Hi Dylan,<br>interesting page. I have been playing with r.cost, r.drain and r.walk too lately and wanted to write something about this topic to the list, so this comes at the right moment :-)<br>I think it is interesting to analyze the effects of the model underlying r.walk. I mean, in r.walk the cost is computed as:<br>
T = a*Dlength + b*Duphill + c*Dmoderatedownhill + d*Dsteepdownhill<br>so this means that there is no distinction between moderate and steep UPhill. The effect is evident if you work on a map that has a "vertical wall". I have tried it on a "toy" map (on a 100x100 region with coordinates from 0 to 100) that is more or less like this (sorry I don't have the r.mapcalc expressions at hand, because I am not at my computer):<br>
100-x where 0<y<30 or 70<y<100 or x<50<br>100-2x where x>=50 and 30<=y<=70<br>(so the "vertical wall" would be on x=50, 30<=y<=70).<br>If you start with this elevation map and use r.walk + r.drain with start and end point respectively (10,50) and (90,50) the path you end up with is just a straight line, right across the "wall" (quite counterintuitive)<br>
(I have tried this with lambda=1 and a friction map made up of ones everywhere)<br>Using r.cost on the slope map, on the other hand, will give you a path that avoids the walls and walks on one of the two slopes on the sides of the map.<br>
This is a consequence of the fact that there is no separate term for steep uphill in r.walk's model.<br> T = a*Dlength + b*Dmoderateuphill + c*Dsteepuphill + d*Dmoderatedownhill + e*Dsteepdownhill<br>This makes r.walk a bit tricky to use and rely on in some cases, I think.<br>
<br>I was investigating how to use this modules to find the easiest/safest path to go from A to B in wilderness areas (something like what you describe in this interesting page: <a href="http://casoilresource.lawr.ucdavis.edu/drupal/node/244">http://casoilresource.lawr.ucdavis.edu/drupal/node/244</a>).<br>
In the end it seems like using r.cost guarantees something more intuitive, even though in most cases you don't have walls in a DEM and you could therefore use r.walk.<br>I wonder what happens if you use the output of r.slope.aspect as a friction map for r.walk... (it feels like giving the same input twice, though).<br>
<br>Regards,<br>Stefano.<br><br><br>2008/2/19, Dylan Beaudette <<a href="mailto:dylan.beaudette@gmail.com">dylan.beaudette@gmail.com</a>>:<br><br> Hi,<br><br> posted a quick comparison between r.walk / r.cost. Page is still sparse on<br>
descriptions / interpretations, but its late!<br><br><br> <a href="http://casoilresource.lawr.ucdavis.edu/drupal/node/544">http://casoilresource.lawr.ucdavis.edu/drupal/node/544</a><br><br> Cheers,<br><br> --<br>
Dylan Beaudette<br> Soil Resource Laboratory<br> <a href="http://casoilresource.lawr.ucdavis.edu/">http://casoilresource.lawr.ucdavis.edu/</a><br> University of California at Davis<br> 530.754.7341<br> _______________________________________________<br>
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