Viewshed analysis is the study of visibility between points on a terrain surface.

Viewshed analysis is used in visual impact assessment.

In viewshed analysis, the visibility of every cell from the observer cell (or numerous observer cells) is computed.

Visibility is calculated by measuring the tangent from the observer's eye to each cell starting from cells closest to the observer. As long as the tangent increases in the line of site from the observer, the cell is visible. If the tangent decreases, the cell is not visible.