How do you find the angle of deflection? , What is the deflection angle that the angle measured in clockwise direction?, How does theodolite measure deflection angle? ,learnwithashiso, Learn with Ashiso

 A deflection angle is the angle in which a survey line makes with the prolongation of the proceeding line. It is designed as Right (R) or Left (L) according as it is measured to the clockwise or to anti- clockwise from the prolongation of the previous line. It's value may vary from 0° to 180°. The deflection angle at Q is  αR and that at R is  θL.

To measure the deflection angles at Q:

1. Set the instrument at Q and level it accurately. 
2. With both plates clamped at 0°, take back sight on P.
3. Plunge the telescope. Thus the line of sight is in the direction PQ produced when the reading on vernier A is 0°.
4. Unclamp the upper clamp and turn the telescope clockwise to take the foresight on R.
5. Unclamp the lower clamp and turn the telescope to sight P again. The verniers still read the same reading as in (4). Plunge the telescope. 
6. Unclamp the upper clamp and turn the telescope to sight R. Read both verniers. Since the deflection angle is doubled by taking both face readings, one- half of the final reading gives the deflection angle at Q.