Page 26 - ebook_soil engineering practical workbook_15112021

2.6 Method of measuring horizontal angles

 The horizontal angle subtended at the instrument station B by the two

signals A and C can be measured as follows.

 Set up the theodolite at station B as previously described and accurately level
it, complete the temporary adjustment. Turn the vertical circle on the left of

the observer.
 Station A and C is a target station fixed on the ground by a three ranging rods

and plumb.
 Levelling the theodolite

 Loosen the upper clamp and bring the horizontal graduated circle vernier
0
approximately to zero (commonly marked 360 ).
 Clamp the upper plate and by upper slow motion screw, bring the zero into
exact coincidence (00’00”).

 Set the telescope to bisect station A and fix the lower clamp. Eliminating the
effect of the parallax.

 Fix telescope clamp and bisect A exactly. Point A should be brought into exact
coincidence with the point of intersection of the horizontal and vertical

cross-hairs by using the vertical circle clamp and tangent screw.
 Now sighting to station A is complete and horizontal graduated circle reading

o
approximately to 00 00’ 00”. Record the reading.
 Loosen the upper plate and turn the telescope in a clockwise direction until

the line of sight to station C. Tighten the upper clamp and by using the upper
tangent screw, bisect point C exactly. Record the reading.

18 | Soil Engineering Practical Workbook