The Geometry of Line-of-Sight and Weapons Fan Algorithms

Intervisibility algorithms, when applied to digital elevation models, are used to compute line-of-sight, weapons fans or viewsheds, and accurate threedimensional perspective views. Whereas data quality, atmospheric effects, vegetation, and buildings contribute to the final result, the geometric model has a major impact. Seven geometric parameters should be explicitly defined: Viewer and target locations, interpretation of viewshed, point interpolation, point selection along radials, viewshed creation, vertical earth curvature, and horizontal earth curvature. The importance of horizontal earth curvature - the determination of straight line distance between observer and target - has not been sufficiently appreciated. Unless Universal Transverse Mercator approximations are valid, geodetic computations should be used. Because digital elevation models available to the military typically have a geographicbased point spacing, many established procedures that implicitly assume a conformal Universal Transverse Mercator grid introduce errors. A spaced radial algorithm produces the best weapons fans.