NPL-352 Manual-09.pdf

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Curvature and Refraction correction
Because the surface of the earth is curved, the vertical difference (VD and Z) at the
measurement point, as referenced to the horizontal plane, inevitably includes some
error. This error is called curvature error. Also, because the density of the air
surrounding the earth decreases with altitude, light is refracted at different rates at
different altitudes. The error caused by this change in refraction is called refraction
error.
Curvature error (A A')
P A'
Horizontal
direction
A
O
Refraction error (A A1')
A1'
A1
P
A'
HD Horizontal dist. (before adj.)
HD' Horizontal dist. (after adj.)
VD Vertical dist. (before adj.)
VD' Vertical dist. (after adj.)
SD Slope dist
VA
Vertical angle
R e
6370 km
k C&R constant (0.132 or 0.200)
HD' HD SD2 sin( 2VA)
= – ----------------------------------- ⎛
2R 1 –
k
-- ⎞
e
⎝ 2⎠
VD' = VD +
HD2 ---------- ( 1 – k)
2R e
Pulse Laser Station NPL-302 Series Instruction Manual 137