A Fast Algorithm for Approximate Viewshed Computation

Given a terrain model, the viewshed from a viewpoint is computed as the set of X-Y positions of a point target such that the target is visible. We introduce a novel approach to the computation of accurate viewshed approximations. Our algorithm relies on the computation of an approximation to the so-called line-of-sight (LOS) function. This function is defined over target X-Y positions, and is the highest elevation of a target so that it is still occluded from the viewpoint. The LOS function summarizes intervisibility information and permits reuse of previous intervisibility computations, resulting in computational efficiencies. Our algorithm gives results very close to the traditional sightline ray method at a substantially smaller computational cost, and is a generalization of the approach developed by Franklin and Ray for the same problem. We describe in detail the application to gridded terrain models, but the approach is similarly applicable to other elevation models.