[GRASS-dev] r.walk issues for 6.4.2?

[Benjamin Ducke]

+1 for backporting the r.walk 6.5 patches
to 6.4.2. They make the module a lot more useful.

Ben

On 10/08/2011 05:29 PM, Michael Barton wrote:
> One of my students was having problems with r.walk not properly
> reflecting surface topography in 6.4.2 svn (from a few weeks ago).
>
> So I checked it out today. My comparison is not quite the same as hers
> as she is on Mac OS X 10.6.8 and I've tested this on 10.7.2 (Lion). But
> we're using the same GRASS builds (done with 10.6.8 on my other computer).
>
> I did not have the same problem she did. I tested with the Spearfish
> demo set. She is using a DEM in a Lambert Conformal Conic projection
> (which I suppose might be a problem for some reason).
>
> However, I did turn up something of interest for the upcoming GRASS
> 6.4.2 release.
>
> In 6.4.1, we did not include the new backlink map option, but I thought
> it was going to be in 6.4.2. So far it has not been backported from 6.5.
>
> Also, running r.walk without the backlink in GRASS 6.4.2 and GRASS 7
> produces significantly different results. Here are the links to a couple
> of outputs of a difference map between r.walk on 6.4.2 minus r.walk on
> GRASS 7 using exactly the same command and files.
>
> r.walk elevation="elevation.dem at PERMANENT" friction="flat1m at PERMANENT"
> output="cost1" start_points="Elkhorn_Peak_site at G7_vector" max_cost=0
> percent_memory=100 walk_coeff="0.72,6.0,1.9998,-1.9998" lambda=1.0
> slope_factor=-0.2125
>
> http://dl.dropbox.com/u/7437464/rwalk_diff.jpg
> http://dl.dropbox.com/u/7437464/rwalk_diff_histogram.png
>
> As you can see, there are very patterned differences in the behavior of
> r.walk between the 2 GRASS versions. The differences are not huge (in
> the range of +3 to -13 minutes), but are still notable. Beyond adding
> the backlink option, do the r.walk algorithms differ significantly
> between them?
>
> Also, has anybody had any problems with r.walk in a Lambert projection?
>
> Michael
> ____________________
> C. Michael Barton
> Director, Center for Social Dynamics & Complexity
> Professor of Anthropology, School of Human Evolution & Social Change
> Arizona State University
>
> voice: 480-965-6262 (SHESC), 480-727-9746 (CSDC)
> fax: 480-965-7671 (SHESC), 480-727-0709 (CSDC)
> www: http://www.public.asu.edu/~cmbarton, http://csdc.asu.edu
>
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-- 
Benjamin Ducke
{*} Geospatial Consultant
{*} GIS Developer

   benducke at fastmail.fm