDEN GROUND - An area which cannot be observed in a photograph because of interference from topographic features.

HIGHLIGHTS - 1) Those portions of a subject from which the greatest amounts of light are reflected. 2) The densest parts of a negative and the lightest parts of a print or transparency.

HIGH-OBLIQUE PHOTOGRAPH - A photograph taken with the camera axis intentionally tilted so as to include the apparent horizon.

HIGH WATER - The maximum elevation reached by rising water. See HIGH WATER MARK*.

HIGHWAY - An undivided road, usually paved, having a built-up driving surface with shoulders. See JEEP TRAIL and ROAD.

HIGHWAY CURVES - Templates for drawing circular curves from their tangents.

HILL - A small portion of the earth’s surface elevated above the surroundings, of lower altitude than a Mountain. In general, an eminence is not considered a mountain unless its elevation, from foot to summit, is well over 1,000 ft.

HINTERLAND - 1) That zone containing the beach flanks and the area inland from the coast line to a distance of five miles. 2) The region lying behind the coast district.

HISTORIC COAST LINE*.

HOLOGRAPHY - The process of recording and reproducing an image using two emulsion coated surfaces and a laser beam which scans the object. No lens is used. The viewer “sees”
 the object in three dimensions and the “object” can be turned to view it from different angles.

HOOK - A Spit which is curved at one end; the curvature may be caused by the action of the waves in rolling material to the sheltered side of the spit

HORIZON - A plane normal to the plumbline at the observers station. See APPARENT HORIZON  and ARTIFICIAL HORIZON.

HORIZON CAMERA - A camera used in conjunction with an aerial surveying camera in vertical photography to photograph the horizon simultaneously with the vertical photographs. The horizon photographs indicate the tilts of the vertical photographs.

HORIZON CLOSURE - The amount by which the sum of a series of adjacent measured horizontal angles around a point fails to equal exactly 360°, the theoretical sum. See CLOSING THE HORIZON.

HORIZONTAL - A plane perpendicular to the plumbline at the point of consideration or origin.

HORIZONTAL CONTROL - Survey information which provides position information used for locating subordinate surveys or mapping.

HORIZONTAL GEODETIC DATUM - An ellipsoid of reference, defined by two dimensions or constants, an initial point on the reference ellipsoid (defined by latitude and longitude) and an azimuth from the initial point to another point. See NORTH AMERICAN DATUM OF 1927.

HORIZONTAL PARALLAX - See ABSOLUTE STEREOSCOPIC PARALLAX.

HORIZONTAL REFRACTION - The lateral effect of terrestrial refraction on an observed direction

HOVERSIGHT - A basic component of an Airborne Surveys System developed by the U.S. Geologic Survey. It consists of a silicone-damped pendulum with a self-contained light source for projecting a beam  of collimated light through a semitransparent mirror. The helicopter pilot can see the light-source image superimposed upon the image of the ground below.

HYDRODIST - A brand of electronic distance measurement device.

HYDROGRAPHIC CHART - A chart showing water features such as; depth, channels, islands, and other aids to navigation.

HYDROGRAPHY - Topography along shore lines, depths of water and submerged features of bodies of water. Also the science or study of bodies of water and related phenomena.

HYPERFOCAL DISTANCE - If an object at a great distance (infinity) be sharply focused, it is found that, without altering the position of the lens, a comparatively near object is still “in focus;” that is, it is rendered without perceptible unsharpness. The distance to this near point is the hyperfocal distance.

HYPERPANCHROMATIC - Films and plates which have a high red sensitivity.

HYPERSENSITIZING - Applied to various methods of increasing the sensitivity of an emulsion; for example, fuming of bathing with ammonia, fuming with mercury, etc.

HYPO - Slang term for sodium hyposulphite, a common fixing agent in photo developing and printing.

HYPSOGRAPHY - See TOPOGRAPHY.

I [Back to Top]

IMC - Image Motion Compensation.

IR - Infrared.

ICONOMETRY - The process of conducting a plan and elevation from the perspective is termed iconometry. It is the reverse of drawing a perspective form plan and elevation. As applied to photographic surveying, iconometry is the process of making a map from photographs.

IDENTIFICATION POSTS - Wood or metal upright stake marked or tagged to help in recovery of survey stations.

IMAGE COMPRESSION - A remote sensing operation which preserved all or most of the information in the image and which reduces the amount of memory needed to store an image or the time needed to transmit an image.

IMAGE ENHANCEMENT     - Any one of a group of operations which improve the detectability of the targets of interest. These operations include, but are not limited to, contrast improvement, edge enhancement, spatial filtering and noise suppression, image smoothing and image sharpening.

IMAGE-MOTION COMPENSATION - A device installed with certain aerial cameras to compensate for the forward motion of an aircraft while photographing ground objects. True image-motion compensation must be introduced after the camera is oriented to the flight track of the aircraft and the camera is fully stabilized.

IMAGE POINT - The image on a photograph corresponding to a specific point on the ground.

IMAGE RAY - Straight line from a ground object, through the camera lens, to the image on the photograph.

IMAGERY - Representation or reproduction of objects recorded on photographic emulsions; visual representation of energy recorded by remote sensing instruments.

IN-AND-OUT STATION (USGS) - A recoverable but unoccupied station incorporated into a traverse by recording a fictitious deflection angle of 180° to reverse the azimuth of the course leading into it, so that the next station coincided with the preceding station and the in-and-out station is used as the backsight for continuing the traverse. In the computations it is treated as an ordinary station in the traverse.

INDEX  CONTOUR - A contour line shown distinctively for easy identification and generally labeled with the elevation value.

INDEX CORRECTION - A correction applied to the reading from any measuring device to compensate for index error. See INDEX CORRECTION*.

INDEX ERROR - A constant instrumental error due to the displacement of the zero or index mark or vernier of an instrument or scale.

INDEX MAP - 1) A map showing location of collections of data which may be other maps, photos, statistical tables or descriptions. 2) A small scale map showing locations or other information about a survey or a project. See ADMINISTRATIVE INDEX MAP, CONTROL INDEX MAP, PHOTOCONTROL INDEX MAP, PHOTOGRAPHY INDEX MAP and SALES INDEX MAP.

INDEX OF REFRACTION - A ratio between the speed of light waves in a medium as compared to the speed in a vacuum.

INDICATED CORNER - A term adopted by the U.S. Geological Survey to designate corner of the public-land surveys whose location cannot be verified by the criteria necessary to class it as a found or existent corner, but which accepted locally as the correct corner and whose location is perpetuated by such marks as fence-line intersections, piles of rock, and stakes or pipe driven into the ground, which have been recovered by field investigation. See OBLITERATED CORNER*.

INDIRECT MEASUREMENT - Any quantitative result determined from its relation to some measurement. A stadia distance, for example, is an indirect measurement.

INDIRECT PHOTOGRAPHY - Photography in which the camera records an image display (television, radar, and so forth).

INERTIAL GUIDANCE - A system for an indirect method of measurement which converts individual accelerations and elapsed time into velocity thence into distance traveled by the measuring system.

INFRARED (IR) - That portion of the electromagnetic spectrum bounded by visible light and by microwave radiation (generally from 0.70 microns to 1000 microns).

INFRARED FILM - Photographic film sensitized to record invisible rays beyond the red end of the light spectrum. It is also sensitive to blue and ultraviolet light and must by used with a red filter to screen out the latter.

INFRARED IMAGERY - A recording in graphic form of radiated electromagnetic energy in the heat (infrared) range of the spectrum.

INFRARED PHOTOGRAPHY - Commonly used term for imagery which is the product of direct-recording camera/infrared film equipment. See FALSE COLOR.

INFRARED RADIATION - Electromagnetic radiation lying in the wavelength interval from about .70 microns to an indefinite upper boundary sometimes arbitrarily set at 1000 microns (0.01 centimeter). At the lower limit of this interval, the infrared radiation spectrum is bounded by visible radiation, whereas on its upper limit it is bounded by microwave radiation of the type important in radar technology. See ELECTROMAGNETIC SPECTRUM.

INFRARED SCANNER - Instrument for obtaining thermal infrared imagery through line scanning techniques. See INFRARED THERMAL SENSING.

INFRARED THERMAL SENSING - Line scanning techniques using infrared scanners with detectors. Usually, the imagery is obtained from selected portions of the 3 to 14 micron region of the spectrum.

INLET - A narrow body of water extending into the land from a larger body of water.

INSTRUMENTAL ERROR - An error arising from imperfect condition of the instrument used. Such an error is usually systematic, but may be accidental or random.

INSTRUMENT STATION - A survey point at which a surveying instrument is set up for making measurements.

INSULAR SHELF - The subaqueous part of an island or archipelago extending from the shore outward beneath the shallow seas to a insular talus where the bottom slopes down rapidly to the ocean depths.

INSULAR TALUS - The steep slope from the lower edge on an insular shelf into the ocean depths. It is characterized by a marked change in gradient and usually begins at a depth of approximately 100 fathoms.

INTERFEROMETER - An apparatus used to produce and measure interference from two or more coherent wave trains from the same source. Interferometers are used to measure wavelengths, to measure angular width of sources, to determine the angular position of sources (as in satellite tracking), and for many other purposes.

INTERIM REVISION (USGS) - A type of limited revision which changes to a published map are determined by reference to current aerial photographs.  Obsolete data are removed from the original drawings (but contours are not corrected), and all new data are combined on a single plate and overprinted in purple. The revised data is not field checked. See MAP REVISION.

INTERIOR ANGLE - An angle between adjacent sides of a closed figure, measured on the inside of the figure.

INTERIOR NODE - See NODAL POINTS.

INTERIOR ORIENTATION - The adjustment of a photograph to a position within a photogrammetric system so that the reconstructed cone of rays is geometrically identical with the cone of rays that entered the camera at exposure.

INTERIOR PERSPECTIVE CENTER - See NODAL POINTS.

INTERMEDIATE BENCH MARK - See BENCH MARK, NONMONUMENTED.

INTERMEDIATE CONTOUR - A contour line drawn between index contours.

INTEROCULAR DISTANCE - See INTERPUPILLARY DISTANCE.

INTERPOLATION - Determination of an intermediate value between fixed or tabulated values from some known or assumed rate or system of change.

INTERPRETATION - The determination of the nature and significance on a qualitative basis of images on aerial photographs and on photographic recordings of other airborne sensors.

INTERPRETER - 1) A person who examines and interprets aerial photographs and other imagery to obtain desired information. 2) A person who deduces the geologic significance of geophysical data.

INTERPUPILLARY DISTANCE - The distance between the pupils of the eyes of an individual.

INTERSECTION - 1) The procedure of determining the horizontal position of an unoccupied point (intersection station) by direction observations from two or more known positions. 2) The procedure of determining the horizontal position of a point by intersecting lines of direction obtained photogrammetrically. The lines of direction may be obtained directly from vertical photographs or by graphic or mathematical analysis of the photographs. 3) The general area where two or more highways join or cross, within which are included the roadway and roadside facilities for traffic movements in that area. See INTERSECTION*.

INTERSECTION STATION - A survey point whose position is determined by directions observed from two or more known stations. See NO CHECK POSITION.

INTERVALOMETER - A timing device for automatically operating the shutter of a camera at specified intervals.

INVAR - An alloy of nickel and steel having a very low coefficient of thermal expansion.

INVERSE COMPUTATION - The computation of the length and azimuths of a line from the coordinates of its end points.

INVERT - The floor, bottom, or lowest part of the internal cross section of a conduit; the flowline.

INVERTED IMAGE - An image which is in reverse order, left to right, as well as from top to bottom.

ISLAND - A body of land extending above and completely surrounded by water at mean high water.

ISLAND SHELF - The zone around an island or island group, extending from the low-water line to the depths at which there is a marked increase of slope to greater depths. Conventionally its edge is taken at 100 fathoms (or 200 meters).

ISLAND SLOPE - The steep slope from the outer edge of an island shelf into great depths.

ISOBAR - A line, generally shown on a weather map, joining points on the earth’s surface having equal barometric pressure (reduced to sea level) at a given time.

ISOBATH - Line on a marine map or chart joining points of equal depth usually in fathoms below mean sea level.

ISOCENTER - The point on a photograph intersected by the bisector of the angle of tilt which is midway between the plumbline and the photograph perpendicular and through the principal point.

ISODIFF - One of a series of lines on a map or chart connecting points of equal correction or difference in datum, especially useful in readjustment of surveys from one datum to another. See ISOLAT and ISOLONG.

ISOGONIC CHART - A chart showing isogonic lines properly labeled with their magnetic declinations. Lines of equal annual change in the declinations are also generally shown.

ISOGONIC LINE - A line joining points on the earth’s surface having equal magnetic declination as of a given date.

ISOLAT - An isodiff connecting points of equal latitude correction.

ISOLINE (USGS) - A line of common scale at the intersection of the planes of two overlapping aerial photographs having common perspective center and equal principal distances, applied generally in Geological Survey to the line of intersection between two components of a set of convergent photographs or the intersection of either wing component with the nominal vertical of trimetrogon photography. In the latter case the isoline becomes an isometric parallel when the nominal vertical is truly vertical.

ISOLONG - An isodiff connecting points of equal longitude correction.

ISOMETRIC PARALLEL - The intersecting line between the plane of a tilted photograph and a horizontal plane having an equal perpendicular distance from the same perspective center. A line parallel to the horizon through the isocenter.

ISTHMUS - A narrow strip of land, bordered on both sided by water, that connects two larger bodies of land.

J [Back to Top]

JEEP TRAIL - A primitive road, usually passable by all-wheel-drive vehicles only.

JUNCTION - The immediate locality in which two or more highway route meet. See INTERSECTION.

JUNCTION CLOSURE (USGS) - The amount by which a new survey line into a junction point fails to give the previously determined position or elevation for the junction points.

JUNCTION POINT - A survey point common to two or more survey lines.

K [Back to Top]

K  - Kelvin.

K FACTOR The B/H ratio. See BASE-HEIGHT RATIO.

KELSH PLOTTER - A brand of stereoscopic plotting instrument of the double-projection type, characterized by the use of contact-size diapositives and a moving illumination system that concentrates light on the portion of the image that is projected to the tracing table.

KELVIN - Thermometer scale equal to the Celsius (formerly called “Centigrade”) degrees plus 273°, thru 0°C = 273°K, 100°C = 373°K. 

L [Back to Top]

LAKE - An extensive sheet of water enclosed by land, occupying a hollow in the earth’s surface. The name is sometimes loosely applied, too, to the widened part of a river, or to a sheet of water lying along a coast, even when it is connected with the sea; there are many graduations, in fact, between bays and lagoons which are almost enclosed and coastal lakes.

LAMBERT CONFORMAL CONIC MAP PROJECTION - A map plotting system in which points on the ellipsoid are mathematically projected onto a cone with its axis identical with the polar axis. The cone surface may be tangent to the ellipsoid or it may cut below the surface (secant) creating two parallels where the scale is exact. The secant form of this projection is the basis of State Plane Coordinate Systems where the zone extends more east-west than north-south.

