A vertical angle is the angle between the inclined line of sight and the horizontal plane through the trunnion axis of the instrument. Prior to the measurement of vertical angle, instrument is required to be leveled with reference to the altitude level. Figure 22.3 shows vertical angles.
The procedure for measuring a vertical angle is as follows:
The temporary adjustment of the instrument is to be done on the station.
Then, leveling of theodolite is to be done using altitude level (the operations involved are same as leveling using plate level).
Loosen the vertical circle clamp, and direct the telescope towards the object whose vertical angle is required to be measured. Clamp the vertical circle, and bisect the point by turning the vertical tangent screw.
Read and record the scale with vernier C and D in Table 22.4
Change the face of the instrument and read the vertical angle again.
The required vertical angle is the average of the values in steps 4 and 5.