Measurement of horizontal angle
● The instrument is set over B.
● The lower clamp is kept fixed and upper clamp is loosened.
● Turn the telescope clockwise set vernier A to 0° and vernier B to approximately 180°
● Upper clamp is tightened and using the upper tangent screw the vernier A and B are exactly set to 0° and 180°
● Upper clamp is tightly fixed, lower one is loosened and telescope is directed towards A and bisect the ranging rod at A.
● Tightened the lower clamp and turn the lower tangent screw to perfectly bisect ranging rod at A.
● Loose the upper clamp and turn the telescope clockwise to bisect the ranging rod at C tightened the upper clamp and do the fine adjustment with upper tangent screw.
● The reading on vernier A and B are noted. Vernier A gives the angle directly and vernier B gives the reading by subtracting the initial reading (180°) from final reading.
Hence, we can change the reading on the horizontal angle while measuring a horizontal angle in theodolite by doing upper clamp is loosened and lower clamp is tightened.