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Friday, April 1, 2011

setting out simple circular curve - rankine's method of tangential deflection angles or single theodolite method


setting out simple circular curve - rankine's method of tangential deflection angles or single theodolite method

Aim:
            To setout the simple curve by single theodolite method.

Instruments required:

            Theodolite, Ranging rods, chain, arrows and pegs

Principle:
Deflection angle:
            The angle between the back tangent and the chord joining the point of        commencement of the curve and the other point on the curve
Procedure:
1.      Prepare a table of deflection angles for the first subchord,normal chord and last subchord.
2.      Set up a theodolite over T1. Direct the telescope to bisect the point of intersection(B), with both plates clamped to zero.
3.      Release the vernier plate and set angle 1 on the vernier using
4.      Point the zero end of the tape at T1 and an arrow held at a distance C1 along it and swing the tape around T1 till the arrow is bisected by the cross hairs to fix point A.
5.      Set the deflection angle .
where, d2 = 1718.9 x C2/R min., so the line of sight is along T1B
6.      With zero end of the tape pinned at A and an arrow held at distance AB = C2 along it and swing the tape around A till the arrow is bisected by the cross hairs thus fixing the point B.
7.      Repeat steps 5 and 6 till the last point T2 is reached.

Check:
            The last point so located must coincide with the point of tangency (T2) fixed independently by measurements from the point of intersection.

Result:

            The given simple curve is thus set out.

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