5. Measuring a deflection angle

A deflection angle is measured from the extension of the previous line to the next line, Figure D-10. It is either to the right (clockwise) or left (counter-clockwise). 

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Figure D-10
Deflection Angle

 

The procedure for measuring a deflection angle is the same as for an angle right (or left) except for when the instrument is reversed. To measure the deflection angle at point B from Point A to point C, after the TSI is set up at point B:

Step (1) Sight point A with the TSI in the reverse position.

Step (2) Rotate the telescope about the HA until it is pointing away from point A along the extension of line AB.

Step (3) Zero the horizontal angle and record it.

Step (4) Release the horizontal circle lock and rotate to point C in the direction of the angle (left in this case).

Step (5) Using the horizontal lock and slow motion, sight accurately on point C.

Step (6) Freeze the angle reading and record it.

Step (7) Compute the deflection angle by subtracting the BS reading from the FS reading. This is the direct angle.

Step (8) Release the horizontal circle lock and rotate to point A.

Step (9) Using the horizontal lock and slow motion, sight accurately on point A.

Repeat steps (2)-(7) to measure the reverse deflection angle.