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Viewshed
Viewshed
Viewshed
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Viewshed of the 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.[1] 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 — 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 belts, and community separators.

Representation

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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

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Main article: Viewshed analysis

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.[2]

History

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Clifford Tandy is credited with coining the term "viewshed" in 1967 by analogy to watershed.[3] The lexicographer Grant Barrett cites a use of the term from 1970 in the Oakland Tribune.[4]

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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,[5] 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.[6] 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.[6]

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.[2]

The 3D-viewshed of a point (X,Y) of the DEM consists of the visible space from that point.[7]

Zone of visual influence

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A zone of visual influence is the area from which a development or other structure is theoretically visible.[8] 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.[9]

See also

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References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Viewshed

View on Grokipedia
from Grokipedia
A viewshed is the geographical area visible from a specific point on a , encompassing all surrounding locations in unobstructed line-of-sight, typically computed using digital elevation models to account for elevation, observer height, and potential barriers such as or structures.
, a core technique in geographic information systems (GIS), enables the mapping and quantification of for applications including for communication towers, wind turbines, and transmission infrastructure, where it identifies optimal locations minimizing visual intrusion or maximizing coverage.
In and , it supports scenic resource assessments by delineating protected visual corridors and evaluating impacts from development, such as in highway alignments or preservation.
Computationally, viewsheds are derived through algorithms that propagate radial sightlines across raster grids, often incorporating curvature and for accuracy over large distances, though challenges persist in integrating dynamic elements like seasonal foliage.

Fundamentals

Definition and Scope

A viewshed is the portion of or visible from a specific observer , encompassing all points in direct line-of-sight without obstruction by elevated features such as hills or structures. This visibility is calculated relative to the observer's height above the surface, typically using digital elevation models (DEMs) to account for topographic variations. In practice, viewsheds are represented as raster maps where cells are classified as visible or non-visible based on whether an unobstructed ray can connect the observer to the target point. The scope of a viewshed is inherently limited by physical constraints, including the observer's (often 360 degrees unless specified otherwise), maximum analysis distance (commonly 5–20 kilometers depending on ruggedness and computational limits), and environmental factors like height or atmospheric conditions that may attenuate . Unlike broader visual assessments, a standard viewshed focuses solely on geometric line-of-sight, excluding subjective elements such as color, texture, or aesthetic quality, though extensions like cumulative viewsheds aggregate from multiple points to quantify exposure frequency. curvature and are incorporated in precise models to extend realistic scope beyond flat- assumptions, ensuring accuracy over large areas. Viewshed delineation excludes areas shadowed by local maxima or minima in profiles along sightlines, with scope varying inversely with —flatter landscapes yield larger viewsheds, while rugged areas restrict them to proximal zones. This binary or probabilistic output forms the basis for advanced analyses but remains grounded in deterministic principles derived from ray-tracing algorithms.

Visibility Principles and Factors

The visibility of a point within a viewshed is determined by line-of-sight (LOS) analysis, which assesses whether an unobstructed straight line connects the observer to the target point without intersecting higher intervening . This principle extends to compute the entire viewshed by evaluating LOS for all raster cells or points within a defined extent, typically using a (DEM) to model heights and radial propagation from the observer. Observer offset, representing the height of the viewpoint above the surface (e.g., level at approximately 1.7 meters or the top of a ), expands the visible area by elevating the starting point of LOS rays. Target offsets account for the height of potential visible features, such as antennas or treetops, allowing over bare-earth if the feature protrudes above obstructing slopes. Terrain profile characteristics, including steepness and variance along LOS paths, critically govern obstructions; rising terrain between observer and target blocks , while local minima or flat areas permit broader sightlines. Digital surface models (DSMs), which incorporate surface features like and buildings, yield more realistic viewsheds than bare-earth DEMs by simulating additional occlusions from non-terrain elements. Distance from the observer attenuates visibility due to Earth's curvature, which algorithms address by adjusting target elevations downward (e.g., via a quadratic approximation of approximately 0.078 meters per kilometer squared) or incorporating atmospheric refraction coefficients (typically 0.13–0.14 for standard conditions). Data resolution influences precision, as coarser grids (e.g., >10-meter cells) overestimate visibility by smoothing micro-relief obstructions, while finer lidar-derived data (e.g., 1-meter) capture subtle barriers like ridges or foliage.

Computation Methods

Core Algorithms

The core algorithm for viewshed computation relies on line-of-sight (LOS) analysis within a (DEM), where visibility from a viewpoint v=(x0,y0,z0)v = (x_0, y_0, z_0) to each target cell p=(xi,yi,zi)p = (x_i, y_i, z_i) is determined by checking whether intervening obstructs the straight-line path. For each target, the algorithm traces the LOS by computing intersections with grid cells along the radial path, interpolating elevations at those points (typically linearly between cell centers), and verifying if the maximum interpolated height exceeds the LOS plane's height at any intersection; if not, the target is visible. This brute-force approach scales as O(n3/2)O(n^{3/2}) for a grid of nn cells, due to the average n\sqrt{n}
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