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<font color="#929292" face="Calibri" style="font-style: normal; font-size: 11px;">_____________________________</font></div>
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<p style="margin-bottom: 0in;"><font color="#929292" face="Calibri" style="font-style: normal; font-size: 11px;"><font><span style="caret-color: rgb(94, 94, 94);">C. Michael Barton<br>
Associate Director, School of Complex Adaptive Systems (</span></font><a href="https://scas.asu.edu/" style="caret-color: rgb(94, 94, 94);">https://scas.asu.edu</a><font><span style="caret-color: rgb(94, 94, 94);">)<br>
Professor, School of Human Evolution & Social Change (<a href="https://shesc.asu.edu">https://shesc.asu.edu</a>)<br>
Director, Center for Social Dynamics & Complexity (<a href="https://complexity.asu.edu">https://complexity.asu.edu</a>)<br>
Arizona State University<br>
Tempe, AZ 85287-2701<br>
USA<br>
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Executive Director, Open Modeling Foundation (</span></font><a href="https://openmodelingfoundation.github.io/" style="caret-color: rgb(94, 94, 94);">https://openmodelingfoundation.github.io</a><font><span style="caret-color: rgb(94, 94, 94);">)<br>
Director, Network for Computational Modeling in Social & Ecological Sciences (<a href="https://comses.net">https://comses.net</a>)<br>
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personal website: <a href="http://www.public.asu.edu/~cmbarton">http://www.public.asu.edu/~cmbarton</a> <br>
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<div>On Nov 30, 2023, at 11:29 AM, Anna Petrášová <kratochanna@gmail.com> wrote:</div>
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<div>Michael, could you please create a PR for the documentation?</div>
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<div dir="ltr" class="gmail_attr">On Wed, Nov 29, 2023 at 3:56 PM Michael Barton via grass-user <<a href="mailto:grass-user@lists.osgeo.org">grass-user@lists.osgeo.org</a>> wrote:<br>
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<div>Thanks Anna and Doug,
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<div>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. </div>
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<div>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</div>
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<div>Michael</div>
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<font color="#929292" face="Calibri" style="font-style:normal;font-size:11px">_____________________________</font></div>
<div style="font-variant-ligatures:normal;font-variant-east-asian:normal;line-height:normal">
<p style="margin-bottom:0in"><font color="#929292" face="Calibri" style="font-style:normal;font-size:11px"><font><span>C. Michael Barton<br>
Associate Director, School of Complex Adaptive Systems (</span></font><a href="https://scas.asu.edu/" target="_blank">https://scas.asu.edu</a><font><span>)<br>
Professor, School of Human Evolution & Social Change (<a href="https://shesc.asu.edu/" target="_blank">https://shesc.asu.edu</a>)<br>
Director, Center for Social Dynamics & Complexity (<a href="https://complexity.asu.edu/" target="_blank">https://complexity.asu.edu</a>)<br>
Arizona State University<br>
Tempe, AZ 85287-2701<br>
USA<br>
<br>
Executive Director, Open Modeling Foundation (</span></font><a href="https://urldefense.com/v3/__https://openmodelingfoundation.github.io/__;!!IKRxdwAv5BmarQ!cD06dCgbgbISy5GfjOmmNAaDn0di5MhfP9j0LhdTycnfvlp8eq4SrZNHlkc4Ekh_TesD_TyM5clf17XA3uJdoE_v$" target="_blank">https://openmodelingfoundation.github.io</a><font><span>)<br>
Director, Network for Computational Modeling in Social & Ecological Sciences (<a href="https://urldefense.com/v3/__https://comses.net__;!!IKRxdwAv5BmarQ!cD06dCgbgbISy5GfjOmmNAaDn0di5MhfP9j0LhdTycnfvlp8eq4SrZNHlkc4Ekh_TesD_TyM5clf17XA3rtQ9dtL$" target="_blank">https://comses.net</a>)<br>
<br>
personal website: <a href="http://www.public.asu.edu/~cmbarton" target="_blank">http://www.public.asu.edu/~cmbarton</a> <br>
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<div>On Nov 29, 2023, at 1:00 PM, <a href="mailto:grass-user-request@lists.osgeo.org" target="_blank">
grass-user-request@lists.osgeo.org</a> wrote:</div>
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<div><span style="font-family:Monaco;font-size:18px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none;float:none;display:inline">Date:
Wed, 29 Nov 2023 10:17:19 -0500</span><br style="font-family:Monaco;font-size:18px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none">
<span style="font-family:Monaco;font-size:18px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none;float:none;display:inline">From:
Anna Petr??ov? <</span><a href="mailto:kratochanna@gmail.com" style="font-family:Monaco;font-size:18px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" target="_blank">kratochanna@gmail.com</a><span style="font-family:Monaco;font-size:18px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none;float:none;display:inline">></span><br style="font-family:Monaco;font-size:18px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none">
<span style="font-family:Monaco;font-size:18px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none;float:none;display:inline">To:
Michael Barton <</span><a href="mailto:Michael.Barton@asu.edu" style="font-family:Monaco;font-size:18px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" target="_blank">Michael.Barton@asu.edu</a><span style="font-family:Monaco;font-size:18px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none;float:none;display:inline">></span><br style="font-family:Monaco;font-size:18px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none">
<span style="font-family:Monaco;font-size:18px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none;float:none;display:inline">Cc:
GRASS developers <</span><a href="mailto:grass-dev@lists.osgeo.org" style="font-family:Monaco;font-size:18px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" target="_blank">grass-dev@lists.osgeo.org</a><span style="font-family:Monaco;font-size:18px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none;float:none;display:inline">>,
GRASS user list</span><br style="font-family:Monaco;font-size:18px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none">
