User's Manual

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92 Trimble TS635 Construction Total Station User Guide
Curvature and refraction corrections
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.
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)
Curvature error (A A')
Refraction error (A A1
')
P
A
'
Horizontal
direction
A
O
P
A1
'
A1
A
'
HD' HD
SD
2
2VA()sin
2R
e
-----------------------------------
1
k
2
---–
©¹
§·
–=
VD' VD
HD
2
2R
e
-----------
1k–()+=