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If you are standing, looking out at a landscape, those points you can see can be described as being intervisible with you. The concept of intervisibility is the basis of one of the many functions contained in most geographical information systems (GIS) designed for use with elevation data. In this entry, the basic algorithm and its extension to the so-called viewshed as well as a number of issues and applications of their use are presented.

The Basic Algorithm

To determine intervisibility, it is essential to have a digital representation of terrain as either a gridded digital elevation model (DEM) or a triangulated irregular network (TIN) and to define two point locations, a viewpoint and a target. At both locations, a height above the terrain should also be ...

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