[GRASS-dev] [GRASS-user] cost surface with negative friction odd behavior

[Anna Petrášová]

Michael, could you please create a PR for the documentation?

On Wed, Nov 29, 2023 at 3:56 PM Michael Barton via grass-user <
grass-user at lists.osgeo.org> wrote:

> 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)
> 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)
> 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-request at lists.osgeo.org wrote:
>
> Date: Wed, 29 Nov 2023 10:17:19 -0500
> From: Anna Petr??ov? <kratochanna at gmail.com>
> To: Michael Barton <Michael.Barton at asu.edu>
> Cc: GRASS developers <grass-dev at lists.osgeo.org>, GRASS user list
> <grass-user at lists.osgeo.org>
> Subject: Re: [GRASS-user] cost surface with negative friction odd
> behavior
> Message-ID:
> <CAE0EDEoFhX75U2r6qC8eK5p8czg2Cd1iWR8J3vX8QiWWE8zNUw at 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-user at lists.osgeo.org> wrote:
>
>
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