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Viewshed identifies the cells in an input raster that can be seen from one or more observation points or lines. Each cell in the output raster receives a value that indicates how many observer points can be seen from each location. If you have only one observer point, each cell that can see that observer point is given a value of one. All cells that cannot see the observer point are given a value of zero. The observer points feature class can contain points or lines. The nodes and vertices of lines will be used as observation points.
Learn how to calculate viewshed using the Spatial Analyst toolbar
Learn how to calculate viewshed using the Viewshed tool
Why calculate viewshed?
Viewshed is useful when you want to know how visible objects might be—for example, From which locations on the landscape will the water towers be visible if they are placed in this location? or What will the view be from this road?
In the example below, the viewshed from an observation tower is identified. The elevation raster displays the height of the land (darker locations represent lower elevations), and the observation tower is marked as a green triangle. The height of the observation tower can be specified in the analysis. Cells in green are visible from the observation tower, while cells in red are not.
Displaying a hillshade underneath your elevation and the output from the Viewshed tool is a useful technique for visualizing the relationship between visibility and terrain.
Not only can you determine which cells can be seen from the observation tower, if you have several observation points, you can also determine which observers can see each observed location. Knowing which observer can see which locations can affect decision making. For example, in a visual quality study for siting a landfill, if it is determined that the proposed landfill can only be seen from dirt roads and not from the primary and secondary roads, it may be deemed a favorable location.
Controlling the viewshed
The image below graphically depicts how a viewshed is performed. The observation point is on the mountain top to the left (at OF1 in the image). The direction of the viewshed is within the cone looking to the right. You can control how much to offset the observation point (for example, the height of the tower), the direction to look, and how high and low to look from the horizon.
There are nine characteristics of the viewshed that you can control:
- The surface elevations for the observation points (Spot)
- The vertical distance in surface units to be added to the z-value of the observation points (OffsetA)
- The vertical distance in surface units to add to the z-value of each cell as it is considered for visibility (OffsetB)
- The start of the horizontal angle to limit the scan (Azimuth1)
- The end of the horizontal angle to limit the scan (Azimuth2)
- The top of the vertical angle to limit the scan (Vert1)
- The bottom of the vertical angle to limit the scan (Vert2)
- The inner radius that limits the search distance when identifying areas visible from each observation point (Radius1)
- The outer radius that limits the search distance when identifying areas visible from each observation point (Radius2)
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