<span style="font-family:Monaco;font-size:18px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:pre-wrap;word-spacing:0px;text-decoration:none"></span><span style="font-family:Monaco;font-size:18px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none;float:none;display:inline"><</span><a href="mailto:grass-user@lists.osgeo.org" style="font-family:Monaco;font-size:18px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" target="_blank">grass-user@lists.osgeo.org</a><span style="font-family:Monaco;font-size:18px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none;float:none;display:inline">></span><br style="font-family:Monaco;font-size:18px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none">
<span style="font-family:Monaco;font-size:18px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none;float:none;display:inline">Subject:
Re: [GRASS-user] cost surface with negative friction odd</span><br style="font-family:Monaco;font-size:18px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none">
<span style="font-family:Monaco;font-size:18px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:pre-wrap;word-spacing:0px;text-decoration:none"></span><span style="font-family:Monaco;font-size:18px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none;float:none;display:inline">behavior</span><br style="font-family:Monaco;font-size:18px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none">
<span style="font-family:Monaco;font-size:18px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none;float:none;display:inline">Message-ID:</span><br style="font-family:Monaco;font-size:18px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none">
<span style="font-family:Monaco;font-size:18px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:pre-wrap;word-spacing:0px;text-decoration:none"></span><span style="font-family:Monaco;font-size:18px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none;float:none;display:inline"><</span><a href="mailto:CAE0EDEoFhX75U2r6qC8eK5p8czg2Cd1iWR8J3vX8QiWWE8zNUw@mail.gmail.com" style="font-family:Monaco;font-size:18px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" target="_blank">CAE0EDEoFhX75U2r6qC8eK5p8czg2Cd1iWR8J3vX8QiWWE8zNUw@mail.gmail.com</a><span style="font-family:Monaco;font-size:18px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none;float:none;display:inline">></span><br style="font-family:Monaco;font-size:18px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none">
<span style="font-family:Monaco;font-size:18px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none;float:none;display:inline">Content-Type:
text/plain; charset="utf-8"</span><br style="font-family:Monaco;font-size:18px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none">
<br style="font-family:Monaco;font-size:18px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none">
<span style="font-family:Monaco;font-size:18px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none;float:none;display:inline">I
think r.walk was not written for negative friction and while I imagine</span><br style="font-family:Monaco;font-size:18px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none">
<span style="font-family:Monaco;font-size:18px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none;float:none;display:inline">some
small (in absolute sense) negative values may work, your negative</span><br style="font-family:Monaco;font-size:18px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none">
<span style="font-family:Monaco;font-size:18px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none;float:none;display:inline">values
are pretty extreme, meaning the resulting travel time through a cell</span><br style="font-family:Monaco;font-size:18px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none">
<span style="font-family:Monaco;font-size:18px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none;float:none;display:inline">would
be negative. That can cause all kinds of issues in the algorithm. So</span><br style="font-family:Monaco;font-size:18px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none">
<span style="font-family:Monaco;font-size:18px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none;float:none;display:inline">I
would say friction should not be negative. I am not sure I would check</span><br style="font-family:Monaco;font-size:18px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none">
<span style="font-family:Monaco;font-size:18px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none;float:none;display:inline">that
in the code, because you would need to check that for each cell and I</span><br style="font-family:Monaco;font-size:18px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none">
<span style="font-family:Monaco;font-size:18px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none;float:none;display:inline">think
it's unnecessary overhead. Maybe just adding a note to documentation</span><br style="font-family:Monaco;font-size:18px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none">
<span style="font-family:Monaco;font-size:18px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none;float:none;display:inline">may
be enough. I haven't looked into the code itself, so this is just my</span><br style="font-family:Monaco;font-size:18px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none">
<span style="font-family:Monaco;font-size:18px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none;float:none;display:inline">guess.</span><br style="font-family:Monaco;font-size:18px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none">
<br style="font-family:Monaco;font-size:18px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none">
<span style="font-family:Monaco;font-size:18px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none;float:none;display:inline">Anna</span><br style="font-family:Monaco;font-size:18px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none">
<br style="font-family:Monaco;font-size:18px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none">
<span style="font-family:Monaco;font-size:18px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none;float:none;display:inline">On
Tue, Nov 28, 2023 at 5:48?PM Michael Barton via grass-user <</span><br style="font-family:Monaco;font-size:18px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none">
<a href="mailto:grass-user@lists.osgeo.org" style="font-family:Monaco;font-size:18px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" target="_blank">grass-user@lists.osgeo.org</a><span style="font-family:Monaco;font-size:18px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none;float:none;display:inline">>
wrote:</span></div>
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