The procedure for setting out a combined curve (consisting of a simple circular curve with transition curve at each end) by method of deflection angles with reference to Figure 39.4 is as follows:
Step 1 : Locate the tangent point T, by measuring back the total tangent length (Tt) along the back tangent, from the point of intersection V.
Likewise, locate the tangent point T' by measuring along the forward tangent the distance Tt from V.
Setting out of the transition curve :
Step 2 : Set a theodolite over the point T. Set the vernier A to zero, and clamp the upper plate.
Step 3 : Direct the line of sight of theodolite to the intersection point V, and clamp the lower plate.
Step 4 : Release the upper plate. Set the vernier A to the first deflection angle (a1).
The line of sight now points towards the first peg on the transition curve.
Step 5 : With the zero of the tape pinned at T and an arrow kept at the mark corresponding to the first length of the chord, the assistant will swing the tape till the arrow is bisected by the line of sight.
Fix the first peg at the arrow point.
Step 6 : Set the vernier A on the second deflection angle (a2) to direct the line of sight to the second peg.
Step 7 : With the zero of the tape pinned at T, and keeping an arrow at the mark corresponding to the total length of the first and second chords, the assistant will swing an arc till the arrow is bisected by the line of sight.
Fix the second peg at the arrow point. It should be remembered that the distance is measured from the point T and not from the preceeding point.
Step 8 : repeat steps (6) and (7) till the last point C on the transition curve is reached.
Setting out of the circular curve :
Step 9 : For setting out the circular curve CC', shift the theodolite to junction point C.
Orient the theodolite with reference to the common tangent CC1 by directing the line of sight towards CT with the vernier. A set at a reading equal to
, and swinging the telescope clockwise in azimuth by
fs (Figure 39.4).
Now the line of sight is directed along the common tangent CC1 and the vernier reads zero.
Step 10 : Plunge the telescope. The line of sight is now directed along the tangent C1C produced.
The deflection angles D1, D2, etc. have been calculated with reference to the tangent C1C produced at C (Figure 38.2).
The line of sight is now correctly oriented, and the reading of the vernier A is zero.
Step 11 : Set the vernier A to the first deflection angle D1, and locate the first peg on the circular curve at a distance of c' from C, where c' is length of the first sub-chord.
Step 12 : Likewise, locate the second peg on the circular curve at the distance c equal to the normal chord from the first peg with the deflection angle D2 at C.
Step 13 : Continue the above process till the junction point C' is reached.
Step 14 : Set out the transition curve T'C' from T' using the same procedure as that for the transition curve TC.