Now that you've completed Section 1 of this unit covering double proportionate or four-way measure, let's continue with Section 2A. In Section 2A of this lesson, you will apply your knowledge of double proportionate methods to a problem with several lost corners. Learn the importance of and basis for index correction in resurvey work, and solve a problem using a new method, three-point control.

In the first part of this lesson, we dealt with the field problem where only one corner had been lost, while in all four cardinal directions, the nearest corners were existent. What if several corners had been missing? Here, all the corners identified by arrows are lost. How should we go about finding them? Or as a last resort, restoring them?

Well, unfortunately there is no magical way to determine the proportionate position for all the lost points at once. Here is where you must apply the insight and perseverance of a master detective and the intelligence and affability of a traveling salesman to do your job. You must ask yourself and any reliable witnesses available -- what about all the other corners which potentially surround each lost corner. Are they all lost?