[GRASS-user] cva run time

[gary]

Per Hamish's suggestion, I have resampled the raster map until I could 
get an output from r.cva. I had to reduce the resolution by a factor of 
8. I don't want to interrupt the run, but that should be around 800x300 
or so cells.

The output of the "sites" CVA was a solid yellow image, which I assume 
means everything was seen. I replaced "type=sites" with "type=all" and 
am rerunning at the moment.



Paul Kelly wrote:
> 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
> 
>