LAMBERT GRID - An information designation of a plane coordinate system based on a Lambert conformal map projection.

LANDFORM - The shape into which a part of the earth’s surface is sculptured by natural forms.

LAND LINE ADJUSTMENT (USGS) - Positioning the public land lines on the topographic map to indicate their theoretical, or approximate location relative to the adjacent terrain and culture, by reconciling the information shown on BLM Plats with the ground evidence of the location of the lines.

LANDMARK - 1) An object of enough interest or prominence in relation to its surroundings to make it outstanding or to make it useful in determining a location or a direction. 2) Any monument or material mark or fixed object used to designate the location of a land boundary on the ground. See LANDMARK*.

LANDSCAPE MAP - A topographic map made to a relatively large scale and showing all details. Such maps are required by architects and landscape gardeners for use in planning buildings to fit the natural topographic features and for landscaping parks, playgrounds, and private estates. These are generally maps of small areas, and a scale is used of 1 inch equals 20 feet to 1 foot equals 50 feet, depending on the amount of detail.

LAND SURVEY - The process of determining land boundaries and areas. See CADASTRAL SURVEY*  and LAND SURVEY*.

LAPLACE AZIMUTH - A geodetic azimuth derived from an astronomic azimuth by use of the Laplace equation which correlates geodetic and astronomic azimuths using observed longitude and known geodetic position.

LAPLACE STATION - One of several stations selected at intervals in a large system of geodetic triangulation or traverse where both astronomic azimuth and longitude observations are made for the purpose of determining the Laplace correction.

LARGE SCALE MAP - A detailed map at scales of 1” = 1000 ft., or even 1” = 100 ft. See SMALL SCALE MAP.

LASER - An acronym for “light amplification by stimulated emission of radiation.” A device producing coherent energy beams in the spectrum of light or near-light frequencies.

LATITUDE - 1) The angle between the plane of the earth’s equator and a normal to surface at the point. 2) The north-south component of a traverse course. See GEODETIC LATITUDE  and ASTRONOMIC LATITUDE.

LEAGUE - See MARINE LEAGUE.

LEAST SQUARES - A mathematical method for the adjustment of observations, based on the theory of probability. In this method, the sum of the squares of all the computed corrections is made a minimum.

LEFT-READING - A descriptive term for a reverted or mirror image.

LEGEND - A description, explanation, table of symbols, and other information, which is printed on a map or chart for a better understanding and interpretation of it.

LEGENDRE’S THEOREM - The lengths of the sides of a spherical triangle (very short by comparison with the radius of the sphere) are equal to the lengths of the corresponding sides of a plane triangle in which the plane angles are derived by reducing each of the spherical angles by approximately one-third of the spherical excess.

LENS, COLOR CORRECTED - A camera lens specially designed for taking color photographs. Chromatic aberration has been corrected to provide substantially optimum resolution for all visible and near infrared wavelengths on the same focal plane.

LENS DISTORTION - An aberration affecting the position of images of the optical axis. The radial component is measured from the principal point. The tangential component is measured normal to a radial line from the principal point.

LENS ELEMENT - One lens of a complex lens system.

LEVEL - 1) Horizontal. 2) A leveling instrument. 3) An attachment (such as a spirit level) to indicate when an instrument or other device is level. See AUTOMATIC LEVEL, CIRCULAR LEVEL, DUMPY LEVEL, ENGINEER’S LEVEL and TILTING LEVEL.

LEVEL CIRCUIT - The measurement of bench mark elevations by spirit leveling in two different directions from one end of the circuit to another. Different turning points are used in each of the two different directions.

LEVEL DATUM - A surface used as a reference for elevation or depth measurements. It may consist only of an assumed elevation value for a given benchmark or may be based on mean sea level.

LEVELING (OR LEVELING)  - 1) The process of measuring the difference of elevation between any two points, usually by spirit leveling. 2) The process of orienting the photogrammetric model to a level surface during absolute orientation. See FLY LEVELING, GEODETIC LEVELING, RECIPROCAL LEVELING, SPIRIT LEVELING, TRIGONOMETRIC LEVELING and WATER LEVELING.

LEVEL SURFACE - A surface which at every point is perpendicular to the plumbline or the direction in which gravity acts.

LIGHT - That form of radiation that is capable of detection by the human eye. Visible radiation (about 0.4 to 0.7 micron in length).

LIGHT SCATTER - Breaking up of light rays by refraction and reflection from particles of moisture or solid matter suspended in the atmosphere.

LIGHT, TRANSMITTED - Light that has traveled through a medium without being absorbed or scattered.

LIMITED REVISION (USGS) - The correction of specified map deficiencies only. Usually this will be confined to the updating of certain major features which would be specified for each project, or it might be limited to certain types of features within portions of a total project area. See MAP REVISION.

LINEAR PARALLAX - See preferred ABSOLUTE STEREOSCOPIC PARALLAX.

LIST OF DIRECTIONS - A tabulation of points or objects observed during triangulation with the horizontal directions referred to an initial.

LITHOGRAPHY - A printing process in which the image to be printed is ink-receptive and all other areas are ink-repellent. See OFFSET PRINTING.

LITTORAL  - Of or pertaining to, a shore, especially a seashore. More specifically applied to the depth zone of the sea floor lying between tide levels.

LOBE - The tongue of land within a meander. When the lobe lies between two stream meanders and is connected with the mainland by a narrow passage, the narrow passage is the neck. The cutting action of the river narrows the neck until finally the river breaks through and forms a new channel or a cutoff.

LOCATION SURVEY - 1) The establishment on the ground of points and lines which have been previously been determined by computation, or by graphical methods, or by description obtained from deeds and maps or other records. 2) In private practice of surveying, survey of a lode mining claim.

LOGICAL CONTOURING - Sketching contours on a map using spot elevations taken at breaks in slope.

LONGITUDE - The dihedral angle between the observer’s meridian and an arbitrary initial meridian (usually Greenwich). See ASTRONOMIC LONGITUDE, GEODETIC LONGITUDE, GREENWICH MERIDIAN and WASHINGTON MERIDIAN*.

LOW-OBLIQUE PHOTOGRAPH - An oblique aerial photograph that does not include the apparent horizon.

LOXODROME - See RHUMB LINE.

M [Back to Top]

m  - Meter.

MAP/MYP  - Management Action Plan/Multi Year Program (A BLM Planning System-not a mapping term).

M & P FACTORS -  Tables used for computing arcs on the meridians and parallels. See M & P FACTOR*.

MSL - Mean Sea Level.

MSS - Multispectral Scanner Subsystem.

MAGAZINE - A container for rolled film or photographic plates attached to the camera body.

MAGNETIC AZIMUTH (OR BEARING) - Azimuth (or bearing) referred to magnetic north or south.

MAGNETIC DECLINATION - The bearing of magnetic-north at a particular date. See MAGNETIC VARIATION and MAGNETIC DECLINATION*.

MAGNETIC NORTH - The direction indicated by the north end of a magnetized needle under influence of the earth’s magnetic field and free of local magnetic disturbance.

MAGNETIC VARIATION - Regular or erratic change in magnetic declination. Not interchangeable with magnetic declination. See MAGNETIC VARIATION*.

MAIN SCHEME STATION - 1) (USGS) One of the principal stations of a triangulation arc or net, observed, computed, and adjusted in accordance with the general specifications for the project, and contributing to the overall strength of the system. 2)  (NGS) A station through which basic data are carried for the continued extension of a survey system.

MAKELINE - A scale line (or one of a corresponding pair of lines) furnished to a process camera operator with instructions for a ratio of enlargement or reduction.

MANUSCRIPT MAP - The original drawing of a map as compiled or constructed from various data, such as ground surveys and photographs. Usually place and feature names are not included.

MAP - A representation on a plane surface, at an established scale, of the physical features (natural, artificial, or both) of a part or whole of the earth (or any celestial body) by means of symbols and labels and with the means of orientation indicated. A map may emphasize, generalize, or omit the representation of certain features to satisfy specific requirements. The type of information which a map is designed primarily to convey is frequently used, in adjective form, to distinguish it from maps of other types. See BASE MAP, COMPILED MAP, ENGINEERING MAP, GENERAL PURPOSE MAP, PLANETABLE MAP,  PLANIMETRIC MAP, PLAINMETRIC BASE MAP, QUADRANGLE MAP, RECONNAISSANCE MAP, SHADED RELIEF MAP, SPECIAL PRINTING MAP, SPECIAL PURPOSE MAP and TOPOGRAPHIC MAP.

MAP OF STANDARD FORMAT - A map with dimensions, layout, lettering, and symbolization in accordance with the specifications for the series.

MAPPING ANGLE - The correction to be applied to geodetic azimuth before plotting an azimuth on a map projection.

MAPPING CAMERA - A camera specifically designed for use in surveying. Generally the term indicates a high precision camera although camera requirements for aerotriangulation may exceed the capability of a mapping camera.

MAP PROJECTION - A system of lines on a plane representing a corresponding system of imagery lines on an adopted terrestrial or celestial datum surface; also, the mathematical concept of such a system. For maps of the earth, a projection consists of (a) a graticle of lines representing parallels of latitude and meridians of longitude, or (2) a grid. A map projection may be derived by geometric construction or by mathematical analysis. Projections derived by mathematical analysis are generally used for maps constructed with survey data. See ALBERS CONICAL EQUAL AREA PROJECTION, LAMBERT CONFORMAL CONIC PROJECTION, MERCATOR PROJECTION, OBLIQUE MERCATOR PROJECTION, ORTHOGRAPHIC PROJECTION, POLYCONIC PROJECTION, STEREOGRAPHIC PROJECTION, TRANSVERSE MERCATOR PROJECTOR, UNIVERSAL TRANSVERSE MERCATOR PROJECTION and POLAR STEREOGRAPHIC PROJECTION.

MAP REVISION (USGS) - Updating, improving, and/or correcting map content at the same scale. A complete revision consists of the correction of all deficiencies in planimetry and relief features, including improvement of accuracy, vertical and/or horizontal, to result in a class 1 map which meets current specifications in all respects. See also STANDARD REVISION, LIMITED REVISION and INTERIM REVISION.

MAP SCALE  - The relationship between a distance on a map and the corresponding distance on the earth. Map scale may be expressed as an equivalence, usually by different units (1/63,360 or 1:63,360); or graphically, as a bar scale. See FRACTIONAL SCALE and REPRESENTATIVE FRACTION.

MAP SERIES - (USGS) A family of maps conforming generally to the same specifications or having some common unifying characteristics. The term ”National Topographic Map Series” is used to designate collectively the several quadrangle and other map series of the United States and its dependencies. The individual series are 7.5 Minute Series - Covers 7.5 minutes of latitude and 7.5 minutes of longitude.

Puerto Rico - 7.5 Minute Series - Bilingual & Metric. 15 Minute Series - Covers 15 minutes of Lat.

& Longitude.

Alaska 1:63,360 series covers 15 minutes in latitude. 1:250,000 series generally covers 1° in latitude

and 2° in longitude.

Metropolitan Area Series - Selected cities

National Park Series

State Series - Available as base maps

Topographic and shaded

United States Series -

International Map of the World Series - U.S. portion

    30 minute series - now being superseded

    1° degree series - now superseded by the 1:250,000 series.

Alaska Reconnaissance Series now being superseded by the 1:250,000 series.

MARGINAL, DATA - Information on the margin of maps, explaining symbols, geographic coordinates, and other data portrayed by the map.

MARGINAL TICK - See GRID TICK.

MARINE LEAGUE - A measure of distance commonly employed at sea, being equal to one-twentieth part of a degree of latitude, or three geographical or nautical miles.

MARK - 1) A definite object ( such as an imprinted metal disk) used to designate a survey point. Usually used with qualifying term such as a station mark, reference mark, azimuth mark, or bench mark. Sometimes refers to the entire survey monument. 2) A call used to indicate the instant of observation. 3) A call sometimes used by the rear chainman to indicate he is on the correct reading.

MARSH - Low-lying wet ground ordinarily covered with water. See SALT MARSH* and SWAMP.

MATCH LINE - 1) The edge of an individual photograph in a mosaic. 2) The line at the edge of a mapped area.

 MEANDER LINE - A traverse along the approximate mean high water line of a permanent natural body of water. See MEANDER LINE*.

MEAN ERROR - An ambiguous term sometimes used to denote average error, error of the mean, or root-mean-square error.

MEAN REFRACTION - The average refraction effect on vertical angles in stated conditions of temperature, humidity, and barometric pressure.

MEAN SEA LEVEL (USGS) - The average height of the sea for all stages of the tide. It is obtained, at any particular coastal location, by averaging the hourly tide heights over a long period. The theoretical tide cycle is 19 years. See MEAN SEA LEVEL*.

MEDIAN - See MEDIAN LINE*.

MERCATOR PROJECTION - A map making system in which points on the ellipsoid are projected onto a cylinder whose axis is in the plane of the central meridian of the map and through the earth’s center. The meridians appear as parallel lines and parallels are at right angles to them. The scale at any point is the same north and south which is achieved by mathematical calculation of parallel spacing. Generally used for maritime purposes.

MERCATOR TRACK See RHUMB LINE.

MERIDIAN - A north-south line; a line of constant longitude; a plane through the earth’s axis.

MERIDIAN CONVERGENCE - See CONVERGENCE OF MERIDIANS.

MERIDIAN, GRID - A line on a grid that is parallel to the central meridian or y axis of the grid system.

MERIDIONAL OFFSETS - Small distances applied to the meridional differences in order to create the curves of the latitudes on a map projection.

MESA - A tableland; a flat-topped mountain or other elevation bounded on at least one side by  a steep cliff.

METER - The new (1952) International Standard Meter is defined as 1,650,763.73 times the wavelength of krypton light where one foot equals 0.3048 meters.

The American Survey Foot retains the old definition of the meter which is 39.37 inches.

METES AND BOUNDS SURVEY*.

MICRODENSITOMETER - Scanning optical machine which measures image densities. See DENSITOMETER.

MICROMETER - 1) Unit of length equal to 10⁶   meter. 2) Reading device on a theodolite.

MICRON (SYMBOL) - Unit of length exactly to 10-⁶ meter, 10-⁴ centimeter, (the term is now replaced by micrometer).

MICROWAVE - Of, or pertaining to, radiation in the microwave region.

MICROWAVE REGION - Commonly, that region of the radio spectrum between approximately 1000 and 300,000 megahertz. Corresponding wavelengths are 30 centimeters to 1 millimeter.

MID-LATITUDE - 1) The latitude of the midpoint of a survey line. 2) The average of the latitude values for the end points of a straight line or a geodesic.

MILITARY GRID - A rectangular grid, coordinate, or reference system placed on a map projection to facilitate location and identification of map data for military purposes. The Universal Transverse Mercator (UTM) grid is used on maps at scales of 1:250,000 and larger between 80° north and 80° south. Beyond the 80° parallels the Universal Polar Stereographic (UPS) grid is used.

MINE SURVEY - A survey to determine the position and dimensions of underground workings and the associated improvements and boundaries. See MINE SURVEY* and MINERAL SURVEY*.

