Method of Measurement of Deflection Angle and Magnetic Bearing

Method of Measurement of Deflection Angle:
The deflection angle is that the angle by which a line is deflected from its original direction. In other words, it's the angle which a survey line makes with the extension of the preceding line. The deflection could also be towards the proper or the left, depending upon whether the angle is measured within the clockwise or anticlockwise direction from the extension of the preceding line. Deflection angles are measured for designing horizontal curves in railways, highways, etc. Let AB be the overall direction of survey. Suppose it's deflected within the direction BC. the line AB is extended up to P. Then angle PBC (ø) is understood because the angle of deflection, and has got to be measured. the subsequent procedure is adopted:

1. The theodolite is about up at B, and centered and levelled properly. Vernier A is about at 0° and B at 180°. The upper clamp is tightened and therefore the lower one loosened. By turning the telescope, the ranging rod at A is perfectly bisected. The lower clamp is then fixed.
2. The telescope is transited and a ranging rod at P is fixed along the prolongation of AB.
3.  The upper clamp is loosened. By turning the telescope clockwise, the ranging rod at C is correctly bisected. The readings on both the verniers are taken. The upper clamp is then tightened. Now, the verniers give the, deflection angle ø.
4. The lower clamp is loosened, and by turning the telescope clockwise the ranging rod at A is again bisected. (At this point , the venier readings are left undisturbed.) The lower clamp is fixed.
5. The telescope is transited. The upper clamp is loosened and by turning the telescope clockwise the ranging rod at C is bisected another time . The readings on the vernier are taken.
6. Thus the deflection angle is doubled. the typical of the 2 verniers is taken. One-half of this average value will give the right value of the deflection angle.

The deflection angle is doubled so as to eliminate errors due to wrong adjustment of the instrument, and those due to eccentricity of the centre.

Method of Measurement of Magnetic Bearing:
Suppose the magnetic bearing of the line AB is to be measured. The procedure is as follows:

1. The theodolite is about up at A, and centered and levelled properly. Vernier A is about at 0° and vernier B at 180°. The upper clamp is fixed.
2. Now a trough compass or tubular compass is fixed on the left standard (4-frame) with a fixing screw. In some theodolites, a circular compass is provided over the vernier between the standards. However, the needle of the compass is released.
3. By loosening the lower clamp, the telescope is rotated until it points to the north (i.e. the magnetic needle coincides with the 0-0' mark). At this point , the position of the telescope is called to be perfectly oriented along the meridian .
4. The lower clamp is fixed and therefore the upper clamp loosened. Then by turning the telescope clockwise, the ranging rod at B is bisected with the help of the upper tangent screw.
5. The readings on both the verniers are taken. The mean of those readings is the magnetic bearing of AB.
6. The face of the instrument is changed, and the magnetic bearing of AB is measured in a similar manner.
7. The mean of the 2 observations will give the right magnetic bearing of the road .