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12.0.0 APPENDIX D: DETERMINING THE HEIGHTS OF TALL STRUCTURES WITH A TRANSIT
Figure 12.1: Using two transit positions to find the height of a tall structure.
1) Set up a transit at point A, at some convenient distance (such as 100') from the structure; note the exact location. Level the instrument. Sight back to the structure, place a target on it to line up with the cross hairs. The imaginary line between the transit crosshairs and the target will be a horizontal datum.
2) Tilt the transit to focus on a center point such as the tip of a weather vane. Note the angle of inclination to the nearest degree and minute possible. Accuracy here is critical.
3) Move the transit to point B, note the distance back from A. Set it up so that the crosshairs line up exactly with the target, i.e., on the datum line. Tilt and focus on the same point, note the new angle of inclination.
4) Calculate side a using the Law of Sines:
5) To the length of CD add the height of the target from the base of the structure; this is the overall height.
NB: If D can be located directly below C, use one transit station at B', the tangent of B'and distance B'D to calculate CD.
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