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
The horizon is the apparent curve that separates the surface of a celestial body from its sky when viewed from the perspective of an observer on or near the surface of the relevant body. This curve divides all viewing directions based on whethe ...
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 communication
Radio is the technology of telecommunication, communicating using radio waves. Radio waves are electromagnetic waves of frequency between 3 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a transm ...
s 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
Urban planning (also called city planning in some contexts) is the process of developing and designing land use and the built environment, including air, water, and the infrastructure passing into and out of urban areas, such as transportatio ...
, archaeology
Archaeology or archeology is the study of human activity through the recovery and analysis of material culture. The archaeological record consists of Artifact (archaeology), artifacts, architecture, biofact (archaeology), biofacts or ecofacts, ...
, and military science
Military science is the study of military processes, institutions, and behavior, along with the study of warfare, and the theory and application of organized coercive force. It is mainly focused on theory, method, and practice of producing mi ...
. 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 — for example, 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 belt
A green belt or greenbelt is a policy, and land-use zone designation used in land-use planning to retain areas of largely undeveloped, wilderness, wild, or agricultural landscape, land surrounding or neighboring urban areas. Similar concepts ...
s, 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
In digital imaging, a pixel (abbreviated px), pel, or picture element is the smallest addressable element in a Raster graphics, raster image, or the smallest addressable element in a dot matrix display device. In most digital display devices, p ...
) 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
Probability is a branch of mathematics and statistics concerning events and numerical descriptions of how likely they are to occur. The probability of an event is a number between 0 and 1; the larger the probability, the more likely an e ...
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.[
]
History
Clifford Tandy is credited with coining the term "viewshed" in 1967 by analogy to '' watershed''.[ The lexicographer Grant Barrett cites a use of the term from 1970 in the '']Oakland Tribune
The ''Oakland Tribune'' was a daily newspaper published in Oakland, California, and a predecessor of the '' East Bay Times''. It was published by the Bay Area News Group (BANG), a subsidiary of MediaNews Group. Founded in 1874, the ''Tribune'' ...
''.
Related concepts
Viewsheds are a specific type of visibility graph.
Isovists are a closely related concept that is more common in the study of architecture
Architecture is the art and technique of designing and building, as distinguished from the skills associated with construction. It is both the process and the product of sketching, conceiving, planning, designing, and construction, constructi ...
. 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.
Total-viewshed map refers to the map, where each point represents the number of Km² visible at that point in the DEM.][
The 3D-viewshed of a point (X,Y) of the DEM consists of the visible space from that point.][
]
Zone of visual influence
A ''zone of visual influence'' is the area from which a development or other structure is theoretically visible.environmental impact assessment
Environmental impact assessment (EIA) is the assessment of the environmental impact, environmental consequences of a plan, policy, program, or actual projects prior to the decision to move forward with the proposed action. In this context, the te ...
. Zones of visual influence have been used extensively in wind farm
A wind farm, also called a wind park or wind power plant, is a group of wind turbines in the same location used to produce electricity. Wind farms vary in size from a small number of turbines to several hundred wind turbines covering an exten ...
development. A map will be created showing the number of wind turbine
A wind turbine is a device that wind power, converts the kinetic energy of wind into electrical energy. , hundreds of thousands of list of most powerful wind turbines, large turbines, in installations known as wind farms, were generating over ...
s 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
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Topography techniques
Urban studies and planning terminology
Environmental impact assessment