Created by jchien on 4/30/14.
This appears to be an implementation of the DEM/Xdraw Viewshed
algorithm from .
1. Franklin, Wm Randolph, and Clark Ray.
"Higher isn’t necessarily better: Visibility algorithms and experiments."
Advances in GIS research: sixth international symposium on spatial data handling. Vol. 2.
Taylor & Francis Edinburgh, 1994.