Pentax W-1502N Series Instruction Manual page 100

Appendix II: Correction for refraction and earth
curvature
Considering the correction of refraction and earth curvature for
distance measurement, the formula for slope distance, horizontal
distance and vertical distance applied in the instrument are as
followings:
The conversion formula for horizontal and vertical distance is as
follows when correction for refraction and earth curvature is not
applied:
VD=SD ∣SIN§∣
HD=SD COS§
NOTE:
The factory setting for the refraction coefficient K is 0.142.
Refer to the section 3.10 to change the value of K.
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