Setting out simple circular curve-
Two theodolite method
To set out the simple curve by two theodolite method.
Instruments Required :
Two Theodolites and Ranging rods.
Principle: The angle between the target and the chord is equal to the angle which that chord subtends in opposite segment.
Given : Chainage of the curve , angle of intersection and Radius of curve (R).
1. Prepare a table of deflection angle for the first sub chord, Normal chord and last sub chord .
2. Set up one theodolite over T1 and another over T2 .
3. Direct the instrument at T1 to the ranging rod at the point of intersection B and bisect it.
4. Direct the instrument at T2 to the first target point T1 and bisect it.
5. Set the verniers of both the theodolites to read zero.
6. Set the first deflection angle (D1) on both theodolites so that the telescopes are in the direction of T1D and T2D respectively.
7. Move the ranging rod until it is bisected by the cross hairs of both the theodolites to locate the point D on the curve .
8. Set the second value of deflection angle on both the theodolites and repeat the step 7 above to get the location of E.
9. Continue the process for obtaining the locations of other points in a similar manner.
Chainage at B, R, f
BT1 = BT2 = R tan f/2
T1T2 = 2R sin f/2
Length of curve T1T2 = pR
Chainage at T1 = Chainage at B – T1B
Chainage at T2 = Chainage at T1 + T1T2
Divide the length of the curve into normal Chords(30m) and subchord (C1,C2)
Deflection angles :
First subchord = 1718.9
Normal chord = 1718.9
Last subchord = 1718.9
The given simple curve is thus set out.