thumb|Viewshed of the [[Gusev (Martian crater)|Gusev crater on Mars from the Mars Exploration Rover (red) overlaid on an elevation map (other colors) – areas in red are visible from the landing site]]

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.

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?).

Viewshed and total-viewshed computation

A terrain can be represented using a regular grid of points called Digital Elevation Model (DEM). Where each point of the DEM is represented by its coordinates X, Y and its height Z.

Viewshed calculation on a large DEM is costly from a computational point of view. This cost is much higher when calculating the viewshed for all the points of the DEM, also called total-viewshed. A faster algorithm for computing the total-viewshed of large DEMs was proposed on.

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, It is usually represented as a map using color to indicate visibility.

Zones of visual influence are used to identify the parts of a landscape that will be affected by a development. They are of particular use to landscape architects in determining visual intrusion as part of an environmental impact assessment. Zones of visual influence have been used extensively in wind farm development. A map will be created showing the number of wind turbines that are visible from a particular area. A cumulative zone of visual influence is used to define the cumulative effects of many developments.

Zones of visual influence are created using GIS tools.

See also

  • Computational archaeology
  • Isovist
  • Scenic highway
  • Visibility graph
  • Visibility (geometry)
  • Euclidean influence zone

References