Geodesic Viewshed

The second method available in ArcGIS Pro to compute viewsheds uses a geodesic operation. Therefore, the tools discussed here must only be used on raster data assigned to a geographic coordinate system.

The Geodesic Viewshed Tool

The Geodesic Viewshed tool provides a newer implementation of viewshed analysis where data in a geographic coordinate system is modelled in 3D to generate visible areas.

First, after opening the Geodesic Viewshed tool, select your input raster. If you see a warning that the raster should have a vertical coordinate system that has heights referenced to a spheroid, you will have to either define your raster’s vertical coordinate system, or if the raster has a vertical coordinate system referenced to a geoid, you will need to reproject the raster to a spheroidal or ellipsoidal datum. To do the former, use the Define Projection tool. To do the latter, use the Project Raster tool. A good option for a vertical coordinate system is the ellipsoidal-based WGS 1984. To modify the elevation values of the raster using the Project Raster tool, the input raster must have a vertical coordinate system defined, the output coordinate system must be selected, and the box marked Vertical should be checked. A Geographic Transformation should automatically populate. If the geographic transformation does not load automatically, you might need to install supplementary coordinate system files.

The Geodesic Viewshed tool has many of the same parameters as the planar versions of the tools; however, all neatly contained in a single tool. Under Viewshed parameters, the Analysis type can be set to Frequency or Observers. When the option is set to Observers, an observer-region relationship table is also output describing the values assigned to the output viewshed raster in relation to input observers. Additional parameters can be set under Observer parameters. These parameters will look similar to those available in the planar versions of the tools. Where each parameter has a dropdown listing Linear Unit, this setting can be changed to Field, where values can be stored in the input observer points. Note that for the Geodesic Viewshed tool, the fields do not need to have the same specific names as in the planar Viewshed tools; instead, any name can be assigned to the relevant field. Note also that when using a Field as input, all measurements default to meters, regardless of the coordinate system. To specify feet, the Field must be formatted as a text string, with units specified as “6 Feet” for example.

The Observer offset parameter is the same as OFFSETA in the planar Viewshed toolset. The Surface offset parameter is the same as OFFSETB. Observer elevation is equivalent to SPOT. Inner and outer radius are equivalent to RADIUS1 and RADIUS2, respectively, and the horizontal and vertical angles are equivalent to AZIMUTH1, AZIMUTH2, VERT1, and VERT2, respectively.

Another notable difference between the Geodesic Viewshed tool and the planar Viewshed tools is that the output visibility rasters are placed over a NoData background, rather than a 0 value background.

In general, the Geodesic Viewshed tool is far more straightforward than the older planar versions of the tools, and likely more accurate. However, care must be taken in properly preparing the input data to ensure the use of an appropriate coordinate system.