Viewshed point visibility test over a terrain profile

It builds a straight line between the observer and target elevations, including their heights above the ground, then checks each intermediate terrain sample against that line. The result states whether the target is visible, identifies the point with the least clearance, and reports the signed vertical clearance there. This makes the calculation transparent and repeatable for surveying, radio planning, site screening, and map-based terrain analysis.

Prepare an ordered terrain profile

Provide terrain samples in travel order from the observer to the target. The first distance must be zero, every later distance must be greater than the previous distance, and the last sample represents the target location. Distances and elevations may use metres, feet, or another linear unit, but all values must use the same unit because the calculation compares them directly. Add observer height and target height as distances above their respective ground elevations. An observer height might represent eye level, an antenna centre, or an instrument mounted on a tripod. Target height can represent a person, a mast, a structure, or zero when visibility of the ground point itself is the question. Include samples at terrain breaks and local high points whenever possible. The test can only evaluate the profile supplied to it; a ridge that falls between widely spaced samples will not be invented or inferred. The ordered input therefore carries both the terrain model and its sampling limitations into the result.

Understand the straight sightline test

The calculation first adds observer height to the terrain elevation at distance zero and target height to the elevation at the final distance. These two absolute elevations define a straight line. At each intermediate sample, linear interpolation gives the elevation of that line at the sample distance. Terrain elevation is then subtracted from sightline elevation to produce clearance. Positive clearance means the line passes above the sampled ground, zero means it touches the sampled ground, and negative clearance means that sample rises above the line and blocks visibility. The target is reported as visible only when every intermediate clearance is zero or greater. The output also returns the critical point, which is the sample with the smallest clearance, along with its distance, terrain elevation, sightline elevation, and clearance. This signed value is useful beyond a simple yes-or-no answer: it shows how much extra height would be needed at the critical sampled location or how narrow the available margin is.

Interpret results within the model limits

Treat the result as a point-to-point test over a supplied profile, not as a complete geographic viewshed raster. It evaluates only the straight vertical section joining two locations. Earth curvature and atmospheric refraction are not applied, so long links may need a separate curved-Earth or effective-radius analysis. Buildings, trees, vehicles, seasonal canopy, and other surface objects are absent unless their heights are already represented in the elevations you provide. The calculation also does not interpolate a continuous terrain surface between samples; denser sampling around crests generally gives a more dependable answer. A visible result with very small positive clearance should therefore be read as sensitive to elevation error and omitted obstacles, while a large positive margin is more robust. A negative minimum clearance identifies definite obstruction within the supplied data, and its critical point helps focus a field check or design change. Run the same deterministic input again and the same output is returned, making it suitable for documented screening workflows and automated comparisons.