The correction for refraction as applied to levelling staff reading is given by
[d-length of sight;
R-radius of the earth]
1
+ \(\frac{1}{7}\)\(\left( {\frac{{{d^2}}}{{2R}}} \right)\)
2
-\(\frac{1}{7}\)\(\left( {\frac{{{d^2}}}{{2R}}} \right)\)
3
+\(\frac{1}{7}\)\(\left( {\frac{{{d^2}}}{R}} \right)\)
4
-\(\frac{1}{7}\)\(\left( {\frac{{{d^2}}}{R}} \right)\)