MINERAL MANAGEMENT MAP SERIES - A BLM compilation of planimetric and Land Status Data consisting of SURFACE MANAGEMENT MAPS and SURFACE-MINERALS MANAGEMENT MAPS.

MISCLOSURE*.

MISMATCH - The condition of map detail displacement along neatlines.

MODEL - A 3-dimensional image of the terrain seen when a pair of overlapping photographs is viewed stereoscopically. When the model is correctly oriented to the horizontal and vertical datums, the terrain is accurately represented in miniature.

MODEL SCALE - The relationship between a distance measured in a model and the corresponding distance on the terrain.

MONOCOMPARATOR - An optical instrument for measurement of coordinates on a photograph which employs one eyepiece (rather than stereo viewing). See COMPARATOR.

MOSAIC - An assembly of aerial photographs whose edges have been feathered and matched to form a continuous photographic representation of a portion of the earth’s surface. Maps can be mosaiced for compilation purposes. See CONTROLLED MOSAIC, UNCONTROLLED MOSAIC, and FILM MOSAIC.

MOSAICKING - The process of making composite photographs or maps.

MOUNTAIN - A mass of land considerably higher than its surroundings, and of greater altitude than a hill; an eminence is often considered a mountain rather than a hill when its elevation from the foot to summit is well over 1,000 ft., but the distinction is arbitrary. The summit area of a mountain is small in proportion to the area of its base; in this respect it differs from a plateau.

MOUNTAIN GROUP - A group made up of several or many mountain peaks, or of short mountain ridges. The Catskill Mountains and the Black Hill are examples.

MOUTH - The exit or point of discharge of a steam into another stream or a lake or sea.

MUD LUMPS - Swellings of bluish-gray clay forming small islands of an acre or more, with the height of 5 to 10 feet above sea level, found at the mouths of the Mississippi; apparently caused by pressure of surface deposits upon buried clays.

MULTIBAND - Simultaneous use of two or more sensors to obtain imagery from different portions of the reflectance portion of the electromagnetic spectrum (most commonly used in connection with black and white photography).

MULTIPLE BAND - Images formed usually simultaneously in more than one portion of the photographic region of the electromagnetic spectrum and analyzed jointly.

MULTIPLEX PLOTTER - A stereoscopic plotting instrument of the double-projection anaglyphic type which uses reduced-scale diapositives, stationary lamphouses with condensing lenses, and projectors designed for an optimum projection distance of 360 mm.

MULTISPECTRAL   - Designates imagery formed, usually simultaneously, in more than one spectral region and analyzed jointly. The simultaneously use of two or more sensors to obtain imagery from different portions of the electromagnetic spectrum.

MULTISPECTRAL SCANNER SUBSYSTEM (MSS) - The ERTS 1 equipment which oscillates a flat mirror between the field of view and the telescope and which gathers data on four bands simultaneously.

MUSKEG - Arctic alluvial areas with insufficient drainage over which moss has accumulated to a considerable depth. These swamps are usually covered with tamarack and fir trees. In places the surface is broken by tall hummocks.

N [Back to Top]

NMAS - United States National Map Accuracy Standards.

NADIR - 1) That point on the celestial sphere intersected by the plumbline extended downward; that point, directly opposite the zenith. 2) The point on the ground or at sea level datum vertically beneath the perspective center of the aerial camera lens, or the photographic image of that point.

NADIRSCOPE - A device for locating the nadir point of each exposure of a stereomodel when absolutely oriented in a direct-projection plotter. The plotted nadir point on the base sheet can be used to facilitate the orientation of succeeding stereomodels or to assist in the rectification of photographs for mosaicking.

NANOMETER - Unit of measure equal to one millimicron or one millionth of a millimeter.

NARROW ANGLE LENS - A lens having an angle of coverage up to 60°.

NATIONAL ATLAS OF THE UNITED STATES - A collection of maps and charts, with descriptive and bibliographic text, arranged to present an accurate graphic concept of the salient physical, historical, political, economic, social, and cultural features of the country.

NATIONAL MAP ACCURACY STANDARDS - See UNITED STATES NATIONAL MAP ACCURACY STANDARDS.

NATIONAL TOPOGRAPHIC MAP SERIES - See MAP SERIES.

NATURAL SCALE - 1) True scale as it exists in nature, without magnification or reduction. Some map projection tables are published at natural scale to permit easy conversion to a variety of map scales. 2) Map scale expressed as a fraction or ratio, independent of linear units of measure. See (preferred) FRACTIONAL SCALE.

NEAR INFRARED - That portion of the electromagnetic spectrum between visible light and thermal infrared with wavelengths from .7 to 4 microns.

NEATLINE - The line on a map at the boundary of the mapped area. Also called sheetline.

NEAT MODEL - The portion of the gross overlap of a pair of photographs that is actually used in photogrammetric procedures. Generally the neat model approximates a rectangle whose width equals the air base and whose length equals the width between flights.

NEGATIVE - In black-and-white photography, imagery in which the light and dark tones of the object are reversed. In color photography, imagery in which the light and dark tones of the object are reversed and the colors are complementary to those of the object.

NEGATIVE ENGRAVING - The operation of making a negative by removing portions of a coating applied to a scale-stable medium. See SCRIBING.

NEGATIVE TITLING - Information recorded on the negative for identification.

NERITIC - 1) Related to shallow water on the margins of the sea, generally that overlying the continental shelf. 2) Related to the shallow sea bottom, generally that of the continental shelf.

NERTIC ZONE - 1) That part of the sea floor extending from the low tide line to a depth of 200 meters. 2) A part of the pelagic division of the oceans with water depths less than 200 meters.

NET - A series of triangulation figures covering an area in such a way that the lengths and relative directions of all lines forming the triangles can be computed successively from a single base.

NETWORK (USGS)  - A group or series of survey lines interconnected to form a number of closed loops or circuits.

NEW YORK ROD - A two piece leveling rod with a movable target. The rodman reads a vernier for target elevations.

NO CHECK POSITION - A horizontal control survey station observed from only two stations; an intersection station. An elevation determined from vertical angles sometimes is used to guard against gross blunders.

NODAL POINTS - Two points associated with a lens system, such that any ray in the object space directed toward the first or front point will emerge in the image space from the second or rear point and parallel to its former position.

NOISE - Any undesired sound; by extension, a noise is any unwanted disturbance within a useful frequency band, such as undesired electric waves in a transmission channel or device.

NOMINAL FOCAL LENGTH - An approximate value of the focal length, rounded to some standard figure, used for the classification of lenses, mirrors, or cameras.

NORMAL - In general, a straight line perpendicular to a surface or to another line; also, the condition of being perpendicular to a surface or line.

NORMAL ANGLE LENS - A lens having an angle of coverage between 60° and 75°.

NORMAL POOL ELEVATION - The level at which a controlled body of water in generally maintained.

NORMAL SECTION - A straight line cut by a plane perpendicular to the surface of the earth ellipsoid at the observer’s station.

NORMAL TENSION - The pull applied to a tape which stretches the metal in an amount equal to the shortening caused by sag.

NORMAL WATER LEVEL (USGS)  - The most prevalent water level in a watercourse, reservoir, lake or pond, generally defined by a shoreline of permanent land-type vegetation. Along large bodies of water, wave action may retard vegetation beyond the normal shoreline. See HIGH WATER MARK*.

NORTH - The primary reference direction relative to the earth. See ASTRONOMIC NORTH, GEODETIC NORTH, GRID NORTH, MAGNETIC NORTH, and TRUE NORTH.

NORTH AMERICAN DATUM OF 1927 (1927 NAD) - This datum is identical with the North American Datum except that the azimuth was changed. It was adopted in 1927 after a readjustment of the triangulation of the entire country, in which Laplace azimuths were introduced. It is now standard geodetic datum on the North American continent.

NORTH AMERICAN DATUM (NAD) - A Geodetic Datum for horizontal surveys superceded by the North American Datum of 1927.

NORTH ARROW - A symbol indicating the direction and the type of meridian to which the control framework of a map or drawing is referenced. Auxiliary arrows may be shown indicating the direction of other meridians which may be of interest to the user of the map.

O [Back to Top]

OCS - Outer Continental Shelf.

OCS/MINERAL MANAGEMENT MAP - A series of maps for use in leasing outer continental shelf lands.

OBJECTIVE LENS - The lens, in a telescope or microscope, which is nearest the object.

OBLATE SPHEROID - The mathematical shape assumed by a homogenous rotating mass, used as an approximation of the earth’s shape.

OBLIQUE MERCATOR PROJECTION - A map plotting system in which points on an ellipsoid are mathematically projected onto a cylinder oriented tangent to an oblique line at the map center; used in one of the Alaska State Plane Coordinate systems.

OBLIQUE PHOTOGRAPH - An aerial photograph taken with the camera axis intentionally tilted from the vertical.

OBSERVATION SCENE - Collection of the image data of one nominal framing area of the earth’s surface; this includes all data from each spectral band of each sensor.

OCCUPY - To set up a survey instrument over a point.

OCEAN - The sheet of salt water which surrounds the great land masses of the earth; it is divided by them into several extensive portions, each known as an ocean, and altogether covers 71 percent of the earth’s surface. The oceans may be divided into three district regions, the Littoral, the Pelagic, and the Abyssal. Alternatively, they may be divided according to depth into four regions: 1) the Continental Shelf, which adjoins the land, 2) the Continental Slope, immediately outside the continental shelf, 3) the deep-sea plain, a wide and almost level area forming most of the ocean floor, and varying in depth from about 2,000 to 3,000 fathoms, and 4) the Deeps.

OFFICIAL OCS PROTRACTION DIAGRAM - A series of maps which define lease blocks on the outer continental shelf.

OFFSET PRINTING - A process of lithographic printing from a flat or cylindrical pressplate in which the inked image on the pressplate is first transferred to a rubber-surfaced cylinder and then printed, or offset, onto a sheet of paper or other material.

OFFSHORE (USGS) - The zone extended seaward for an indefinite distance from the limit of mean low water (Atlantic and Gulf coasts) and from mean lower low water (Pacific coast).

OPEN TRAVERSE - A traverse which does not close upon itself or another known point.

OPEN WINDOW NEGATIVE - A negative having open areas, used as a mask where screens, rulings and tints are to be printed in the open areas.

OPTICAL AXIS - The straight line which passes through the center of curvature of a lens element. In a compound lens, if the centers of the curvature of all the components lie in one straight line, this line is the optical axis of the lens.

OPTICAL COLOR COMBINER - Instrument which produces false or “true” color images by linearly combining a few black-and-white films of the same scene. The films are usually obtained from multi-band or time-sequential photography. The films are put in projectors which are all focused on the same screen and which have various color filters placed in front of their lens. The brightness of the projector’s lamp in each projector can be changed independently thereby changing chromaticity balance.

OPTICAL DENSITY - Photographic transmission density.

OPTICAL FLAT - A surface, usually of glass, specially ground and polished plane within a fractional part of a wavelength of light, used to test the flatness of other surfaces. See PLANE PARALLEL PLATE.

OPTICAL PLUMB (ALSO OPTICAL PLUMMET) - A device for optically determining a vertical line of sight, serving the same function as a plumb bob.

OPTICAL SQUARE - See PENTA PRISM.

ORIENTATION - 1) The act of establishing the correct relationship in direction with reference to the points of  the compass. 2) The state of being in correct relationship in direction with reference to the points of the compass. 3) A map is in orientation when the map symbols are parallel with their corresponding ground features. 4) A planetable is in orientation when lines connecting positions on the planetable sheet are parallel with the lines connecting the corresponding ground objects. 5) A surveyor’s instrument is in orientation if the horizontal circle reads 0° when the line of collimation is parallel to the direction it had at an earlier (initial) position of the instrument, or to a standard line of reference. If the line of reference is a meridian, the circle will show azimuths referred to that meridian. 6) A photograph is in orientation when it correctly presents the perspective view of the ground directly in front of the observer; or when the images on the photograph appear in the same direction from the point of observation as do the corresponding map symbols. See ABSOLUTE ORIENTATION, RELATIVE ORIENTATION and INTERIOR ORIENTATION.

ORIGIN - The point in a system of coordinates which serves as an initial point in computing and where X = 0 and Y = 0.

ORTHOGRAPHIC PROJECTION - A map plotting system whereby parallel lines project from points on the sphere to a plane tangent to the sphere at the map center; not practicable for plotting survey data.

ORTHOMETRIC CORRECTION - A systematic correction which must be applied to a measured difference of elevation because level surfaces at different elevations are not exactly parallel.

ORTHOMETRIC ELEVATION - An elevation value to which the orthometric correction has been applied.

ORTHOPHOTOGRAPH - A photograph derived from perspective photographs and equivalent to a photograph made by orthographic projection. In a perfect orthophotograph, there are no displacements of images because of tilt or relief.

ORTHOPHOTOMAP - A photomap prepared from a orthophotograph or a precisely controlled assembly of orthophotographs. It is generally published in standard map format. See PHOTOMAP.

ORTHOPHOTOMOSAIC - As assembly of orthophotographs, usually precisely controlled, to form a uniform-scale photographic representation of a portion of the earth’s surface.

ORTHOPHOTOSCOPE - An instrument for converting conventional perspective photographs into orthophotographs by differential rectification. It features a double-projection anaglyphic instrument and a movable exposing slit that is used to scan the ground surface of the projected stereomodel in a systematic pattern. It records the orthographically corrected imagery of the blue-filtered diapositive on film insensitive to red light.

OUTER CONTINENTAL SHELF - Under Public Law 212 (the Outer Continental Shelf Lands Act) it is that portion of the continental shelf which lies seaward of state boundaries as defined in Public Law 31 (the Submerged Lands Act).

OVERLAP - The amount by which one photograph duplicates the area covered by another photograph, usually expressed as a percentage.

OVERLAY - 1) A drawing on a transparent medium to be superimposed on another drawing. 2) A transparent medium on which flight lines and/or area to be photographed are plotted to be superimposed on a map, thus avoiding defacing the map.

OVERSHEET - A transparency or a print of a map compilation used for recording supplemental information.

OXBOW - A crescent-shaped lake formed in an abandoned river bend which has become separated from the main stream by a change in the course of the river.

P [Back to Top]

PADS - Position azimuth determination system.

PI - Photo Interpretation.

PUG - Punkt Ubertragungs Gerδte. See POINT MARKING AND TRANSFERRING DEVICE.

PZS - Pole-Zenith-Star or Pole-Zenith-Sun.

PANCHROMATIC - Photographic emulsion sensitive to all colors of light.

PANEL - An element of a target used for ground control or point identification during aerial photography.

PANELING - Marking points on the ground with material of suitable contrast, size, and shape to provide images which can be positively identified on aerial photographs.

PANEL POINT - A photo image of a paneled point.

PANTOGRAPH - A precision mechanical instrument for reproducing at some desired scale, generally at a reduction, details from stereomodels, photographs, maps, drawings, etc.

PARALLACTIC ANGLE - See ANGULAR PARALLAX.

