Now lets refer to figure 12-5 to see how to reduce a slope distance using the difference in elevation between two stations.

In figure 12-5, the EDM transmitter is located at station A and has an h.i. equal to AD. The reflector, at station B, has an h.i. equal to BE. The ground elevations at A and B are known and the difference between these elevations is designated D_A-B. To reduce the slope distance, s, you first determine the difference in elevation between D and E. This can be done using the following equation:

In other words, h.i. of EDM + h.i. of reflector.

Now, looking again at figure 12-5, you see that CDE is a right triangle; therefore, since the slope distance was observed and recorded using the EDM, and having calculated ADE, the horizontal distance, CD, can be simply determined using the Pythagorean theorem.