Figure 12-2.—Electromagnetic distance-measuring equipmentin use.REDUCTION OF SLOPE DISTANCEAs you learned in the EA3 TRAMAN, to reduce theslope distance of a line to horizontal distance, you needto know either the vertical angle of the line measuredfrom the instrument or the difference in elevationbetween the ends of the line. With that information youcan use the equations that you studied in chapter 12 ofthe EA3 TRAMAN to reduce the slope distance. Asapplied to chaining or transit-tape operations, thecalculations are simple; however, as applied to EDMoperations, the procedures are frequently a little morecomplicated, as you will see below. The methods ofslope reduction that we will discuss in this chaptershould be used only for slope distances that are less than2 miles in length or for observed vertical angles that areless than 5 degrees. For a discussion of slope reductionwhen distances of over 2 miles or vertical angles greaterthan 5 degrees are encountered, you should studycommercial publications, such as Surveying Theory andPractice, by Davis, Foote, Anderson, and Mikhail.Slope Reduction Using the Vertical AngleWhen the slope distance and the vertical angle areobtained from separate setups of an EDM and atheodolite, additional information is required forreducing the slope distance. This information includesthe heights above the ground (h.i.) of the EDMtransmitter and the reflector or remote unit, the h.i. ofFigure 12-3.—An electro-optical distance-measuring instrument (Geodimeter).12-3