Figure 7-2.—Difference in elevation in trigonometry leveling.Figure 7-3.—Lines of indirect levels.In figure 7-2, a transit is setup and leveled at A. Theintersection of your line of sight with the rod (point C).rodman holds a rod on B. The instrumentman trains theComputing the DE consists of multiplying the measuredtelescope on C, which is an easily read value (usually adistance by the proper trigonometric function of thefull foot) on the rod. With the telescope trained on C, themeasured angle (sine, when slope distance (OC) isvertical angle (a) is read. Then either the horizontalmeasured; tangent, when horizontal distance (OD) isdistance or the slope distance between the instrumentmeasured).and rod is determined. Now one side and one angle of aThe following paragraphs discuss typical situationsright triangle (OCD) are known. From your knowledgethat you will encounter in trigonometric leveling. Youof trigonometry, you know that the other sides and anglewill see in each of these situations the reamer in whichcan be computed. However, in trigonometric leveling,the computed DE is applied to determine the HI andyou are concerned only with determining the length ofrequired elevations.the side opposite the measured angle (side CD). Thelength of this side is the difference in elevation (DE).1. DEPRESSION ANGLE BACKSIGHT (fig.As-you can see in figurebetween the height of7-2, the DE is the distance7-3, view A). The rod is on point B below the instrumerit.instrument (HI) and theThe measured vertical angle (a) is a depression (minus)7-3