What This Calculates
Over long distances, the Earth’s curvature causes a straight sightline to drop below the horizon by a measurable amount. This is useful for surveying, radio line-of-sight planning, and long-distance photography.
The Formula
Drop ā dĀ² / (2R), where d is the horizontal distance and R is Earth’s mean radius (about 6,371 km). This is a small-angle approximation valid for distances much smaller than Earth’s radius, and ignores atmospheric refraction.
For general educational and planning purposes.
Last reviewed August 2026