Subtense Bar-GRADIENT JOINING INSTRUMENT STATION AND TARGET OF SUBTENSE BAR - Computer Programming

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## Friday, April 1, 2011

Subtense Bar-GRADIENT JOINING INSTRUMENT STATION AND TARGET OF SUBTENSE BAR

Aim :

To determine the gradient of the line joining the instrument station and left target of a subtense bar.

Instruments required :

Subtense bar, theodolite, arrow, pegs, levelling staff and tape.
Given : R.L. of B.M. = 100.000m

Procedure :

1.      Set up the theodolite on the line which is fixed by viewing through sight vane of alidade of the subtence bar.
2.      Sight apex of the left target of the subtense and note down the vertical angle by keeping the horizontal angle as zero.
3.      Sight the apex of the other target and note down vertical and horizontal angles.
4.      Note down the B.M. reading
5.      Measure the instrument height using tape.

Calculation :

A = Left target of subtense bar
B = Right target of subtense bar
O = Instrument (Theodolite) station
q = Horizontal angle subtended by two targets

From DOAB
we get
D tan q/2 = S/2
D =  ½ S cot q/2
H = height of target with reference to H.I.
H = D tan a where a = Vertical angle
R.L. of Left target A = R.L. of B.M. + Staff reading + H
R.L. of the instrument station = R.L. of the H.I. - Instrument height
Vo = R.L. of left target - R.L. of Instrument station