PARALLAX - The apparent displacement of the position of an object with respect to a reference point or system caused by a shift in the point of observation. See ABSOLUTE STEREOSCOPIC PARALLAX, X PARALLAX AND Y PARALLAX.

PARALLAX BAR - See STEREOMETER.

PARALLAX DIFFERENCE - The difference in the absolute parallaxes of two points imaged on a pair of photographs. Customarily used in the determination of the difference in elevations of the objects.

PARALLEL - 1) The relationship between two lines in the same plane which will not meet if extended. 2) A line on the earth (or a map) having the same latitude at every point.

PARALLELOGRAM OF ZEISS - See ZEISS PARALLELOGRAM.

PARALLEL PLATE - See PLANE PARALLEL PLATE.

PASS - A low and passable gap through a mountain barrier.

PASSIVE SYSTEM - A sensing system that detects or measures radiation emitted by the target.

PASS POINT - A point selected on imagery of the same object or point on the ground appearing on three or more photographs, used to relate adjacent stereomodels, or strips horizontally (and usually also vertically) in aerotriangulation; also, the corresponding point on the base sheet used to provide a basis for stereomodel positioning and orientation in map compilation. Pass points are usually chosen at classical points. See CLASSICAL POINT and TIE POINT.

PATTERN - 1) In a photo image, the regularity and characteristic placement of tones or textures. 2) The relations between any more-or-less independent parameters of a response; e.g., the pattern in the frequency domain of the response from an object.

PEAK - A top of a mountain or hill, standing above the level of the range or the surrounding country.

PEDIMENT - In arid and semi-arid regions, the gently sloping plain strewn with boulders that borders the mountains.

PEELCOAT - A transparent plastic material with an opaque coating which can be cut and peeled to prepare open windows for printing masks.

PEG TEST - A method of adjustment of a leveling instrument.

PENCIL OF LIGHT - A ray of light.

PENINSULA - A stretch of land almost surrounded by water.

PENTA PRISM - A five sided prism with angles such that any entering ray is reflected at 90°. See RIGHT ANGLE PRISM.

PEN-TYPE GRAVER - A pin vise with an angled head used in scribing.

PERMANENT MARK - A readily identifiable, relatively permanent, recoverable mark which designates precisely the location of a survey point.

PERSONAL ERROR - An error caused by an observer’s personal habits, mental or physical reactions, or inability to perceive dimensional values exactly. It may be accidental or systematic. A systematic personal error is termed a “personal equation.”

PERSPECTIVE CENTER - The point of origin or termination of bundles of perspective rays.

PERSPECTIVE PHOTOGRAPH - A photograph taken with any apparatus in which all object rays pass through a single point and all image rays also pass through a single point.

PERSPECTIVE PROJECTION - The projection of points by straight lines through a given point to an intersection with the plane of projection. Unless otherwise indicated, the point of projection is understood to be at a finite distance from the plane of projection.

PERSPECTIVE RAY - A straight line from a ground object through the lens to the image on the photograph.

P-FACTOR - Any of the considerations such as atmospheric and terrain conditions, photographic system capabilities, and photogrammetric system capabilities which influence the determination of a maximum flight height for photography consistent with a given requirement for compilation of planimetry. Generally used in a collective sense and referred to as “the P-factor” to denote the combined effect of these considerations.

PHILADELPHIA ROD - A sliding two piece leveling rod with a target.

PHOTOALIDADE - A photogrammetric instrument having a telescopic alidade, a plateholder, and a hinged ruling arm, all mounted on a supporting frame. It is used for plotting lines of direction and measuring vertical angles to selected features on oblique and terrestrial photographs.

PHOTO BASE - Distance between principal points of adjacent aerial photos, usually measured on one photograph after conjugate principal points are transferred.

PHOTOCLINOMETRY - Determination of slopes from study of brightness in one photographic image.

PHOTOCONTROL DIAGRAM - See PHOTOCONTROL INDEX MAP.

PHOTOCONTROL INDEX MAP (USGS) - A map showing the location and identifying data of the centers of the aerial photographs, the existing basic and supplemental control, the planned basic and supplemental control, and the quadrangle and (or) project boundaries. Sometimes an outline of the area covered by each aerial photograph is shown.

PHOTOCONTROL POINT - Any station in a horizontal and (or) vertical control system that is identified on a photograph and used for correlating the data shown on the photograph.

PHOTODELINEATION - The declineation of features on a photograph, usually with colored pencil.

PHOTOGRAMMETRIC CONTROL - Basic surveys established by photographic methods, used to position lower order surveys, mapping or photogrammetric surveys.

PHOTOGRAMMETRIC ELEVATION (USGS) - An elevation established by photogrammetric means. See T-ELEVATION.

PHOTOGRAMMETRIC SYSTEM - A combination of instruments, facilities, and procedures used in producing maps and surveys by photographic analysis including cameras, printers, photographic materials, plotting instruments, control pattern, and personnel.

PHOTOGRAMMETRY - The science or art of obtaining reliable measurements by means of photographs.

PHOTOGRAPH - A general term for a positive or negative picture made with a camera on sensitized material, or prints made from a camera original.

PHOTOGRAPHIC MAP - 1) A photographic copy of an assembly of individual aerial photographs which are arranged along the flight line in their proper relative positions. 2)  An overlay containing the delineated boundary of each photograph, keyed to a base map, and depicting the location and area of coverage of each photograph and/or flight strips of photographs.

PHOTOGRAPH PERPENDICULAR - The perpendicular from the interior perspective center to the plane of a photograph.

PHOTOGRAPH PYRAMID - A surface formed by three image points on a photograph as a base and the apex of a pyramid at the perspective center.

PHOTOGRAPH SCALE - The ratio of the distance between two points on a truly vertical photograph, having no appreciable relief, and corresponding distance on the ground. If the photograph is titled or if the terrain has appreciable relief, the photograph scale is not uniform. The nominal scale of vertical photographs is generally taken as the ratio between the focal length of the aerial camera and the flight height above mean ground. The scale of oblique photographs varies continuously in the direction of tilt, so that the term “photograph scale” represents an approximation only.

PHOTOGRAPHY - The art or process of producing images on sensitized material by the action of light. The proper term for the product in this process is photograph or photographs.

PHOTOGRAPHY INDEX MAP - A map of showing the location and numbers of the flight strips and photographs.

PHOTOIDENTIFICATION - The process of 1) showing the photographic location of ground points in relation to surrounding image detail, and 2) identifying, on the ground, points marked and described on the photographs. See PINPOINTING.

PHOTOIMAGE MAP - See PHOTOMAP.

PHOTOINDEX - An uncontrolled mosaic of photographs in shingle overlay, rephotographed at a reduced scale. It is generally used as a source of information for details of available photographic coverage.

PHOTOINTERPRETATION - The examination of photographic images for the purposes of identifying objects, determining their nature, describing them, and evaluating their significance.

PHOTOMAP - A map prepared on a base containing photoimagery which is retained as a part of the representation. It may incorporate special cartographic treatment, photographic line enhancement, color separation, or a combination of these. See ORTHOPHOTOMAP.

PHOTOMECHANICAL ETCHING - A process in which positive film copy is exposed by arc light onto sensitized paint-coated plastic. The paint is hardened where the light strikes and is unaffected in the unexposed areas. The nonhardened paint areas are removed by a solvent and hand rubbing. The resultant product resembles a scribed negative.

PHOTOMORPHIC IMAGE - A system component used in classification of land which combines airphoto information such as drainage density, photographic texture, settlement patterns and density of structures.

PHOTOMOSAIC - See MOSAIC.

PHOTOTHEODOLITE - An instrument combining the function of theodolite and camera, used in terrestrial photogrammetry.

PHOTOTOPOGRAPHY - 1) (USGS) Mapping in which topographic detail is obtained by photogrammetric methods. 2) (DOD) Mapping in which detail is plotted entirely from photographs at suitable ground stations.

PHOTOTRIANGULATION - The process for the extension of horizontal and (or) vertical control whereby the measurements of angles and (or) distances on overlapping photographs are related into a spatial solution using the perspective principles of the photographs. Generally, but not always, this process involves the use of aerial photographs and is called aerotriangulation.

PHOTOTRIG - A procedure for determining elevations trigonometrically wherein horizontal distances are determined photogrammetrically and vertical angles are either measured instrumentally in the field or are obtained from measurements on terrestrial photographs.

PHOTOTRIG TRAVERSE - A vertical angle traverse employing phototrig methods.

PHOTOTYPE - The product of a photographic process in which a master copy of a machine-composed lettering is made on film. Positive or negative copies are processed from the master image onto adhesive-backed stripping film. See STICKUP.

PICTOMAP - A color reproduction of a standard photomosaic on which the imagery has been converted into interpretable colors and symbols by means of tonal masking techniques.

PICTURE AND CONTROL POINT - See preferred PHOTOCONTROL POINT.

PICTURE POINT CONTROL - Generally, a survey station identified on a photograph. See preferred term PHOTOCONTROL POINT.

PIN POINTING - The process of identifying photoimages by ground comparison which results in thrusting a fine needle through one photograph and identifying the hole on the reverse side. See PHOTOIDENTIFICATION.

PITCH - 1) Rotation of an aircraft in a nose up-nose down direction. 2) Rotation of the camera or coordinate system about the Y axis - (Phi O).

PLANE COORDINATES - In general, coordinates specifying the location of points on a plane. In surveying use, the “plane” is usually a projection of the earth’s surface such as a developed cone or cylinder.

PLANE PARALLEL PLATE - A flat optical element having both surfaces ground flat and precisely parallel.

PLANE SURVEYING - A branch of surveying in which the surface of the earth is considered a plane. For small areas, precise results may be obtained with plane-surveying methods, but the accuracy and precision of the results decrease as the area surveyed increases in size.

PLANETABLE - A devise for plotting survey data directly from field observation. A planetable consists of a drawing board on a tripod with some type of sighting instrument, generally a telescopic alidade, to measure and plot angles graphically.

PLANETABLE MAP - A map made by planetable methods. The term includes maps made by complete field mapping on base projections and by field contouring on planimetric bases.

PLANETABLE SHEET - See FIELD SHEET.

PLANIMETER - A device for measuring small areas by mechanical integration.

PLANIMETRIC BASE MAP - A map prepared from aerial photographs by photogrammetric method as a guide or base for contouring. Manmade features as well as water features are shown.

PLANIMETRIC MAP - A map which presents only the horizontal positions for the features represented; distinguished from a topographic map by the omission of relief in measurable form. Planimetric maps may show bench marks and/or elevations of significant features.

PLANIMETRY - The details of a map shown in plan, generally the culture and water features.

PLASTIC RELIEF MAP - A topographic map printed on plastic and vacuum molded to show relative terrain elevations.

PLATEAU - An extensive, level or mainly level area of elevated land. See MESA.

PLATE COORDINATES - The X and Y coordinates of control points, pass point or other images on a photograph, generally related to the fiducial marks.

PLATEHOLDER, DIAPOSITIVE - A frame or holder for the diapositive plate, specifically designed to facilitate the recovery of correct interior orientation of the diapositive in a particular type of photogrammetric plotting instrument.

PLATEN - The screen, usually a disk with a reflecting plane surface, on which the projected images are viewed in stereoscopic plotting instruments of the double-projection type. 2) The surface in the exposure chamber of the camera against which the film is held tight at the instant of exposure. This is usually a precise plane surface, but in some cameras it is curved.

PLATE PERPENDICULAR - See PHOTOGRAPH PERPENDICULAR.

PLATFORM - The vehicle on which a remote sensor is mounted.

PLAYA - An area of land, such as a Salt-Marsh or an Alkali Flat, which is temporarily filled with water and becomes a shallow, muddy lake after exceptionally heavy rainfall, or the flooding of a river, but dries up again in hot weather.

PLAYBACK - Later transmission of data which was recorded locally at the time of the occurrence.

PLUMB POINT - The nadir point in an aerial photograph.

PLUMMET - A plumb bob or plumbline.

PLUS - A fractional part of 100 feet used in designating a point on a standard survey line at the plus distance from an even station. See STATION.

POINT - 1) A position or location in a reference system determined by survey. 2) A headland or pointed land area projecting into a body of water.

POINTING ERROR - The sum of the discrepancies (usually small) in direction to a theodolite sighting caused by vision defects, heat waves, eye fatigue, parallax or optical illusion.

POINT MARKING - The procedure of designating discrete photographic locations for pass points, tie points, or control stations by physically marking the emulsion of  a print, diapositive, or glass-plate negative. When corresponding image points are marked on overlapping photographs, the procedure is referred to as point transferring.

POINT MARKING AND TRANSFERRING DEVICE - A precision instrument for physically marking photographic emulsions to establish discrete points on a single photograph or corresponding points on a stereopair. Provision for stereoscopic viewing is either built in or added to enable accurate marking and transferring. See PUG and SNAP MARKER.

POINT TRANSFERRING - See POINT MARKING.

POLARIZED LIGHT - Light rays which have been modified such that the transverse vibrations are restricted to a single plane.

POLAR PLANIMETER - See PLANIMETER.

POLAR STEREOGRAPHIC PROJECTION - A stereographic map projection where the point of tangency is at the pole. Meridians appear as radial lines and parallels are concentric circles.

POLE-ZENITH-STAR - The astronomical triangle formed on the celestial sphere.

POLYCONIC PROJECTION - A map projection having the central geographic meridian represented by a straight line, along which the spacing for lines representing the geographic parallels is proportional to the distances between the parallels; the parallels are represented by arcs of circles which are not concentric, but whose centers lie on the line representing the central meridian, and whose radii are determined by the lengths of the elements of cones which are tangent along the parallels. All meridians except the central one are curved. The projection is neither conformal nor equal area, but it has been widely used for maps of small areas because of the ease with which it can be constructed.

POLYESTER FILM - A plastic material (such as polyethylene terephthalate) of high dimensional stability, widely used as a supporting base in aerial films, map manuscripts, and color-separation drawings.

PORRO-KOPPE  PRINCIPLE - The system for elimination of known camera lens distortion by observing the photograph (or projecting it) using a lens with the same distortions as the camera.

PORRO PRISM - A prism that deviates a ray of light 180° - essentially two 90° prisms cemented together.

POSITION - 1) Data which define the location of a point with respect to a reference system. 2) The place occupied by a point on the surface of the earth. 3) The coordinates which define the location of a point on the geoid or ellipsoid. 4) A prescribed setting (reading) of the horizontal circle of a direction theodolite which is to be used for the observation on the initial station of a series of stations to be observed.

POSITION=AZIMUTH DETERMINATION SYSTEM - Military designation for inertial positioning and azimuth reference now termed the Auto Surveyor.

PRAIRIES - 1) The gently undulating, almost flat, generally treeless, grassy plains. 2) Open, grassy areas as small as 160 acres within forested regions of the northwest USA.

PRECISION - (USC & GS) Degree of refinement in the performance of an operation or in the statement of a result.

