TRANSITION SPIRAL: TRANSIT
ATINTERMEDIATE STATION
Referring to the transition spiral in the previous calculation procedure, assume that lack of
visibility from the TS makes these setups necessary: Points 4, 5, 6, and 7 will be located
with the transit at point 3; points 8 and 9 and the SC will be located with the transit at
point 7. Compute the orientation and deflection angles.
Calculation Procedure:
- Consider that the transit is set up at point 3 and a backsight
is taken to the TS; find the orientation angle
In Fig. 18, assume that the spiral has been staked up to P with the transit set up at the TS
and that the remainder of the spiral is to be staked with the transit set up at P. Deflection
angles are measured from the tangent through P (the local tangent). The instrument is ori-
ented by back-sighting to a preceding point B and then turning the angle (^8) b. The orienta-
tion angle to B and deflection angle to a subsequent point F are
Sb = (2Lp + Lb)(Lp-Lb)^ (38)
Sf= (2Lp +Lf)(Lf-Lp)^ (39)
If B, P, and F are points obtained by dividing the spiral into an integral number of
arcs, these equations may be converted to these more suitable forms:
(^8) h = (2np + nb)(np-np) -J^ (38a)
(^8) r (2np + nf)(nf -np) -^ (396)
where n denotes the number of arcs to the designated point.
Tangent through P
Main
tangent
FIGURE 18. Deflection angles from local tangent to spiral.