||It has been suggested that Zone of Visual Influence be merged into this article. (Discuss) Proposed since February 2013.|
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 calculated for areas of particular scenic or historic value that are deemed worthy of preservation against development or other change. Viewsheds are often calculated for 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.
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?).
Viewsheds are a specific type of visibility graph.
Isovists are a closely related concept that is more common in the study of architecture. Viewsheds and isovists are sometimes said to be equivalent, however others have found differences between them. It has been argued that isovists are more focused on representing space whereas viewsheds are about the visibility of features. Also, the problems they are used in have different scales. Planners use viewsheds where terrain heights come into play whereas architects do not typically take that into account with isovists.
The area from which a structure can be seen may be called the "Zone of Visual Influence". This can be referred to as the viewshed as well, though.
- Visual Impact Assessment for Highway Projects (PDF). Washington, DC: U.S. Department of Transportation, Federal Highway Administration, Office of Environmental Policy. September 1990. p. 26–33, 123. Publication number FHWA-HI-88-054.
- Nijhuis, Steffen; van Lammeren, Ron; Antrop, Marc (September 2011). Exploring the Visual Landscape - Introduction. Research in Urbanism Series 2. p. 30. doi:10.7480/rius.2.205. ISSN 1879-8217.
- Barrett, Grant (March 27, 2006). "viewshed". A Way with Words. Retrieved March 10, 2015.
- Turner, A; Doxa, M; O'Sullivan, D; Penn, A (2001). "From isovists to visibility graphs: a methodology for the analysis of architectural space". Environment and Planning B: Planning and Design 28 (1): 103–121. doi:10.1068/b2684.
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