Thanks Anna and Doug, I did not expect it to work (thought it would be useful if it did). Rather I was surprised by the fact that r.walk DID run and that it gave very odd results.
Adding to the docs is a good idea. Even better would also to have a friction map with a negative value (min<0) raise an error in r.walk, saying that all values in a friction map must be ≥ 0 Michael _____________________________ C. Michael Barton Associate Director, School of Complex Adaptive Systems (https://scas.asu.edu<https://scas.asu.edu/>) Professor, School of Human Evolution & Social Change (https://shesc.asu.edu) Director, Center for Social Dynamics & Complexity (https://complexity.asu.edu) Arizona State University Tempe, AZ 85287-2701 USA Executive Director, Open Modeling Foundation (https://openmodelingfoundation.github.io<https://openmodelingfoundation.github.io/>) Director, Network for Computational Modeling in Social & Ecological Sciences (https://comses.net) personal website: http://www.public.asu.edu/~cmbarton On Nov 29, 2023, at 1:00 PM, grass-user-requ...@lists.osgeo.org wrote: Date: Wed, 29 Nov 2023 10:17:19 -0500 From: Anna Petr??ov? <kratocha...@gmail.com<mailto:kratocha...@gmail.com>> To: Michael Barton <michael.bar...@asu.edu<mailto:michael.bar...@asu.edu>> Cc: GRASS developers <grass-dev@lists.osgeo.org<mailto:grass-dev@lists.osgeo.org>>, GRASS user list <grass-u...@lists.osgeo.org<mailto:grass-u...@lists.osgeo.org>> Subject: Re: [GRASS-user] cost surface with negative friction odd behavior Message-ID: <cae0edeofhx75u2r6qc8ek5p8czg2cd1iwr8j3vx8qiwwe8z...@mail.gmail.com<mailto:cae0edeofhx75u2r6qc8ek5p8czg2cd1iwr8j3vx8qiwwe8z...@mail.gmail.com>> Content-Type: text/plain; charset="utf-8" I think r.walk was not written for negative friction and while I imagine some small (in absolute sense) negative values may work, your negative values are pretty extreme, meaning the resulting travel time through a cell would be negative. That can cause all kinds of issues in the algorithm. So I would say friction should not be negative. I am not sure I would check that in the code, because you would need to check that for each cell and I think it's unnecessary overhead. Maybe just adding a note to documentation may be enough. I haven't looked into the code itself, so this is just my guess. Anna On Tue, Nov 28, 2023 at 5:48?PM Michael Barton via grass-user < grass-u...@lists.osgeo.org<mailto:grass-u...@lists.osgeo.org>> wrote:
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