A 1,360 ft river crossing is levelled reciprocally from both banks; the mean of the two differences removes the systematic error, and the half-difference is compared with the predicted curvature and refraction.
Apply· about 22 minutes by hand· 6 steps
Given
Benchmarks A and B on opposite banks of a river, 1,360 ft apart. Elevation of A = 596.482 ft (US survey feet); B is to be determined. Readings to 0.001 ft.
Set-up on the A bank, instrument close to A: reading on the near rod at A = 3.472. Three readings on the far rod at B: 8.256, 8.261, 8.260.
Set-up on the B bank, instrument close to B: reading on the near rod at B = 9.318. Three readings on the far rod at A: 4.614, 4.619, 4.618.
Combined curvature and refraction correction, in feet, is 0.0206 times the square of the distance expressed in thousands of feet.
Both observations were made within the same half hour, with the instrument moved directly between banks.
Required
The difference in elevation from A to B from each bank.
The accepted difference in elevation and the elevation of B.
The magnitude of the systematic error removed, compared against the predicted curvature and refraction.
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