Precision relates to the quality of execution, and is distinguished from accuracy which relates to the quality of the result. The term precision not only applies to the fidelity with which required operations are performed, but by custom has been applied to methods and instruments employed in obtaining results of a high order of accuracy. Precision is exemplified by the number of decimal places to which a computation is carried and a result stated. In a general way, the accuracy of a result should determine the precision of its expression. Precision is of no significance unless accuracy is also obtained. See ACCURACY.

PRELIMINARY EDITION (USGS) - A map printed and distributed for temporary use with the proviso that it will later be superseded.

PRELIMINARY SURVEY - Collection of data in the field as a basis of studies on a proposed project.

PRESSPLATE (USGS) - A thin metal plate containing a printable image processed from a color-separation drawing.

PRESSURE PLATE - A flat glass plate used for holding film in a plane position by pressing the film against another flat surface. See PLATEN.

PRIMARY STATION Se MAIN-SCHEME STATION.

PRIME VERTICAL - The vertical circle whose plane is perpendicular to the plane of the celestial meridian. The two intersections of the prime vertical with the horizon are the east and west points.

PRINCIPAL DISTANCE - The perpendicular distance from the internal perspective center of a photographic system to the plane of a particular finished negative or print.

PRINCIPAL LINE - The trace of the principal plane on the aerial photograph. It is normal to the axis of tilt and passes through both the nadir and the principal point.

PRINCIPAL PLANE - The vertical plane containing the photograph perpendicular of a tilted photograph.

PRINCIPAL POINT - 1) The foot of the perpendicular from the internal perspective center to the plane of the photograph. 2) Intersection with the earth’s surface of a line which is an extension of the optical axis of a return beam vidicon camera. This point differs from the format center by the boresight angle error from a nominal alignment. 

PRINCIPAL STATION - See MAIN SCHEME STATION.

PRINT - A reproduction of another photograph, a map, a drawing, or other subject material. See CONTACT PRINT, PROJECTION PRINT and RATIO PRINT.

PRINTER - A device for producing reproductions. See DIAPOSITIVE PRINTER, CONTACT PRINTER and PROJECTION PRINTER.

PRINT FILM - Sensitized emulsion on transparent base manufactured for the special purpose of printing  photographs, particularly color photographs.

PRINTING PLATE - A plate from which impressions are taken in a printing press. See PRESS PLATE.

PROBABLE ERROR - There is a 50-50 chance that any measurement will have an error less than the probable error. The probable error equals 0.6745  times the standard error. See STANDARD ERROR and ROOT MEAN SQUARE ERROR.

PROCESS CAMERA - See COPYING CAMERA.

PROFILE - A vertical section of the surface of the ground or underlying strata along any fixed line.

PROFILE LEVELING - The determination of elevations at selected points along an alinement for construction or map accuracy testing.

PROFILING - See PROFILE LEVELING.

PROJECTION DISTANCE - The distance from the external node of a projection lens to the plane onto which the image is projected.

PROJECTION PRINT - A photographic print made by projecting the imagery of a transparency onto a sensitized surface.

PROJECTION PRINTER - A precision diapositive printer in which a lens is placed between the negative and diapositive plate to produce a scale-changed diapositive. For some projection printers, the nominal printing ratio is expressed by the ratio of the nominal focal length of the camera (in millimeters) to the principal distance of the stereoplotter to be used (also in millimeters). Other projection printers, designated as “universal” are capable of producing diapositives in a wide range of printing ratios (i.e., from various camera focal lengths to a variety of stereoplotter principal distances).

PROJECTION TABLES - Mathematical values used for plotting various map projections.

PROJECTOR - An instrument by which an image can be projected onto a viewing surface.

PROJECTOR, REFLECTING - An instrument by means of which the image of an aerial photograph can be projected onto a map. By varying the position of the projector lens, the scale of the projected image can be varied, and, by tilting the table top, compensation can be made for any tilt in the photograph.

PROMONTORY - A headland; a cliff or crag projecting into the sea.

PSEUDOSCOPIC IMAGE (Pronounced Soodoscopic) - The apparent reversal of relief.

PSYCHROMETER - An instrument for measuring relative humidity of the atmosphere. It consists of two similar thermometers, the bulb of one being kept wet. Air is forced past the thermometers by means of  a spring-wound or battery-driven fan motor or by hand whirling.

PUBLICATION SCALE - The scale at which a map is intended to be published.

PUGGING - See preferred term POINT MARKING.

PUG POINT-TRANSFER DEVICE - A brand of point transfer device having a binocular stereoscopic viewing system and a pair of mechanical drills for making holes of specified sizes in the emulsion.

PULL UP - A tracing of selected map detail with line weights and symbol sizes suitable for clarity when reduced to the scale of the map under revision.

Q [Back to Top]

QUADRANGLE – A four-sided figure, bounded by parallels of latitude and meridians of longitude, used as an area unit in mapping. The dimensions are not necessarily the same in both directions.

QUADRANGLE MAP – A map of  a quadrangle. See STANDARD QUADRANGLE MAP.

QUADRANGLE REPORT (USGS) – A brief history of the mapping of a specific quadrangle. It is assembled in a folder beginning with the specification sheet. It accompanies the mapping material through each phase of production (basic control surveys, supplemental control surveys, photogrammetric compilation, field interpretation and completion, and map finishing) and is filed with the map material. The narrative summary for each operational phase stresses conditions that may affect later phases.

QUADRIBRACH – The four armed base of a survey instrument which carries opposed leveling screws.

QUADRIPOD – A four-legged stand for support of an observing platform, signals, etc.

QUANTIZING – The process by which each grey-tone in a digitized image is assigned a different value according to a mathematical model.

R [Back to Top]

REV – Return Beam Vidicon.

RF – Representative Fraction.

RADAR – Radio detection and ranging. An electronic instrument that indicates the distance from a transmitter to a reflecting object by measuring elapsed circuit time of travel of ultra-high-frequency radio waves of known propagation velocity.

RADAR ALTIMETER – An electronic instrument which determines height above terrain during flight.

RADAR MAP – A map produced by the application of radar techniques.

RADIAL – A line or direction from the radial center to any point on the photograph.

RADIAL CENTER – The selected point on a photograph from which radials to various image points are drawn or measured. The radial center can be either the principal point, the nadir point, the isocenter, or a substitute center. Also called center of radiation or center point.

RADIAL DISTORTION – See LENS DISTORTION.

RADIAL LINE METHOD – A system for updating minor detail on a map.

RADIAL LINE PLOTTER – A method of transferring data from stereo viewed photographs via mechanical linkage to a plotter position. A Kail plotter

RADIAL TRIANGULATION – A method of locating photogrammetric control points on a control base by graphic or mechanical means. In this method, radials originating from the radial center of each photograph are used for extending the triangulation by intersection and resection.

RADIANT ENERGY – Energy which is transferred by electromagnetic waves without corresponding transfer of matter.

RADIATION – 1) The process of locating points using their directions and distances from a known point. 2) The process of propagation of electromagnetic energy through any medium or through space. 3) The process of particle emissions, especially radioactive particles. 4) Energy transmission through wave motion.

RADIOMETER – Instrument for detecting and usually also for measuring radiant energy.

RADIOMETRIC – Concerned with the combined electronic and optical transmission of data.

RAISED BEACH – A beach which has been raised by earth movement to form a narrow Coastal Plain; it is often bounded by inland cliffs. If more than one rise has taken place, there may be raised beaches at different levels.

RANDOM ERROR – Errors that occur without known physical law or pattern. In theory, small errors occur more frequently than large errors and there are as many positive errors as negative errors.

RANGE – In general, two points in line with the point of observation. The following are practical examples of a range: 1) The line defined by the side of a building or by a fence may be extended visually to its intersection with a survey line; the point of intersection thus determined is said to be in range with the side of the building or with the fence. 2) In hydrographic surveying, a range formed by two shore objects, if suitably located, aids in keeping a boat moving in a straight line – the line defined by the range. 3) In navigation, specially constructed aids mark ranges defining channels which are to be followed by vessels to keep them clear of dangers. 4) Boundary lines across water areas and boundary corners in water areas where permanent marks cannot be established are sometimes defined by intersection of range lines or by a range line and distance from a mark, the range lines being marked by permanent monuments on the land. See RANGE*.

RANGE TIE – See RANGE.

RASTER – Pattern followed by the electronic beam exploring element scanning the screen of a cathode ray tube.

RATIO PRINT – A projection print made at a specified magnification or reduction ratio.

RAVINE – A long, narrow depression in the earth’s surface, rather smaller than a valley but larger than a gully. Several gullies often lead to a ravine, and several ravines to a valley.

REAL TIME – Time in which reporting events or recording of events is simultaneous with the events.

RECIPROCAL LEVELING – leveling across a wide river (or other open barrier) by establishing a turning point of each bank of the river from one side and taking a backsight on each to determine the height of instrument on the other side. By using the mean of the two HI determinations, the effects of curvature and refraction on the long sights are compensated.

RECIPROCAL VERTICAL ANGLES – Vertical angles measured over a line at both ends in trigonometric leveling to reduce the effects of curvature and refraction.

RECONNAISSANCE – A general examination of a region with reference to its main features, usually as a preliminary to a detailed survey. It may be performed in the field or office, depending on the extent of information available.

RECONNAISSANCE MAP – The plotted results of a general examination or survey of the main features, or certain specifications, of a region, usually low order of accuracy and usually as a preliminary to more detailed surveys, examinations, or mapping.

RECONNAISSANCE SURVEY – A preliminary survey, usually executed rapidly and at low cost; a “windshield” survey.

RECOVER – To visit a survey station, identify its mark as authentic and in the original location, and verify the description.

RECTANGULAR COORDINATES – See PLANE COORDINATES and COORDINATE SYSTEM.

RECTANGULAR SPACE COORDINATES – The perpendicular distances of a point from planes defined by each pair of a set of three axes which are mutually perpendicular to each other at a common point of origin. In photogrammetry, space coordinates are also termed survey coordinates, and are the x-coordinates and y-coordinates which define the horizontal position of a point on a ground system, and the z-coordinate, which is the elevation of the point with reference to the ground system.

RECTIFICATION – The process of projecting a tilted photograph to produce the equivalent of an untilted photograph taken at the same exposure station except for effects of relief displacement. See TRANSFORMATION.

RECTIFIED PHOTOGRAPH – A photograph in which tilt displacements have been removed by transformation.

REDUCTION TO CENTER – The correction to be applied to a direction or angle observed from a set up at a point other than the station mark. See ECCENTRICITY.

REDUCTION TO SEA LEVEL – The application of a correction to a measured horizontal length on the earth’s surface (at any elevation) to convert it in its projected or corresponding length at sea level.

REEF – A submarine elevation of rock or coral forming a menace to navigation. See SHOAL and REEF*.

REFERENCE MARK – A permanent supplementary mark near a survey station to which it is related by an accurately measured distance and direction, and (or) a difference in elevation. See REFERENCE MONUMENT*.

REFERENCING – The process of tying a survey station to a supplementary mark or other natural or artificial features by distances and directions.

REFLECTING PROJECTOR METHOD – A system for updating maps from more recent source data. A variable ratio projector is used.

REFRACTION – The deflection from a straight path undergone by a light ray or other electromagnetic wave in passing obliquely from one transparent medium into another in which its velocity is different.

REFRACTION, ATMOSPHERIC – The bending of the path of light or other electromagnetic wave energy passing through air of varying density. See INDEX OF REFRACTION and MEAN REFRACTION.

REGISTER (USGS) – The relative positioning of components of a composite map. A map is in register when each component is in correct position relative to the others. The term is sometimes incorrectly referred to as registry or registration. See REGISTER*.

REGISTERING – Alignment process by which two images or two digital images of the same ground area are positioned coincident with respect to one another so that their respective grey tones at any (x,y) coordinate or any resolution cell represents the sensor output for the same part of the same object.

REGISTER PUNCH (USGS) – A manually or mechanically operated device consisting of a precision punch and die assembly which cuts systematically uniform holes in component guides and manuscripts.

REGISTER STUDS – Precisely machined metal or plastic studs which, when inserted into prepunched holes, provide accurate register of component guides and manuscripts.

RELATIVE APERTURE – The ratio of the equivalent focal length to the diameter of the entrance pupil in a photographic or telescopic lens. Also termed “f-number” or “speed.”

RELATIVE HUMIDITY – The ratio of water vapor present in air to the maximum amount possible at that temperature.

RELATIVE ORIENTATION – Determination of the position and attitude of one of a pair of overlapping photographs with respect to the other.

RELATIVE SETTING (DMA) – In tilt analysis of oblique photography, the dihedral angle between the two planes passing through the principal point of the opposite obliques, the principal point of the vertical photograph, and the common exposure station. This angle is measured on the vertical photograph as the angle between the two isolines, or as the deflection angle between the perpendiculars from the principal point of the vertical photograph to the two isolines.

RELATIVE SWING (DMA) – In the tilt analysis of oblique photographs, the angle of rotation of the oblique camera about its own axis with respect to the plane of the vertical photograph, measured on the oblique photograph by the angle between the isoline and a line joining the fore and aft fiducial marks.

RELATIVE TILT – The tilt of a photograph with reference to an arbitrary plane, not necessarily a horizontal plane, such as that of the preceding or subsequent photograph in a strip. Also defined as the angle between the photograph perpendicular and a reference direction, such as the photograph perpendicular of the preceding or subsequent photograph in a strip.

RELIEF – Variations in the elevation of the ground surface, also features of height above a plain or reference datum. On a topographic map, relief is depicted by hachures or shading, or, more accurately, by contours or by spot elevations or both.

RELIEF DISPLACEMENT – Displacement of images radially inward or outward from the nadir point of the photograph. Relief displacement of images is caused by differences in elevation of the corresponding ground objects whether below or above, respectively, the elevation of the ground nadir.

RELIEF FEATURES – Landforms. For convenience, relief features are classified in three orders of magnitude: the first order comprising the continents and ocean basins; the second comprising mountains, plains, and basins, which are of constructional nature; and the third comprising smaller features formed by destructive forces – erosional features, such as gullies and valleys; residual features, such as peaks and monadnocks; and depositional features, such as deltas and moraines.

RELIEF MODEL – A representation of the physical features (natural or artificial) of the whole or a part of the earth’s surface, with topography physically expressed in three dimensions. The vertical scale is generally larger than the horizontal scale for accentuation.

REMOTE SENSING – Imaging or recording of physical phenomena at a distance by detecting the electromagnetic radiant energy which the phenomenon either reflects or emits.

REPEATING INSTRUMENT – A theodolite or transit so designed that successive measures of a horizontal angle may be accumulated and read on the graduated circle.

REPRESENTATIVE FRACTION – Map scale expressed as a fraction, as 1/24,000 or as a ratio 1:24,000.

REPRODUCTION – 1) The summation of all the map making processes involved in printing copies from an original drawing. The principal processes are photography, lithography or engraving, and printing. 2) A copy of an original by any of the processes of duplication or printing.

RESEAU – A network (or grid) of fine lines accurately ruled on a glass plate. Also, the glass plate bearing the network. In photogrammetric practice, the term generally refers to the network of lines ruled on the focal-plane plate of certain cameras to provide a means of measuring film distortion.

