Measurement of horizontal angle by theodolite

Procedure

•  Instrument was set up at station ‘O’ and temporary adjustment perform to level it accurately.

• Then the verneirs on upper plate, named as verneir ‘A’ and verneir ‘B’ adjusted to 0⁰ and 180⁰ respectively. At this stage the vertical circle was to the left of the observer. Then both plates are clamped to rotate the instrument above its outer axis.

• The lower clamped was loosened and it was pointed towards the left hand side of the ‘P’. The lower clamped was tighten and the object ‘P’ was bisected accurately by using accurate the lower tangent screw. The readings of verneir A and B should be checked and such their should be no change in the previous readings it is noted that the left hand side object was bisected first because of the fact that the graduation on the scale place runs in clockwise direction.

• The upper clamp was loosen and the instrument was rotated clockwise above the inner axis to bisect the object ‘Q’. The upper clamp is then clamped and the object ‘Q’ was bisect accurately by using upper slow motion screw.

• The reading on the both verneir were taken the reading of verneir ‘A’ gives directly the angle ‘P’ and ‘Q’ while the vertical angle ‘P’ and ‘Q’ on verneir ‘B’ can be obtained by reducing 180⁰ from the reading.

• The mean value of two readings give the angle POQ with one face.

• The face was changed by transiting the telescope and the whole process was repeated that mean values of angle POQ was obtain with the face.

• Average horizontal angle was thus obtained by taking the mean value obtain with two faces.

Diagram

Observation table

object	face	Readings on vernier		Mean Angle
		A	B
Light Poles		00 0’ 0”	1300 0’ 0”	139 29' 30”
		2200 30’ 40”	400 30’ 40”
		00 0’ 0”	1200 0’ 0”	179059’10”
		1660 43’ 40”	3460 2’ 0”
		00 0’ 0”	1800 0’ 0”	179 59’40”
		1770 42’ 20”	3570 42’ 20”
		00 0’ 0”	1800 0’ 0”	1800 0’ 0”
		1530 21’ 40”	3300 41’ 40”

Result

Angle obtained by horizontal method = 68⁰ 12^’ 30”