Measurement of Horizontal Angle
The procedure is described for measuring horizontal angle θ = PQR at station Q (Ref. Given fig)
1. Set the theodolite at Q with vertical circle to the left of the line of sight & complete all temporary adjustments.
2. Release upper and lower both clamps and turn upper plate to obtain 0° on the main scale. Then clamp main screw and by using tangent screw gets zero reading exactly. At this stage vernier A reads 0° and vernier B reads 180°.
3. Through telescope take line of sight to signal at P and lock the lower clamp. For exact bisection, use tangent screw.
4. To bisect signal at R, Release the upper clamp and swing telescope. Lock upper clamp and use tangent screen to obtain exact bisection of R.
5. Read verniers A and B. The reading of vernier A directly gives desired angle PQR, whereas
180° is to be subtracted from the reading of vernier B to obtain the angle PQR.
6. Transit (move by 180° in vertical plane) the telescope to make vertical circle to the right of telescope. Repeat steps 2 to 5 to obtain two more values for the angle.
7. The average of four values found for θ, give the horizontal angle. Two values get with face left and 2 obtained with face right position of vertical circle are known one set of readings.
8. If more precision is needed the angle may be measured repeatedly. For example release lower clamp after step 5, sight signal at P, then lock lower clamp, release upper clamp & swing the telescope to signal at Q. The reading of vernier doubles. The angle measured by vernier B is also doubled. Any no. of repetitions may be made and taken the average. With face right similar readings are then taken. Average angle is found finally and is taken as desired angle 'Q'. This is called method of repetition.
9. There is another method of getting precise horizontal angles. It is known method of reiteration.
If from a station a number of angles are to be measured this technique is used. With zero reading of vernier A signal at P is sighted perfectly and lower clamp and its tangent screw are locked. Then by sighting Q, θ1 is measured and noted down. Then by unlocking upper clamp θ2, θ3 and θ4 are measured and bisecting signals at R, S and P. Angles are calculated and checked to see that sum is 360º. Both verniers are read and similar procedure is carried out by changing the face in each case (face left and face right).
