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

[Michael Barton]

I got these differing results with elevation.dem in Spearfish, a UTM projection. The Lambert issue is a different one.

I'm not sure why r.walk should not work in a Lambert projection since the horizontal and vertical units are all in meters.

Michael
______________________________
C. Michael Barton 
Director, Center for Social Dynamics & Complexity
Professor of Anthropology, School of Human Evolution & Social Change
Arizona State University
Tempe, AZ  85287-2402
USA

voice: 	480-965-6262 (SHESC), 480-727-9746 (CSDC)
fax:          480-965-7671(SHESC), 480-727-0709 (CSDC)
www: 	http://csdc.asu.edu, http://shesc.asu.edu
		http://www.public.asu.edu/~cmbarton

On Oct 10, 2011, at 12:33 AM, Markus Metz wrote:

> On Sat, Oct 8, 2011 at 5:29 PM, Michael Barton <Michael.Barton at asu.edu> 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?
>> 
> There should be no differences between GRASS 7 and GRASS 6.4, at least
> in my tests there are no differences.
> 
> The problem you experience has most likely nothing to do with Lambert
> projection, instead I suspect with the resolution of 1km posted in the
> ticket. Even though the results appear to make sense in geographic,
> this is pure accident. In fact, r.walk is not supposed to work with
> geographic because r.walk assumes meters as both horizontal and
> vertical units.
> 
> The formula given in the manual and implemented in (all versions of)
> r.walk is IMHO weird because the c coefficient results in negative
> time required to cross a moderate downhill slope, but walking humans
> can not travel backwards in time, and the Dijkstra search requires for
> good reason positive costs. With large horizontal resolutions, e.g.
> 1km, the time required to cover the distance from one cell to the next
> cell is much larger than most encountered slopes. At least the r.walk
> algorithm behaves likes this, which results in a sort of manhattan
> distance map as obtained in a flat landscape.
> 
> Markus M