Create Viewshed -- Exceeds Physical Viewing Distance

jpgis

Issue: the Create Viewshed tool has an option for the maximum viewing distance that exceeds the physical limit of viewing distance on the horizon.

For example, using an observer height of 6ft, and a target height is 20ft, the distance at which a 6ft person can view an object 20ft high is approx 3.4 miles. (Use this calculator if you want: https://www.cactus2000.de/uk/unit/masshor.shtml) However, even if using a max viewing distance of 9 miles, the tool will calculate out to 9 miles, which is physically impossible.

Is the tool not geodetic? 

XuguangWang

Thank you for the question. Currently the Create Viewshed tool is based on a planar algorithm. However, it does use a curved Earth approach, which means that the maximum viewing distance will be limited by the Earth's curvature.

1. The best way to verify the maximum distance, is run the tool in the sea (with an altitude of 0 everywhere). On the land, there will be terrain with altitudes > 0, potentially causing visibility in distant locations more than what is predicted by the Earth's curvature.

2. Please double check if 3.4 miles is a correct distance for the example (an observer height of 6ft, and a target height is 20ft). The calculator from the link gives me around 8 miles.

Thanks.