A "viewshed" is a region visible to an observer. "Visibility" refers to line of sight, but the same concept and calculations apply to transmission of signals in other media: radio and microwave frequency waves, sonar, and radar.
Viewsheds can help answer many different kinds of questions:
- How to situate infrastructure, such as pipelines and highways, in out-of-the way places that do not destroy the quality of natural scenery
- Where to site transmission towers to avoid spatial gaps in reception
- How to characterize regions under surveillance (or not under surveillance)
The simplest viewshed calculations suppose that light moves in straight lines in a Euclidean space (the earth is not curved and no refraction occurs). This is a good approximation for distances of several kilometers or miles. Corrections for the earth's curvature and for optical refraction in the earth's atmosphere are needed for accurate results over longer distances.