How this calculator works
The drop and horizontal distance form the perpendicular legs of a right triangle. The sightline is its hypotenuse. This flat-ground model ignores curvature, atmospheric refraction and instrument error.
Formula
tan(theta) = drop / run; theta = atan(drop / run); drop = run x tan(theta); run = drop / tan(theta).
Worked example
A 10 m drop over 20 m horizontally gives atan(0.5) = 26.5651 degrees and a 22.3607 m sightline.
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Frequently asked questions
Do I measure from eye level?
Yes. Subtract the target elevation from the observer eye or instrument elevation.
Is this the angle from vertical?
No. It is measured downward from horizontal. The angle from vertical is its complement.
Can I use a level sightline?
A zero drop gives a zero angle in angle mode. Inverse distance calculations require a positive angle below 90 degrees.