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

[Michael Barton]

Helena,

Have you tried it 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











On Oct 9, 2011, at 3:36 PM, Helena Mitasova wrote:

> I am not sure whether it is relevant, but for our r.walk example here (nc_spm_08 data set)
> http://courses.ncsu.edu/mea582/common/GIS_anal_grass/GIS_Anal_granal2.html
> 
> the results from GRASS6.4.1 and GRASS7 are identical (I haven't tried it for 6.4.2, I assume it is same as 6.4.1?)
> 
> Helena 
> 
> On Oct 8, 2011, at 11:29 AM, 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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>