[GRASS-user] cva run time

[Paul Kelly]

On Sat, 22 Jul 2006, gary wrote:

> I was hoping that setting max_dist to be small would reduce the data. 
> Otherwise, I need to figure out how to truncate my DEM.

Ha ha no I remember from my experiences with r.los several years ago that 
the code performed the calculation anyway, then at the last minute checked 
if the point was within the max distance and if not discarded the result!! 
So max_dist doesn't affect the processing time at all and really just 
operates as a mask that you could create anyway afterwards with r.mapcalc 
if you needed it.

There is a fairly promising fast algorithm available that someone with an 
interest in the subject and good C programming skills could check out:
A fast algorithm for approximate viewshed computation
David Izraelevitz (TEC/Eng. Res. and Development Center, U.S. Army Corps 
of Engineers, SRI International)
Photogrammetric Engineering and Remote Sensing, v 69, n 7, Jul 1, 2003, pp 
767-774
Would be a lot of work though to do it well.

Paul