Work out curvature and refraction correction instantly with clear inputs, formula shown and shareable results.
Over long sights the earth curves away from the level line, making staff readings too large by 0.0785 d² metres with distance in kilometres. Atmospheric refraction bends the line of sight downward and partly offsets it, conventionally by one seventh, so the combined correction is about 0.0673 d² and is subtracted from the observed reading.
Curvature
c = 0.0785 d² metres, d in kilometres
Combined
Combined = curvature x (1 - refraction coefficient), about 0.0673 d²
c = 0.0785 d² metres, d in kilometres. Over long sights the earth curves away from the level line, making staff readings too large by 0.0785 d² metres with distance in kilometres.
When backsight and foresight distances are equal, because the error cancels. That is why balancing sight distances is standard levelling practice and why the correction matters mainly for trigonometric heighting.
This calculator takes 2 inputs: Sight distance, Refraction coefficient. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.