RESECTION – Literally “to cut again;” 1) The graphical or analytical determination of a position, as the intersection of at least three lines of known direction to corresponding points of known position. 2) The determination of the horizontal position of a survey station by observed directions from the station to points of known positions. Also, the line drawn through the plotted location of a station to the occupied station. 3) The determination of the position and/or altitude of a camera, or the photograph taken with that camera, with respect to the exterior coordinate system;

RESOLUTION – The minimum distance between two adjacent features, or the minimum size of a feature which can be detected by a photographic or other image-recording system. See GROUND RESOLUTION.

RESOLUTION CELL – The smallest element of a digitized image having a grey tone assigned to the area.

RESOLVING POWER – A mathematical expression of lens definition, usually stated as the maximum number of lines per millimeter that can be resolved (that is, seen as separate lines) in the image.

RESOURCE PHOTOGRAPHY – Aerial photographs flown for the purpose of management of the Natural Resource lands.

RESTITUTION – The photogrammetric process of determining the true position of ground objects, the image of which appears displaced or distorted on aerial photographs.

RESURVEY – To survey again, determining new or improved values. See RESURVEY*.

RETICLE – A system of wires, hairs, threads, etched lines, or the like, placed in a telescope perpendicular to its axis and at its principal focus, to define the line of sight of the telescope, to permit specific pointings to be made on a target or signal, or readings to be made on a rod or scale. Also reticule.

RETICLE RING – The ring across which the wires, hairs, etc., of a reticule are stretched, or which supports the glass diaphragm on which the lines are etched.

RETICULATION – A network of minute depressions or corrugations in a negative, produced – either accidentally or intentionally – by any treatment resulting in rapid expansion and shrinkage of the swollen gelatin. Reticulation may be produced by solutions which are too warm or too alkaline, or by forced drying in an air current which is too hot.

RETICULE – See RETICLE.

RETURN BEAM VIDICON CAMERA (RBV) – a remote sensing subsystem containing three individual cameras operating in different spectral bands: blue/green, green/yellow and red/IR.

REVERTED IMAGE – An image in which detail is in reverse order, from left to right, as compared with the corresponding detail of the object. The order of detail from top to bottom remains unchanged. A mirror image.

REVISION – The process of updating a map, chart or publication to reflect current field status of an area.

RHUMB LINE – A line on the surface of the earth making the same angle with all meridians; a loxodrome or loxodromic curve spiraling toward the poles in a constant true direction. Parallels and meridians, which also maintain constant true directions, may be considered special cases of the rhumb line.

RIGHT ANGLE PRISM – A hand tool which defines a 90^o line of sight by means of a pentaprism. Double pentaprisms provide accurate line of sight to left as well as right.

RIGID GRAVER – A tool having two feet and a needle point for scribing.

RIVER – A water course with a substantial stream of fresh water generally retaining some flow during dry weather in its natural state.

RIVER CROSSING – The process of carrying a line of leveling across a wide stream when no suitable bridge is available. Special observations are required from both sides of the stream.

ROAD – A rural route for vehicles usually traversable by passenger cars and by trucks. See JEEP TRAIL and HIGHWAY.

ROD LEVEL – a spirit level attached to a leveling rod to indicate when the rod is vertical.

ROELOFS PRISM – An optical device attached to the objective end of a theodolite to provide a means of pointing precisely on the center of the sun. Four overlapping images define a small square which are used as a point target.

ROLL – 1) Rotation of an aircraft about its longitudinal axis. 2) Rotation of camera or coordinate system about the X axis – (omega – ?).

ROLLING GROUND or LAND – Any undulating land surface; a succession of low hills giving a wave effect to the surface. A land surface much varied by many small hills and valleys.

ROOT MEAN SQUARE ERROR – The square root of the sum of the squares of individual errors (deviations from the mean) divided by the number of errors. See STANDARD ERROR.

ROUNDOFF – The addition or subtraction of a small amount (one-half or less in terms of the last retained significant figure) to reduce the number of significant digits in a computed or measured quantity.

ROUTE SURVEY – Survey for construction of linear work such as highways, canals, pipeline, powerlines etc.

S [Back to Top]

SETER – Survey Equipment Technical Evaluation Report.

SIA – Stereo Image Alternator.

SLAR – Side Looking Airborne Radar.

SLR – Side Looking Radar.

SADDLE – A low point on a ridge or crest line, generally a divide between the heads of streams flowing in opposite directions.

SAFELIGHT – A light of such intensity and color range that it will not actinically affect the specific light-sensitive materials being handled and developed in a darkroom.

SAG CORRECTION – The difference between the effective length of a tape (or part of a tape) when supported continuously throughout its length and when supported only at the ends or at a limited number of independent points.

SALES INDEX MAP (USGS) – State maps for public distribution showing, by diagram or by listing, maps offered for sale by the Geological Survey. Quadrangle outlines, names, dates of survey, and authorship are shown by overprint. River surveys, are indicated by overprinting the river courses; lists of special maps available and items of general information are printed on the back.

SALT MARSH – See SALT MARSH*.

SAND DUNE – A mound, ridge, or hill of loose sand, heaped up by the wind.

SAND SPIT – A narrow sand embankment, created by an excess of deposition at its seaward terminus, with the end away from the point of origin terminating in open water.

SATELLITE GEODESY – The surveying discipline which uses earth orbiting man-made satellites to obtain geodetic data.

SATURATION – Point at which additional input energy to the sensor results in no increase in sensor output.

SCALE CONVERSION – The changing of map materials from one scale to another, usually by photographic methods and usually without changing the graphic content.

SCALE FACTOR – In the State coordinate systems scale factors are applied to geodetic lengths to obtain grid lengths.

SCALING – The adjustment of a stereomodel to a scale of known ratio to the ground distances.

SCANNING DENSITOMETER – Device used to convert image data from film or photographic format to electronic video signal format. Usually the film is placed on a glass cylinder which rotates and slowly translates. A fine beam of light is focused on the film, passed through the film and is detected by a photomultiplier where it is amplified to a usable video signal.

SCATTERING – Diffuse reflection of electromagnetic energy due to moisture and dust particles in the air.

SCHEIMPFLUG CONDITION – One of the two conditions of direct optical projection that must be satisfied to achieve maximum sharpness in a projected image. It requires that the object plane, the principal plane of the lens, and the image plane all intersect in a common line.

SCRIBED DRAWING – A scribing guide on which the negative scribing has been completed. See COLOR-SEPARATION DRAWING.

SCRIBING – The art of technique of cutting with specially designed tools through a photographically opaque coating applied to a transparent base, usually plastic. Portions of the coating are removed, creating a negative image on the lines, letters, and symbols desired for reproduction.

SCRIBING COPY – See COLOR-SEPARATION GUIDE.

SEA LEVEL DATUM – A determination of mean sea level that has been adopted as a standard datum for heights or elevations. The Sea Level Datum of 1929, the current standard datum for geodetic leveling in the United States, is based on tidal stations along the coasts.

SEA LEVEL FACTOR – The factor applied to ground measurements to reduce measurements to sea level datum.

SECANT CONE – A cone which cuts another surface in two places.

SECONDARY STATION – A survey station established in conjunction with the main scheme, but observed with a lower order of accuracy and precision, to increase the density of control.

SECOND ORDER – Designation of survey work of next to the highest category of precision and accuracy. See CONTROL SURVEY CLASSIFICATION.

SECOND ORDER LEVELING – Differential leveling which closes on First Order Bench Marks using specified equipment and methods which achieve closures of 0.035 ft./mile or less.

SECOND ORDER MAGNITUDE RELIEF FEATURE – Mountains, plains and basins of constructional nature.

SECOND ORDER TRAVERSE – A closed survey which has a closing error of 1 in 10,000 or less which also complies with specifications for angle, distance and azimuth observations.

SECOR – Acronym for Sequential Collation of Range. An electronic distance measurement system for satellite observation.

SECTION (U.S.C. & G.S.) – That portion of a line of levels which is recorded and abstracted as a unit. See SECTION*.

SELF LEVELING LEVEL – See AUTOMATIC LEVEL.

SELF READING ROD – A level rod which has a movable tape which can be set to read elevation values from a given HI.

SEMIANALYTICAL AEROTRIANGULATION – A category of aerotriangulation methods in which the positions and/or elevations of ground stations are obtained by the mathematical adjustment of model, section, or strip coordinates derived from models oriented in a stereoplotting instrument. See BZ CURVE.

SEMI-MATTE PRINT – A non-glossy paper with only a faintly lustrous surface.

SEMITANGENT – The line segment between the Point of Curvature and the Point of Intersection on a curved route survey.

SENSOR – Detecting device which collects and conveys some interpretable data; the component of an instrument that converts an input signal into a quantity which is measured by another part of the instrument.

SENSOR PLATFORM – Vehicle on which a remote sensor is mounted and carried aloft; an aircraft platform, space platform, etc.

SERIES CONVERSION (USGS) – The preparation of a 15-minute map from source materials originally used to publish 7½ -minute maps, with approximate cartographic changes.

SET-UP – 1) The transit or leveling instrument’s location for observation. 2) A correction in precise taping.

SHADED-RELIEF MAP – A map on which the hypsography is made to appear three dimensional by the use of graded shadow effects. A shaded-relief map may also contain contours or hachures in combination with the shading.

SHOAL – A submarine elevation over which water is shallow, which is detached from the shore and composed of any material other than rock or coral and which is a menace in navigation. See REEF.

SHORE – The land which is covered and uncovered by the rise and fall of the normal tide.

    It is the strip of land between the mean high-water and mean low-water lines. In its strictest use, the term applies only to land along tidal waters.

SHORELINE, OCEAN – The line along which the land surface meets the water surface of a lake, sea, or ocean. Strictly speaking, it is not a line, but a narrow strip or area, embracing that part of the land surface which comes in contact with wave action both above and below the surface of the water. The term does not apply on tidal flats or marshes which are overflowed by the tides, but essentially to strips where the land surface has an appreciable slope toward the water. See COASTLINE.

SHORELINE, RIVER – The shoreline is the line which is washed by the water wherever it covers the bed of the river within its banks. It lies along the bank at the means level attained by the waters of the river when they reach and wash the bank without overflowing it.

SHUTTER – A mechanism for controlling the interval of time during which light is allowed to pass through an optical or photographic system.

SIDE CANYON – A small ravine draining into a main river.

SIDELAP – The area common to two adjacent strips of aerial photography.

SIDE-LOOKING RADAR (SLAR) – A radar system using a stabilized antenna oriented at right angles to the aircraft’s flight path, using for radar mapping at night or through cloud cover.

SIDE SHOT – Observations taken from a survey station to a point not on the continuous line of survey. A point whose position or elevation was so determined.

SIDE SIGHT – A sight to a point not on the line of traverse. It may be a side shot or an observation to locate an in-and-out station, an azimuth mark, or an intersected point.

SIGNAL – A natural or artificial object or structure located at or near a survey station and used as a sighting point or target for survey measurements.

SIGNAL-TO-NOISE RATIO – Ratio of the value of the signal to that of the noise.

SIGNATURE – Unique reflectance or emission response from a particular object or environmental association.

SIMULTANEOUS LEVEL LINE – See DOUBLE RODDED LINE.

SKETCHMASTER – A trade name for a mounted semisilvered mirror used for tranformation of aerial photographs in updating maps.

SKEW MERCATOR – See OBLIQUE MERCATOR.

SLANT RANGE – A line of sight distance between two points of different elevation.

SLOTTED TEMPLET – A card with precision slots cut along radial lines determined from pass points; used in radial triangulation.

SLOUGH – A swampy ditch. See BAYOU.

SLOTTED TEMPLET TRIANGULATION – Radial phototriangulation using slotted templets. Also called a laydown.

SMALL SCALE MAP – A map covering a very large area as a 1:1,000,000 base map. See LARGE SCALE MAP.

SNAP MARKER – A point transfer device designed to be used with a mirror stereoscope. A steel ball is struck by a spring-loaded hammer to make a small indentation about 100 microns in diameter in the emulsion.

SONAR – 1) A sonic device used in hydrography for the detection and location of underwater objects. 2) A system for determining depths by measuring the interval of time between transmission of an underwater sonic or ultrasonic signal and return of its echo.

SONNE CAMERA – A system for taking continuous strip photography by moving the film across a slit at a speed corresponding to the ground speed.

SOUND – 1) A relatively long arm of the sea or ocean forming a channel between an island and a mainland or connecting two larger bodies, as, a sea and the ocean, or two parts of the same body; usually wider and more extensive than a strait. 2) To measure or ascertain the depth of water, as with sounding lines.

SOUNDING – Measuring the depth of water with a lead line or by other means.

SOURCE MATERIAL – Data of any type required for production of maps and charts.

SPACE COORDINATES – See RECTANGULAR SPACE COORDINATES.

SPADING – Removing scribe coating with a wide flat blade.

SPATIAL MODEL – The three-dimensional image seen by stereoscopic methods.

SPECIAL PRINTING PLANIMETRIC MAP (USGS) – Standard 7 1/2 – and 15-minute series and 1:250,000-series topographic maps on which the contours and woodland data are omitted.

SPECIAL PURPOSE MAP (USGS) – A map designed to meet limited objectives emphasizing or representing certain classes or types of information. Typical examples are the geologic map of the United States; the shaded-relief map of Massachusetts, Rhode Island, and Connecticut, and a State index map.

SPECTRAL CHARACTERISTIC – Relation, usually shown graphically, between wavelength and some other variables.

SPHERICAL COORDINATES – Pairs of angular values such as latitude and longitude or right ascension and declination which locate points on a sphere in reference to fixed, or defined, great circles.

SPHERICAL EXCESS – The amount by which the sum of three error free angles of a triangle on a sphere exceeds 180 degrees.

SPHERICAL TRIANGLE – A triangle, on the surface of a sphere, whose sides are arcs of three great circles.

SPHEROID – Any figure differing slightly from a sphere. In geodesy, it is a mathematically defined figure closely approximating the geoid in form especially used in connection with gravity observation. See ELLIPSOID.

SPHEROIDAL EXCESS – The amount by which the sum of three error-free angles of a triangle on a spheroid exceed 180 degrees. See SPHERICAL EXCESS.

SPIRIT LEVELING – The determination of elevations by use of a leveling rod and an instrument incorporating a spirit level to establish a horizontal line of sight; the term has now been broadened to include leveling by means of other types of precise levels, such as an automatic level.

SPIT – A narrow, low-lying tongue of sand or gravel, or small point, projecting into the sea. It differs from a bar in that it is attached to the land at one end, and it is often formed by shore currents across the entrance to a bay.

SPLINES – Flexible curve rulers which are adjusted to the points of a curve to be drawn and held in place by lead weights.

SPOT ELEVATION (USGS) – An elevation shown on a topographic map to supplement the information shown by contour lines and bench marks. Where the exact location of the elevation is not evident, it is indicated by a brown cross.

SPUR – A submarine prolongation of a mountain range onto or across a continental or insular shelf.

SPUR LINE – A survey line connected at only one end to similar surveys of equal or higher order.

