Let ABC be the required angle between the sides BA and BC. An iterative method is generally used for accurate and precise work. In this method, the angle value is mechanically added several times and the correct angle measurement value is obtained by dividing the accumulated reading by the number of repetitions.To measure a small horizontal angle ABC, the following procedure is adopted:
The mean of the two values on the angle obtained on face left and face right gives the required value of the angle ABC.
Keeping the face of the instrument left, centre and level it accurately over the ground point B.
Set the vernier to read zero. Loosen the lower plate and swing the telescope in azimuth to sight the left-hand point A. Using the lower tangent screw, bisect the point A accurately.
Read both the verniers and take the mean of the two readings.
Loosen the upper plate and swing the telescope in clockwise direction until point C is brought in the field of view. Using the upper tangent screw bisect the point C accurately.
Read both the verniers and take the mean of the readings. The difference of the mean readings for points C and A gives the approximate value of the angle.
Unclamp the lower plate and turn the telescope in clockwise direction until point A is again sighted. Clamp it and bisect it accurately with the lower tangent screw.
Loosen the upper plate and swing the telescope in clockwise direction and again bisect point C accurately using the upper tangent screw. The verniers will now read double the value of the angle ABC.
Repeat the process until the angle ABC is repeated the required number of times, say 5 times.
Read both the verniers. The accumulated reading is obtained by taking the difference of the two mean readings to stations C and A.
Divide the accumulated angle by the number of repetitions to get the correct value of the angle ABC on face right.
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