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Viewshed
A viewshed is the geographical area that is visible from a location. It includes all surrounding points that are in line-of-sight with that location and excludes points that are beyond the horizon or obstructed by terrain and other features (e.g., buildings, trees). Conversely, it can also refer to area from which an object can be seen.〔 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 areas of particular scenic or historic value that are deemed worthy of preservation against development or other change. Viewsheds are often spaces that are readily visible from public areas such as from public roadways, public parks or high-rise buildings. The preservation of viewsheds is frequently a goal in the designation of open space areas, green belts, and community separators.
==Representation==
A viewshed can be represented by raster data indicating the visibility of a viewpoint for or from an area of interest. In a binary representation, a cell (shown graphically as a pixel) with a value of 1 (or "true") indicates that the viewpoint is visible from that cell, while a value of 0 (false) indicates that the viewpoint is not visible. 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?).
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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