Question

Explain the setting of curve by Rankine’s deflection angle method.

Answer 1

Rankine’s deflection angle Method:- 

1) Set up theodolite at first tangent point, T1 and level it carefully, set the vernier A to 360⁰

2) bisect the ranging rod at the point of intersection B.

3) Set the vernier to A to the first deflection angle Δ1, the telescope will now direct along T1D.

4) Pin down the zero end of the chain or tape at T1 holding arrow on the chain at a distance equal to the length of first sub chord swing the chain around T1 and bisect the arrow by the diaphragm hairs to fix the position on first point, D on the curve.

5) Loosen the upper plate and set the vernier to the second deflection angle Δ2 the line of sight will view direct along T1E.

6) Pin down the zero end of the chain or tape at D and swing the other end. Bisect the arrow held at the other end, the second point on the chain E is thus located.

7) Thus establish points on the curve until the end of the curve is reached. The last point located must coincide with the tangent point T2 already located. The slight difference can be adjusted on the last few pegs.

8) If the error is beyond the permissible limits the whole work must be checked or repeated. 

9) In case of the left hand curve, each of the calculated values of the deflection angle (Δ1, Δ2 etc) should be sub subtracted from 360⁰ . The angle thus obtained will be set on the vernier of the theodolite for setting out the curve.