State the procedure of measurement of horizontal angle by repetition method.
1 Answer
Answer :
The horizontal angle is measured very precisely using method of repetition as follows :
1) Set-up the theodolite over station O and level it properly.
2) Set the vernier A and vernier B at 0°0’0” and 180°0’0” respectively on the horizontal graduated scale. This is done by loosening the upper clamp and moving the upper plate until the zero of the vernier plate A coincides with the zero of the main scale. Tighten both the clamps.
3) Loosen the lower clamp and bisect point A, the readings in vernier A and vernier B should be 0°0’0” and 180°0’0” respectively. Minor adjustment in reading is done by lower tangent screw.
4) Loosen the upper clamp and bisect point B. Point B is accurately bisected using upper tangent screw. Tighten both the clamps and note the readings in both verniers.
5) Tighten the upper clamp and rotate the telescope either in clockwise or anticlockwise until it bisects point A. The readings in the vernier should remain the same. Loosen the upper clamp and bisect point B. Point B is accurately bisected using upper tangent screw. Tighten both the clamps and note the readings in both verniers. This time vernier A will be twice of the earlier angle.
6) In the same way take the angle for the third time.
7) Read the final angle. The average angle by face left will be the accumulated angle divided by 3.
8) Change the face of the theodolite and repeat the same procedure.
9) The mean of both angles gives the horizontal angle AOB.