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A viewshed is an area that is visible from a specific location. Viewshed analyses are a common function of most GIS software. The analysis uses the elevation value of each cell of the DEM to determine visibility to or from a particular cell. The location of this particular cell varies depending on the needs of the analysis. For example, a Viewshed analysis is commonly used to locate communication towers or determining the view from a road. Viewsheds can be calculated using an individual point such as a tower or multiple points such as a line representing a road. When analyzing a line segment, each of the vertices along the line is calculated to determine its visible area. The process can also be reversed. For example, when locating a landfill, the analysis can determine from where the landfill is visible to keep it hidden from view. A viewshed analysis can be performed using one of many GIS programs, such as GRASS GIS (r.los, r.viewshed), SAGA GIS (Visibility), TNT Mips, ArcMap, Maptitude, ERDAS IMAGINE. A viewshed is created from a DEM by using an algorithm that estimates the difference of elevation from one cell (the viewpoint cell) to the next (the target cell). To determine the visibility of a target cell, each cell between the viewpoint cell and target cell is examined for line of sight. Where cells of higher value are between the viewpoint and target cells the line of sight is blocked. If the line of sight is blocked then the target cell is determined to not be part of the viewshed. If it is not blocked than it is included in the viewshed (Kim et al., 2004). Wikipedia

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