viewshed

Viewshed is a concept used in geography, GIS, and landscape planning to describe the portion of terrain that is visible from one or more observers. It defines the spatial extent from which a point of interest, such as a lookout, a tower, or a building, can be seen given the terrain and any other obstructions. A viewshed is typically computed relative to an observer height above the terrain and may incorporate target heights to determine visibility.

Computationally, viewshed analysis uses a digital elevation model (DEM) and line-of-sight algorithms to test visibility from the observer location to each cell or point in a region. For each target location, the algorithm checks whether the straight line from the observer to that location intersects the terrain at any higher point. Results are often expressed as a raster or map indicating visible and non-visible areas. Common variants include single-viewshed (one observer), multi-viewshed (several observers), and cumulative or overlap viewsheds for assessing shared visibility. Some analyses also consider horizon viewsheds, which limit visibility to the line of sight toward the horizon.

Applications of viewshed analysis are wide-ranging. They include urban planning and landscape design, telecommunications and broadcast network siting, solar and wind resource assessment, military and security planning, cultural heritage management, wildfire risk analysis, and ecological studies of habitat visibility. Outputs are used to inform where to place structures, observe landscapes, or maximize or minimize visibility.

Limitations and considerations include data quality and resolution of the DEM, the accuracy of observer and target heights, and the exclusion of dynamic factors such as vegetation, buildings, atmosphere, and seasonal changes. Real-world visibility may differ from modeled results due to these factors.