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> wrote: > In teaching a class about modeling movement, we talked about how to > represent a cost surface when some areas of terrain (e.g., water or paved > roads), could be crossed more rapidly than walking across unmodified > topography. Since a friction map for r.walk adds cost in seconds/meter to > the cost of walking across unmodified terrain, I wondered if a friction map > with **negative costs** would decrease the movement costs across a > landscape. > > Using the SC demo data set, I tried this with a set of maps I prepared for > class. Previously, I had created a friction map from the landclass96 map > such that cells with water (class 20) have a value of 90 sec/m, dense > vegetation (classes 7-19) have a value of 20 sec/m, and the rest of the > cells have a value of 0 sec/m. I generated a cost surface with r.walk using > elevation and this friction map with a starting point from the Dorothea Dix > Hospital. > > r.walk --overwrite -k elevation=elevation@PERMANENT > friction=landclass96_friction output=dd_hospital_seconds_friction > outdir=dd_hospital_directions_friction start_points=DD_hospital memory=1000 > > This behaved as expected and created a cost surface that represented > different degrees of difficulty in crossing vegetated and inundated > terrain. > > To explore what would happen with a friction surface with negative > numbers, I simply subtracted the previous friction map from 0 so that dense > vegetation = -20, water = -90, and the rest of the cells = 0. Then I used > it with r.walk and the elevation DEM. > > r.walk -k elevation=elevation@PERMANENT > friction=landclass96_friction_negative > output=dd_hospital_seconds_negativefriction > outdir=dd_hospital_directions_negativefriction start_points=DD_hospital > memory=2000 > > There was no error, but the resulting cost surface is very strange. > > The data range of the original cost surface with the positive values > friction map is: > > Range of data: min = 0 max = 103883.003593944 (max of 28.8 hours to > reach the hospital given added costs of walking through dense vegetation or > avoiding water). > > However, the data range of the cost surface made with the friction map > with negative values is completely weird. > > Range of data: min = -166202271.811971 max = -154320.938078961 (-46167 > to -43 hours) > > Since a friction map simply adds cost to the cost surface, if a negative > friction map did not work, I might expect it to raise an error or have no > impact (values < 0 become 0). But the values in the resulting cost surface > don't make any sense at all. > > Does anyone have any thoughts on this? > > I will try to attach attach the original friction map made from the SC > landclass96 map, the cost surface made with elevation and friction map, and > cost surface made with elevation and negative friction map. > > 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 > > > _______________________________________________ > grass-user mailing list > grass-u...@lists.osgeo.org > https://lists.osgeo.org/mailman/listinfo/grass-user >
_______________________________________________ grass-dev mailing list grass-dev@lists.osgeo.org https://lists.osgeo.org/mailman/listinfo/grass-dev
