I'm looking for implementations of an algorithm called "Available Sky". ArcGIS (Spatial Analyst or GRID) is preferred, but solutions in GDAL, SAGA GIS or others are perfectly acceptable.
The description I have is "a method to quantify the influence of terrain on GPS radiocollar performance by creating a variable called "Available Sky" (Rodgers et al. 1997). … AS is the proportion of sky available to GPS radiocollar through direct line of site in all directions and at all angles without terrain obstructions (disregarding forest cover). … locations on mountain tops have high AS values … conversly locations in valley bottoms are low due to mountain ridges on either sides [lateral obstructions]" — paraphrased from 'GPS radiotelemetry error and bias in mountainous terrain', Robert G. D'Eon, Robert Serrouya, Graham Smith, Christopher O. Kochanny; Wildlife Society Bulletin 2002.
The paper goes on to describe, in outline only, a process of comparing a base elevation model with a coarser "sky" raster set x100m higher than the highest point of the dem. The process is to calculate direct line of sight for each dem point to each Sky point, arriving at an AS value is the proportion of the total number of sky points visible from that location.
Best Answer
It seems like there should be a way to derive AS from a skyline graph created using the ArcGIS 10.0 3D analyst. If you have a skyline (3D polyline) that surrounds an observation point, it should be able to step through each vertex on the skyline and find some portion of a sphere that is visible.
Or, if you moved each vertex so that it is one unit distance from the observation point, but having same direction from it, it seems the shadow volume would correspond to the AS.