A two-peg test on a level is reduced to a collimation error per unit sight length, expressed in seconds of arc, and then used to correct a pair of unbalanced sights and recover a turning-point elevation.
Apply· about 20 minutes by hand· 6 steps
Given
Two firm pegs A and B set 300.0 ft apart on nearly level ground. All readings in feet to 0.001 ft.
Position 1, instrument set midway between the pegs: rod reading on A = 5.284, rod reading on B = 4.117.
Position 2, instrument set on the line 5.0 ft outside peg A, so that the sight to A is 5.0 ft and the sight to B is 305.0 ft: rod reading on A = 4.652, rod reading on B = 3.462.
The instrument is later used on a job where the sights could not be balanced: a backsight of 5.216 was read on BM-14, elevation 742.186 ft, at a sight length of 250 ft, and a foresight of 8.940 was read on TP-3 at a sight length of 480 ft.
Elevations are carried to 0.001 ft.
Required
The true difference in elevation between the pegs.
The collimation error of the instrument, expressed per 100 ft of sight and in seconds of arc, with its sense.
The corrected elevation of TP-3.
Work it through yourself before reading on - the solution below shows every step, so there is no way to skim it without giving the answer away.
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