STABLE BASE – A film used in cartography having a high dimensional stability over a period of many years.

STADIA – A technique of distance measurement wherein the observer reads the intercept subtended on a graduated rod between two hairs or marks on the reticle of the telescope, the distance to the rod being proportional to the rod intercept. A rod specially designed for this use is called a stadia rod; the marks on the reticle are called stadia wires; such a survey is called a stadia survey, etc.

STANDARD – An exact value (a physical entity or abstract concept) established and defined by authority, custom, or common consent to serve as a reference, model, or rule in measuring quantities, establishing practices or procedures for evaluating results. See STANDARD*.

STANDARD-ACCURACY MAP – A map which complies with the National Map Accuracy Standards.

STANDARD-CONTENT MAP (USGS) – A map that represents natural and manmade features according to current standards. Topographic instructions specify classes and dimensions of various map features shown so that uniform treatment is attained throughout a map series. Maps conforming to these specifications are considered to have standard content.

STANDARD ERROR – Also standard deviation. The root-mean-square value based on the differences between the average error and the individual errors of a set. It is a measure of the precision of a single observation.

STANDARD MAP – See STANDARD QUADRANGLE MAP.

STANDARD MERIDIAN – A meridian on a map projection along which scale is as stated.

STANDARD PARALLEL – A parallel of latitude which is used as a control line in the computation of a map projection. See STANDARD PARALLEL*.

STANDARD QUADRANGLE  (USGS) – A quadrangle of a specific series, conforming to the systematic pattern of the series.

STANDARD QUADRANGLE MAP (USGS) – A designation for one of a series of quadrangle maps conforming in content and format to the standards in effect at the time the map was produced. See MAP OF STANDARD FORMAT, STANDARD ACCURACY MAP and STANDARD CONTENT MAP.

STANDARD REVISION (USGS) – The updating and correction of a map to reflect changes that have occurred since the original survey or latest revision. The accuracy of the existing map is preserved, and current specifications pertaining to map symbolization, content, and format are generally applied. See MAP REVISION.

STANDARD TENSION – The pull on a survey tape at which the tape was standardized.

STATE BASE MAP – A base map of a state as a unit.

STATE COORDINATE SYSTEMS – The plane-rectangular coordinate systems established by the United States Coast and Geodetic Survey, one or more for each state in the United States, used for defining positions in terms of plane-rectangular (x and y) coordinates. Also called State Plane Coordinate Systems.

STATE PLANE COORDINATE SYSTEMS – The plane-rectangular coordinate systems established by the U.S. Coast and Geodetic Survey, one for each State in the Union, used for defining positions of geodetic stations in terms of plane-rectangular (x and y) coordinates.

The two principal systems in use in the United States are the Lambert Conformal Conic Map Projection and the Transverse Mercator Map Projection. See OBLIQUE MERCATOR PROJECTION.

STATION – 1) A point whose position has been or will be determined by surveying methods. 2) Any survey point whose position is given by its accumulated distance from a starting point. 3) A 100 foot distance between points on route surveys; a unit of measure.

STATOSCOPE – An altimeter.

STEREO BASE – A line representing the distance and direction between complementary image points on a stereo-pair of photos correctly oriented and adjusted for comfortable stereoscopic vision.

STEREOCOMPARATOR – A stereoscopic instrument for measuring photograph coordinates of image points.

STEREOCOMPILATION – 1) The procedure of producing a map from aerial photographs by means of stereoplotting instruments. 2) The map data produced with stereoplotting instruments.

STEREOGRAM – A stereoscopic pair of photographs or drawings correctly oriented and mounted or projected for stereoscopic viewing.

STEREOGRAPHIC PROJECTION – A map plotting system in which points on the ellipsoid are projected onto a plane parallel to a tangent plane from a point on the opposite side of the sphere from the point of tangency. The polar case is most useful. See POLAR STEREOGRAPHIC.

STEREOIMAGE ALTERNATOR – A device for viewing stereoscopic models in direct-viewing double-projection plotting instruments, consisting of synchronized, rapidly rotating shutters placed in both the projection field and the viewing field. The shutters in the projection field cause the imagery of the left and right diapositives to be projected on the tracing table in rapid alternation. The shutters in the viewing field, rotating in phase with those in the projection field, permit each eye to see only the imagery of the corresponding (left or right) diapositive for attainment of the stereoscopic effect.

STEREOMETER – A measuring device comprising a micrometer movement by which the separation of two index marks can be changed in order to measure parallax difference on a stereoscopic pair of photographs. Also termed parallax bar.

STEREO PAIR – Two aerial photographs taken from separate positions of the same area with sufficient overlap to permit stereoscopic viewing.

STEREOPLANIGRAPH –Brand name of a universal stereoplotting instrument.

STEREOPLOTTERS – A stereoscopic plotting instrument of several types: Stereometers, Paper Print Plotters, Direct Viewing Projection Plotter, Mechanical Projection Plotters, Optical Double Projection Plotters.

STEREOSCOPE – An optical instrument used for viewing two properly related photographs or diagrams simultaneously to obtain the mental impression of a three-dimensional model. See ZOOM STEREOSCOPE.

STEREOSCOPIC PLOTTING INSTRUMENT – An instrument for compiling a map or obtaining spatial solutions by observation of stereoscopic models formed by stereoscopic pairs of photographs.

STEREOSCOPIC PRINCIPLE – The formation of a single, three-dimensional image by binocular vision of two photographic images of the same terrain taken from different exposure stations.

STEREOSCOPY – The science and art that deals with the use of binocular vision for observation of a pair of overlapping photographs or other perspective views, and with the methods by which such viewing is produced.

STEREOTEMPLET – A slotted templet derived from a stereoscopic model and made in two parts, each covering the same area. The composite templet is adjustable in scale and is representative of the horizontal plot of the model.

STEREOTEMPLET TRIANGULATION – A mechanical analog system of horizontal aerotriangulation that features the adjustment of pass points to horizontal control by means of stereotemplets.

STEREOTRIANGULATION – A triangulation procedure that uses a stereoscopic plotting instrument to obtain the successive orientations of the stereoscopic pairs of photographs into a continuous strip. The spatial solution for the extension of horizontal and (or) vertical control using these strip (or flight) coordinates may be either graphical or computational procedures often called bridging.

STICKUP – Adhesive-backed film or paper on which letters or symbols have been photographed or printed. Also, the operation of applying such lettering and symbols to color-separation guides. See PHOTOTYPE.

STREET – a public thoroughfare (generally urban) over 20 feet in dedicated width.

STRENGTH OF FIGURE – A number expressing the relative strength of a triangle as a function of its shape; a smaller number indicates greater relative strength. The strength of figure is independent of the size of the triangle or of the precision of angle measurements.

STRIP – A series of consecutive overlapping serial photographs or stereomodels in a flight line that may be treated as a unit in aerotriangulation.

STRIP ADJUSTMENT – The adjustment of consecutive photographs, models, or groups of these into a homogeneous strip. The adjustment may be horizontal or vertical or both; it is often an intermediate step in analytical of semianalytical aerotriangulation. See BLOCK ADJUSTMENT.

STRIP CAMERA – See SONNE CAMERA.

STRIP WIDTH – The average dimension, measured normal to the flight line, of a series of neat models in the flight strip. Strip width is generally considered as equal to width between flights.

SUBDIVISION SURVEY – A land survey, usually filed in County records, dividing land into three or more separate lots and providing access thereto. See  URBAN SUBDIVISION* and SUBDIVISION OF SECTION SURVEY*.

SUBLITTORAL – Refers to the benthonic zone extending from low tide level to the edge of the continental shelf or some comparable depth of water. It may be separated into inner and outer zones at some depth ranging from about 50 to about 300 feet.

SUBMARINE BULGE – Fanlike sedimentary deposit, presumed to have been formed by turbidity currents, on the outer continental slope at the mouth of a submarine canyon. Term is applicable where data are inadequate to distinguish between deltas and alluvial fans.

SUBMARINE CANYON – Steep valleylike submarine depression crossing the continental margin region, except for isolated portions of outer ridges, less than 1 to more than 10 miles wide, less than 60 to more than 6000 feet deep.

SUBMERGED LANDS ACT*.

SUBSTITUTE CENTER – A photo image used instead of the principal point because it is easily identified.

SUBTENSE BAR – A horizontal held bar of precisely determined length, used to measure distances by observing the angle it subtends at the distance to be measured.

SUPER WIDE ANGLE LENS – A lens having an angle of coverage greater than 100Ί.

SUPPLEMENTAL CONTOUR – See preferred term SUPPLEMENTARY CONTOUR.

SUPPLEMENTAL CONTROL – Vertical (and sometimes horizontal) control of selected photo-identifiable points to be used in photogrammetric compilation.

SUPPLEMENTAL STATION (USGS) – A secondary triangulation station generally established to obtain an elevation by vertical angles for photogrammatic mapping.

SUPPLEMENTAL BENCH MARK – See BENCH MARK, NONMONUMENTED.

SUPPLEMENTAL CONTOUR – A contour line drawn between basic-interval contour lines on a map of an area that has wide variation in slope. Supplemental contours are only used where it is necessary to portray local features not adequately shown by basic contours.

SUPPLEMENTARY CONTROL – See preferred SUPPLEMENTAL CONTROL.

SURFACE MANAGEMENT MAPS – Part of the Mineral Management Series which combines surface ownership (Land Status) and planimetric or Topographic data.

SURFACE-MINERAL MANAGEMENT MAPS – Part of the Mineral Management Series which combines sub-surface ownership (Land Status) and planimetric or Topographic data.

SURGE ZONE – The region between the breaker zone and the 50-60 feet depth contour, where the effect of sea waves and swell produces oscillatory surges causing sediment transport and abrasive erosion.

SURVEILLANCE PHOTOGRAPHY – Aerial photography for the purpose of detection of trespass or other infraction of laws.

SURVEY (USGS) - 1) the orderly process of determining data relating to the physical or chemical characteristics of the earth. The list of orderly processes which can be properly termed “surveys” is long and may be divided into classes according to the type of data obtained, the methods and instruments used, and the purposes to be served. Examples: geodetic survey, topographic survey, hydrographic survey, land survey, geologic survey, geophysical survey, soil survey, mine survey, engineering survey. 2) The associated data obtained in a survey. The data obtained in a particular project may be designated by the name of the project, as “the topographic survey of the District of Columbia.” 3) An organization engaged in making a survey. Such an organization is often given an official name which includes the word “survey.” Examples: “The United States Geological Survey,” “The Massachusetts Geodetic Survey.” See MINERAL SURVEY*, PRELIMINARY SURVEY, RECONNAISANCE SURVEY, SUBDIVISION SURVEY, TOWNSITE SURVEY* and TRANSIT-STADIA SURVEY.

SURVEYING – The science or art of making the measurements necessary to determine the relative position of points above, on, or beneath the surface of the earth, or to establish such points. See PLANE SURVEYING.

SWALE – A slight, marshy depression in generally level land.

SWAMP – Low lying land saturated with moisture and overgrown with vegetation but not covered with water. See SWAMP*.

SWING – 1) the correction applied to an observation made to an eccentric signal. 2) The rotation of a photograph in its own plane around the photograph perpendicular.

SWING OFFSET – The perpendicular distance from a point to a transit line found by moving the tape in an arc until a minimum horizontal distance is obtained.

SWIVEL GRAVER – A tool for scribing curved lines.

SYMBOL – A diagram, design, letter, or abbreviation placed on maps and charts, which by convention, usage, or reference to a legend is understood to stand for or represent a specific characteristic or object.

SYSTEMATIC ERROR – An error that occurs according to a known pattern or law. Systematic errors can be compensated for partially by corrections and appropriate measuring techniques.

T [Back to Top]

T.B.M. (USGS and USC & GS) – Temporary bench mark. See TURNING BENCH MARK.

TIR – Thermal infrared.

TP – Turning point.

TABLELAND – Land elevated much above the level of the sea and generally offering no considerable irregularities of surface. 2) A flat or undulating elevated area; a plateau or mesa.

TABLEMOUNT – A seamount (roughly circular or elliptical in plan) generally deeper than 100 fathoms, the top of which has a comparatively smooth platform. Syn: SEAMOUNT: GUYOT.

TACHYMETRY, TACHEOMETRY – Surveying method used to denote the procedures for obtaining horizontal distances and differences in elevation by indirect methods, which are based on the optical geometry of the instruments employed. Sometimes referred to an optical distance measurement, telemetry, or stadia.

TALUS – A collection of fallen disintegrated material which has formed a slope at the foot of a cliff.

TANGENT – 1) That part of a traverse or alinement included between the point of tangency of one curve and the point of curvature of the next curve. 2) Sometimes applied to a long straight line of a traverse, especially on a route survey, whether or not the termini of the line are points of curve. See TANGENT* and SEMITANGENT.

TANGENTIAL DISTORTION – See LENS DISTORTION.

TANGENT PLANE – A plane that touches a curved surface of double curvature at one and only one point or that touches a curved surface of single curvature along one or more parallel straight lines which are elements of the surface, without intersecting the surface.

TANGENT SCREW – A knob mounted on a fine threaded screw giving a tangential movement for making the final setting to an instrument of precision, as a surveyor’s transit.

TAPE – A steel, fabric or Invar ribbon used for direct distance measurement.

TAPE CORRECTION – A quantity or quantities applied to a taped distance to eliminate or reduce errors due to the physical condition of the tape and to the way in which it is used.

TARGET – A symmetrical pattern centered over a point to be recorded on a photograph. See PANEL.

TARGET IDENTIFICATION OR RECOGNITION – Process by which targets from image format data are identified by means of a decision rule.

TARGETING – The distinctive marking of a ground point with material of any kind, placed in a symmetrical contrasting pattern about the point to facilitate the identification and precise recovery of that point on an aerial photograph. Also called paneling.

TELESCOPE LEVEL – A sensitive spirit level attached to a transit’s telescope, with its axis parallel to the telescope axis.

T-ELEVATION – A photogrammetrically determined elevation, shown on map manuscripts. The elevation figure is followed by the letter “T,” as 1057T, to indicate that the elevation is unchecked. See SPOT ELEVATION.

TELLUROMETER – A trade name of an electronic measuring system.

TEMPERATURE CORRECTION – An amount added algebraically to a measurement to account for length changes due to thermal variance from standard conditions.

TEMPLET – 1) A representation of a photograph or a stereomodel for use in aerotriangulation. 2) The design-cross-section of a road, canal, etc. See SLOTTED TEMPLET TRIANGULATION.

TEMPLET LAYDOWN – See RADIAL TRIANGULATION.

TEMPLET MATCHING – An operation used to find out how well two photographs or images match one another.

TEMPORARY BENCH MARK – See BENCH MARK, NONMONUMENTED.

TERRACE – 1) A flat, level or nearly level, narrow area of land bordering a river or lake, bounded on at least one side by a definite steep slope rising upward from it, and on the other sides by downward slopes. 2) A low embankment of ridge of earth constructed across a slope to control surface runoff and minimize soil erosion. 3) Sloping ground cut into a succession of benches and steep inclines for purposes of cultivation.

