Suppose it is required to measure the horizontal angle PQR
- Setup the theodolite over station Q and level it carefully.
- Set up the Vernier A to zero of the horizontal circle the horizontal circle usually graduated from 00 to 3600
In order to set Vernier A to the zero, loose the upper plate until the arrow of the Vernier. A approximately coincide with the zero of the horizontal circle. Clamp both plate together and by operating the slow motion screw bring the arrow into exact coincidence with the zero of the circle. Both plates are now clamped.
- Loose the lower clamp turn the instrument and then bisect the ranging rod at P. tighten the clamp and bisect P exactly by using the lower slow motion screw. The point of intersection of cross hair should be brought into exact coincidence with the station of the point rod.
- Check the reading of Vernier it should be same as above.
- Now unclamp the upper plate and turn the telescope in clockwise direction until it bisect the ranging rod at R. Tight the upper clamp by using the upper tangent screw. Bisect the ranging rod R exactly.
- Read both Vernier, the reading on Vernier A given the value of angle PQR, whereas reading on Vernier B must be deducted from 1800 in order to get value of the angle PQR.
- The mean of the two Vernier reading will give required value of PQR.
- Change the phase of the instrument and repeat the whole process . the mean of two Vernier reading given required value of PQR.
- On the important work take both face observation to eliminate the error due to imperfect adjustment of the instrument.
- The mean of both face reading given exact value of angle PQR which is free from all instrument error.