is 0. When the telescope is pointed up at a higher feature (elevated), the vertical angle increases from 0

and is called a plus vertical angle. These values increase from 0, to +90, when the telescope is pointed

straight up. As the telescope is depressed (pointed down), the angle also increases in numerical value.

A depressed telescope reading showing that it is below the horizontal plane is called a minus vertical

angle. These numerical values increase from 0, to -90, when the telescope is pointed straight down.

d. To determine the difference in elevation between two points, set up and level the instrument at

one point. Hold the rod at another point. Point the telescope at an easily read value (a full meter) on the

rod, and measure the vertical angle (Figure 4-12). Determine the horizontal distance (ED) between the

instrument and rod by taping, obtaining a stadia reading, or by triangulation. You may also determine

the slope distance (EB) using EDME. Whichever distance (ED or EB) is selected, one side and one

angle () of a right triangle should be determined. Using this information, compute the other sides and

angle. For trigonometric leveling, only the side opposite the measured angle, the difference in elevation,

is computed.

(1) The computation consists of multiplying the measured distance by the proper trigonometric

function of the measured angle (sine, if the slope distance [EB] is measured; tangent, if the horizontal

distance [ED] is measured). Where AE is the height of the instrument above point A, and BC is the

height of the line of sight above point C, the difference in elevation between A and C is AE + BD - BC.

(2) This method of determining the difference in elevation should be limited to horizontal

distances less than 300 meters when moderate precision is sufficient and to proportionately shorter

distances when higher precision is required.

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