4. Ground to Grid Angle
To convert angle B7-Jerry-A1 to grid, we need the arc-to-chord correction for lines Jerry-A1 and Jerry-B7. We have the former, now must compute the latter. Although line Jerry-B7 is longer it is oriented more east-west than Jerry-A1 so the magnitude of its arc-to-chord correction should be smaller..
Since we've already shown that Equations H-11 and H-12 give that same results, we'll use the latter to compute the correction.
As expected, because of the line orientation, its arc-to-chord correction is smaller.
Figure K-8 is a sketch of the angle with all parts in place.
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| Figure K-8 Angle Correction |
The grid angle B7'-Jerry-A1', βG , is:
The total effect of the backsight and foresight arc-to-chord corrections amounts to only 00.20" which may be small enough to disregard.