TERRAIN – An area of ground considered as to its extent and topography.

TERRAIN FACTORS – Terrain factors consist of land forms, drainage features, the ground, the vegetation, and the cultural features or manmade changes in the surface of the earth.

TERRESTRIAL CAMERA – A phototheodolite; a camera for use on the ground.

TERRESTRIAL PHOTOGRAPH – A photograph of a portion of the earth’s surface taken by a camera located on the ground.

TEXTURE – A distribution of grey tones and photographic features in a small area which produces easily recognizable discrete tonal features.

THEMATIC MAP – A map that portrays selected phenomena and/or factual contents.

THEORETICAL  CORNER – A USGS term which designated corners on the map for which no evidence was found on the ground and which were located by land line adjustment.

THERMAL RADIATION – Electromagnetic radiation emitted by any substance as a consequence of the thermal excitation of its molecules.

THERMAL RESOLUTION – Image resolution expressed as a function of the minimum temperature difference between two objects of phenomena.

THETA – A Greek letter much used in mathematics. Usually the mapping angle in a Lambert Map projection.

THIRD ORDER LEVELS – Differential leveling which closes on higher order work with no lines longer than 30 miles. Maximum closing error 12 mm multiplied by square root of distances in kilometers.

THIRD ORDER MAGNITUDE RELIEF FEATURE – Features formed by destructive forces-erosional features, (such as gullies and valleys), residual features (such as peaks and ridges) and depositional features (such as deltas and moraines).

THIRD ORDER TRAVERSE – A closed survey with a maximum closing error of 1 in 5,000 which meets minimum standards for angle measurements and azimuths.

THIRD ORDER WORK – The lowest order of control survey for which monumentation is authorized. See CONTROL SURVEY CLASSIFICATION.

THREE-POINT METHOD – See RESECTION.

THREE POINT PROBLEM - The three point method. See RESECTION.

THREE WIRE LEVELING – A method for reading a level rod employing three cross wires which compares the average of the three readings with the middle wire reading, especially useful with a rod graduated in yards.

TICK MARK – A small mark placed along a line or edge of a map to indicate a point of measurement. See GRID TICK.

TIDAL BENCH MARK – A bench mark set to reference a tide staff at a tidal station. The elevation of the bench mark is determined with relation to the local tidal datum.

TIDELANDS – All coastal areas that are situated above mean low tide and below mean high tide, particularly as such areas are alternately uncovered and covered by the ebb and flow of the ordinary daily tides.

TIE – A survey connection from a point of known position to a point whose position is desired or a connection to close a survey on a previously determined point.

TIE POINT – 1) Point of closure of a survey, either on itself or on another survey. 2) A pass point common to two or more strips in aerotriangulation.

TIE STRIP – An overlay containing all planimetric and relief features in the area along the edge of a map or chart. It is used to insure matching of these features on adjoining sheets.

TILT – The angle at the perspective center between the photograph perpendicular and the plumbline. See X TILT and Y TILT.

TILT DISPLACEMENT – Displacement of images on a tilted photograph radial from its isocenter. Tilt displacement is outward or inward with respect to the isocenter, according to whether the images are on the low or high side of the isometric parallel (the low side is closest to the earth, or the object plane).

TILTING LEVEL – A leveling instrument in which the final leveling of the telescope is obtained by small controlled amount of rotation of the telescope about a horizontal axis.

TIP – Tilt of the aircraft at exposure of a photograph in which one wing is lower than the other.

TITLING – 1) That information lettered on photographic negatives for identification purposes. Supplementary information may be added for special purposes. 2) The placing of such information on the negatives.

TOLERANCE – The allowable variation from a standard or from specified conditions.

TONE – Each distinguishable variation in shade of grey between black and white.

TONGUE – A low narrow strip of land, projecting into a body of water.

TOPOGRAPHIC EXPRESSION (USGS) – The effect achieved by shaping and spacing contour lines so that topographic features can be interpreted with ease and fidelity. Good expression is achieved by delineating the contours in appropriate relationship to each other, with due consideration of the scale and contour interval of the map. To achieve appropriate relationship, the contours sometimes must be adjusted (without exceeding accuracy tolerances) to (1) show features that otherwise might be missed; (2) emphasize significant characteristics of the terrain; or (3) omit minor features that are relatively unimportant or that might lead to an incorrect interpretation of the terrain.

TOPOGRAPHIC FEATURE – A relief feature generally of third-order but sometimes of second-order magnitude. See RELIEF FEATURE.

TOPOGRAPHIC LICENSE – See CARTOGRAPHIC LICENSE.

TOPOGRAPHIC MAP (USGS) – A map which presents the horizontal and vertical positions of the features represented; distinguished from a planimetric map by the addition of relief in measurable form. A topographic map generally shows the same features as a planimetric map, but contour lines or comparable symbols are used to show mountains, valleys, and plains; and depth curves are used to show depths in bodies of water.

TOPOGRAPHIC SURVEY – A survey which has for its major purposes the determination of the configuration (relief) of the surface of the earth (ground) and the location of natural and man-made objects thereon.

TOPOGRAPHY – The features of the surface of the earth considered collectively as to form. A single feature, such as a mountain or valley, is termed a “topographic feature.” Topography is subdivided into hypsography (the relief features), hydrography (the water and drainage features), and culture (manmade features).

TOWER, SURVEYING – A structure used to elevate a surveying instrument so that lines of sight are above obstacles. A tower usually has two independent components, one built within the other; the inner component supports the instrument, and the outer component supports the observer and a signal.

TRACING PAPER METHOD – A system for updating minor detail on a map using newer aerial photos. Mutually identified image points are marked on tracing paper and the map position determined by graphical resection.

TRACING TABLE – An instrument used in conjunction with a direct-viewing double-projection plotter to observe the stereomodel, to make precise measurements in it, and to delineate mapworthy detail, including contours, on a map manuscript. The instrument includes a viewing screen (see platen) for observing corresponding imagery, a reference mark (see floating mark), and a vertical leadscrew with a scale or digital counter.

TRAIL – A pathway marked clearly enough to be followed on the ground or on photographs but not suitable for passenger cars.

TRANSFORMATION – The process of projecting (mathematically, graphically, or photographically) a photograph, or points on it, from its plane onto another plane by translation, rotation, and/or scale change. See RECTIFICATION.

TRANSFORMED PRINT – See TRANSFORMING PRINTER.

TRANSFORMING PRINTER (USGS) – A projection printer specifically designed for the transformation of intentionally tilted photographs according to preset parameters. The most common application is in the conversion of low-oblique photographs into equivalent “vertical” photographs, assuming a nominal value for the obliquity of the original photographs.

TRANSIT-STADIA SURVEY –A survey in which horizontal and vertical directions or angles are observed with a transit and distances are measured by stadia.

TRANSIT TRAVERSE – A traverse in which the angles are measured with a transit or theodolite and the lengths with a metal tape. See ELECTRONIC TRAVERSE.

TRANSPARENCY – A photographic print on a clear base, especially adaptable for viewing by transmitted light. Also, the light-transmitting capability of a material.

TRANSPONDER – A combined receiver and transmitter whose function is to transmit signals automatically when triggered by an interrogating signal.

TRANSVERSE – A map projection in which a meridian is treated as an equator.

TRANSVERSE LOW-OBLIQUE PHOTOGRAPHY – Twin-low oblique photography in which the two cameras of a couple are oriented so that the plane containing their optical axes is at right angles to the flight line.

TRANSVERSE MERCATOR MAP PROJECTION – A conformal map plotting system in which points on the ellipsoid are mathematically projected onto a cylinder whose axis is oriented 90^o (transverse) to the axis of the ellipsoid. The cylinder surface may meet the ellipsoid at the map center or it may cut below the surface (secant) creating two meridians where the scale is exact. The secant form of this projection is the basis of State Plane Coordinate Systems where the zone extends north-south more than east-west.

TRAVERSE – a sequence of lengths and directions of lines connecting a series of stations, obtained from field measurements and used in determining positions of the stations. See CLOSED TRAVERSE, OPEN TRAVERSE, ELECTRONIC TRAVERSE and TRANSIT TRAVERSE.

TRIANGLE CLOSURE – An error of angular closure; the amount by which the sum of three measured angles of a triangle fails to equal 180^o plus the spherical excess.

TRIANGLE OF ERROR – The triangle formed when three plotted rays intersect.

TRIANGULAR DIVISION METHOD – A system for updating minor detail on a map using newer aerial photos. Equivalent areas are divided into homologous triangles.

TRIANGULATION – A method of surveying in which the stations are points on the ground at the vertices of a chain or network of triangles. The angles of the triangles are measured instrumentally and the sides are derived by computation from selected sides or bases, whose lengths are obtained by direct measurement on the ground or by computation from other triangles. See ARC TRIANGULATION and AREA TRIANGULATION.

TRIANGULATION STATION – A marked point on the earth whose position is determined by triangulation.

TRIANGULATION TOWER – See TOWER, SURVEYING.

TRIBRACH – The three-arm base of a surveying instrument which carries the foot-screws used in leveling the instrument. See QUADRIBRACH.

TRIBUTARY – A river or stream which contributes its water to a main river by discharging it into the latter, from either side, and at any point along its course.

TRIGONOMETRIC LEVELING – The determination of differences of elevation from observed vertical angles and measured or computed horizontal or inclined distances.

TRILATERATION – A method of determining horizontal positions by measuring the lengths of triangle sides, usually with the use of electronic instruments.

TRIMETROGON CAMERA – An assembly of three cameras equipped with wide-angle Metrogon lenses, in which one of the cameras is vertical and the other two are 60 degree obliques.

TRIMETROGON MAPPING – A method of compiling map data from aerial photographs taken with an assembly of three cameras, one aimed vertically downward, the others to either side of the flight line at 60^o from the vertical. This name was chosen originally because the cameras contained Metrogon wide-angle lenses. The angular coverage in a plane normal to the flight line exceeds 180^o, providing overlap between the vertical and both oblique photographs as well as inclusion of both horizons.

TRIVET – A low support for a surveying instrument which is used where a tripod cannot be used.

TRUCK MOUNTED TOWER (USGS) – A surveying tower hinged to a motor truck which is equipped with a mechanical device for quickly hoisting the tower to or lowering it from a vertical position.

TRUE – Correct or actual values as differentiated from relative, measured or fictitious values, i.e., true north vs magnetic north, true sun vs fictitious sun, true altitude vs measured altitude. Depending on the use, “true” can have an exact meaning. The generally accepted interpretation is that true north means a meridian determined with respect to the earth’s axis by astronomic methods.

TRUE HORIZON – The trace on an oblique photograph of a horizontal plane passing through the perspective center.

TRUE NORTH – The direction of the earth’s axis of rotation. See preferred ASTRONOMIC NORTH.

TUNDRA – One of the level or undulating treeless plains characteristic of artic regions, having a black muck soil and a permanently frozen subsoil.

TURNING BENCH MARK – A bench mark set during continuous leveling and used as a turning point.

TURNING POINT – A temporary point on which the rod is held, after a foresight has been made on it, while the instrument is moved to another station so that a backsight can be made on that elevation.

TWIN-LOW OBLIQUE PHOTOGRAPHS (USGS) – Photography with a twin-camera arrangement consisting of a pair of aerial cameras coupled rigidly together and exposed simultaneously. In Geological Survey practice wide-angle precision cameras are used with their respective optical axes in a common vertical plane, making an angle of 20^o with a plumbline and 40^o with each other.

UPS – Universal Polar Stereographic.

U.S.A.S.I. – U.S.A. Standards Institute (Formerly A.S.A.).

UTM – Universal Transverse Mercator.

U.V. – Ultra Violet.

ULTRAVIOLET ABSORBING FILTER – A haze cutting filter used mainly in photography with color films to avoid excessive bluishness and loss of contrast in the pictures; usual designations are U.V.; Haze; Wratten 2A.

ULTRAVIOLET RADIATION – Electromagnetic radiation of shorter wavelength than visible radiation but longer than X-Rays; roughly, radiation in the wavelength interval between 10 and 4000 angstrom units.

ULTRA-VIOLET RAYS – Radiant energy in the ultraviolet portion of the electromagnetic spectrum.

ULTRA-WIDE-ANGLE LENS – A lens having a coverage in excess of 100^o. See SUPER WIDE ANGLE LENS.

UNCONTROLLED MOSAIC – A mosaic in which the photographs have not been positioned by reference to horizontal control.

UNDA – That part of the floor of the ocean which lies in the zone of wave action, and in which the bottom is repeatedly stirred and reworked by storm waves.

UNDAFORM ZONE – That part of the ocean floor which lies in the zone of wave action and in which, therefore, the bottom is repeatedly stirred and reworked by storm waves.

UNDERGROUND MARK – A surveying mark set and plumbed below the center of a surface mark and separated therefrom so as to preserve the station in case of accident to the surface mark.

UNDERWATER CONTOUR – A contour shown in an inundated area. See DEPTH CURVE.

UNITED STATES NATIONAL MAP ACCURACY STANDARDS – 1) Horizontal accuracy; For maps at publication scales larger than 1:20,000, 90 percent of all well-defined features, with the exception of those unavoidably displaced by exaggerated symbolization, will be located within 1/20 inch (0.85 mm) of their geographic positions as referred to the map projection; for maps at publication scales of 1:20,000 or smaller, 1/50 inch (0.05 mm). 2) Vertical accuracy: 90 percent of all contours and elevations interpolated from contours will be accurate within one-half of the basic contour interval. Discrepancies in the accuracy of contours and elevations beyond this tolerance may be decreased by assuming a horizontal displacement within 1/50 inch (0.50 mm). Commonly referred to as Map Accuracy Standards.

UNIVERSAL PLOTTER – A stereoplotter capable of compiling contour maps from vertical or oblique aerial photography as well as terrestrial photography. A true universal plotter will accept photographs from any focal length camera.

UNIVERSAL POLAR STEREOGRAPHIC – A map designed for military use based on a Polar Stereographic Projection covering to the poles from 84^o N. and 80^o S. latitudes.

UNIVERSAL PROJECTION PRINTER – See PROJECTION PRINTER.

UNIVERSAL TABLE FRAME – A supporting frame for stereoplotters, specially designed to accommodate the projectors of any of the various types of direct-viewing double-projection plotters.

UNIVERSAL TRANSVERSE MERCATOR PROJECTION – A special case of the transverse Mercator projection. Abbreviated as the UTM grid, it consists of 60 north-south zones, each 6^o wide in longitude.

UNIVERSAL WATER CHARTS – A series of blank charts at 1:1,000,000 scale, published for each 4^o band of latitude. They are used for aerial navigation over water or for plotting positions, distances, and courses in land travel over unmapped areas, such as in Antarctica.

UPDATING – Addition of recent changes to a map. See SKETCHMASTER, RADIAL LINE METHOD, TRIANGULAR DIVISION METHOD and REFLECTING PROJECTOR METHOD.

V [Back to Top]

W [Back to Top]

X [Back to Top]

Y [Back to Top]

Z [Back to Top]