Definition- a deflection angle is the angle which a survey line makes with the prolongation of proceeding line.

Deflection angle vary from 00 to 1800 but never more than 1800.

Deflection angle measured clockwise direction from prolonged survey line is known as right deflection angle.

Deflection angle measured anti-clockwise direction from prolonged survey line is known as left deflection angle.

Procedure-

1. Set up the theodolite at Q and level it accurately.
2. with both plate clamped, the Vernier A reading 3600 take back side on p (i.e. bisect ranging rod at exactly)
3. Transit the telescope to direct the line of sight produced by PQ.
4. Loose the upper plate and turn to telescope clockwise to take fore side on R. the mean of two Vernier reading give the approximate value of deflection angle at Q.
5. Loose the lower clamp and turn the telescope horizontally to Backsight on P. the Vernier will read the same reading as in above step and the telescope inverted.
6. Transit the telescope unclamped the plate and again bisect R read both Vernier. Find the mean of final Vernier reading.