viewshed

 He who from the top of AEtna casts his eyes leisurely around, is affected chiefly by the extent and diversity of the scene. Only by a rapid whirling on his heel could he hope to comprehend the panorama in the sublimity of its oneness. But as, on the summit of AEtna, no man has thought of whirling on his heel, so no man has ever taken into his brain the full uniqueness of the prospect; and so, again, whatever considerations lie involved in this uniqueness, have as yet no practical existence for mankind.
 http://xroads.virginia.edu/~HYPER/poe/eureka.html

A viewshed is an area of land, water, or other environmental element that is visible to the human eye from a fixed vantage point. The term is used widely in such areas as 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.

In digital imaging, a viewshed may be a binary raster indicating the visibility of a viewpoint for or from an area of interest. A pixel with a value of unity indicates that the viewpoint is visible from that pixel, while a value of zero indicates that the viewpoint is not visible from the pixel. In certain disciplines, such as radio communications, 'visibility' may be probabilistic, therefore viewshed may be represented with non-integer values.
In radio, a viewshed is the area where a specific combination of transmitter, antenna, and terrain allow reception of signal.

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Avebury// googleearth