Object: To measure the horizontal angle by repetition method.

Apparatus:  Theodolite, tripod, ranging rods. The repetition method is used to measure horizontal angle very accurately. In this method the angle is added a number of times. The total is divided by the number of readings to get the angle. The angle should be measured clockwise in the face left and face right position, with three repetition at each face. The final reading of the first observation will be the initial reading of the second observation, and so on. The following procedure is adopted.

Procedure:

1. Suppose it is required to measure Angle AOB.
2. Set up the instrument over 0 (i. e. centred and levelled)
3. Keep the telescope in the normal position, with the vertical circle on the left.
4. Release both the upper and lower clamps.
5. Set the vernier A to zero (i. e. 360°) and the vernier B to 180°.
6. Tighten the upper clamp, bring it exactly to zero by using upper tangent screw.
7. Take also the reading of vernier B (it must be exactly at 180″)
8. The ranging rod at A is perfectly bisected. Now the lower clamp is tightened.
9. The upper clamp is loosened, and the ranging rod or object at B is bisected by turning the telescope clockwise.
10. The reading on both the verniers are taken. Angle AOB
11. Unclamp the lower plate, and turn the telescope clockwise

To sight the station A again. Tighten the lower clamp.

12. Use the lower tangent screw for exact bisection of point A.
13. Check the vernier readings have not changed.
14. Release the upper clamp and turn the telescope clockwise to sight the object at B. Tighten the upper clamp
15. Bisect the object at B exactly using the upper tangent screw.
16. The vernier A will give the value which will be about twice the angle

17. Repeat steps 11, 12, 13 and 14.
18. Divide the final reading by three to get the value of AngleAOB.
19. Change face of instrument to the face right. The telescope will be in inverted condition.
20. Repeat above procedure. Determine another value of the angle AOB.
21. Determine average value of the angles obtained with the face left and right.
22. Use the given table 6.1 for recording the observations.

Precautions:

1. The bubble may be re-centred after each turning of the angle before sighting the initial point.
2. It should not be re-centred between the back sight on the station A and the fore sight on station B.

3. An even no. of total repetition should be taken. 4. When two sets are taken, start the second set after setting the vernier A to 180°.
Remark’s                      If Any