C.K PITHAWALLA COLLEGE OF ENGINEERING AND TECHNOLOGY

SURVEYING

PREPARED BY : BRANCH : GUIDED BY : MITAL J. DHOLAWALA HEMANGI MAHUVAGARA - 130090106025 KEYUR MANIYA - 130090106026 FORAM MODI - 130090106027 RAVIN NAIK - 130090106028 BRIJESH NAKRANI - 130090106029 BRANCH : CIVIL GUIDED BY : MITAL J. DHOLAWALA

TRIGONOMETRIC LEVELLING

METHOD OF OBSERVATION Direct method Reciprocal method

Direct method: This method is useful where it is not possible to set the instrument over the station whose elevation is to be determined. In this method, the instrument is set on the station on the ground whose elevation is known, and the observation of the top of the tower is taken. Sometimes, the instrument is set on any suitable station and the height of instrument axis is determined by taking backsight on B.M. Then, the observation of the top of the tower is taken.

In this method, the instrument can not be setup on the top of the tower and observation can not be taken of the point on the ground. If the distance between two points s large, combined correction = 0.0673D^2 for earth’s curvature and refraction is required, where D = distance in kilometer.

STEEP SLOPE: If the ground is quite steep, the method of indirect levelling can be used with advantage.

Steps: Set up the instrument at a convenient station O1 on the line PR. Make the line of collimation roughly parallel to the slop of the ground. Clamp the telescope. Take a backsight PP’ on the staff held at P. Also measure the vertical angle t1 to P’ Determine R.L. of P’ as, R.L. of P’ = R.L. of P + PP’ Take a foresight QQ’ on the staff held at the turning point Q, without changing the vertical angle t1. Measure the slope distance PQ between P and Q. R.L. of Q = R.L. of P’ + PQ sin t1 – QQ’ OR R.L. of Q = R.L. of P +PP’ + PQ sint1 – QQ’

Shift the instrument to the station O2 midway between Q and R Shift the instrument to the station O2 midway between Q and R. Make the line of collimation roughly parallel to the slope of the grounds. Clamp the telescope. Take a backsight QQ” on the staff held at the turning point Q. Measure the vertical angle t2. Take a foresight RR’ on the staff held at the point R without changing the vertical angle t2. Measure the sloping distance QR. R.L. of R = R.L. of Q” + QR sint2 – RR’ Thus, R.L. of R = (R.L. of P + PP’ + PQ sin t1 – QQ’) + QQ” + (QR sin t2 – RR’)

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