We know that earth flattens in the polar direction and this curvature of earth is responsible for the departure of horizontal line from a level surface. To counter this error, orthometric corrections is applied.
This departure in feet, Cf is calculated as
Cf= 0.667M2=0.0239F2
This departure in meters, Cm is calculated
Cm =0.0785K 2
where M = distances in miles from the point of tangency to the earth.
F= distances in thousands of feet from the point of tangency to the earth.
K = distances in kilometers from the point of tangency to the earth.
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