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

[Michael Barton]

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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