Viewshed

[1] A viewshed is not necessarily "visible" to humans; the same concept is used in radio communications to indicate where a specific combination of transmitter, antenna, and terrain allow reception of signal.

Viewsheds are commonly used in terrain analysis, which is of interest to urban planning, archaeology, and military science.

In urban planning, for example, viewsheds tend to be calculated for areas of particular scenic or historic value that are deemed worthy of preservation against development or other change.

The preservation of viewsheds is frequently a goal in the designation of open space areas, green belts, and community separators.

In certain disciplines, such as radio communications, "visibility" may be probabilistic and therefore the viewshed may be represented with non-integer values.

Viewsheds for multiple points, lines, or areas may have counts or fractional values for queries involving "how much" or "how many" (e.g., how much of a highway is visible?).

A terrain can be represented using a regular grid of points called Digital Elevation Model (DEM).