Viewsheds on terrains in external memory

Visibility is one of the fundamental problems on terrains. It is at the core of many applications, such as planning the placement of communication towers or watchtowers, planning buildings and roads such that they have a good view or such that they do not spoil somebody else's view, and finding routes on which you can travel while seeing a lot, or without being seen. A variety of problems pertaining to visibility have been researched in computational geometry and graphics, as well as in GIS and geospatial engineering.