Hi Victor, I am not sure I understand your concern. Have you tried running r.walk with the original raster with negative values? Theoretically, I don't see why r.walk couldn't work with negative elevation, although I haven't tried it. It should work the same if you add a constant value as you suggest. Perhaps you want to e.g. use an absolute value of the elevation?
Anna On Wed, Dec 20, 2023 at 5:59 PM Victor Lundström via grass-user < [email protected]> wrote: > Hi everyone, > > I've run in to a problem that I hope to get some help with. > I'm preparing a set of rasters to perform species distribution modelling. > For one > of my predictors, I have chosen to use r.walk in order to record how far > away my occurrence > records are to the nearest shoreline. Here's the catch though.. I will be > generating my walk-distance > rasters by using the GEBCO bathymetry data set. Here in lies the problem. > Seeing as it is bathymetry, my raster will have negative values. My initial > thought was simply to run the bathymetry raster through r.mapcalc in this > way: > > r.mapcalc "expression=prehist_dem = bathymetry + 866" > > In this example, "866" references the lowest depth recorded (i.e. -866m). > Using the expression above, I have > now removed any negative value in the raster so that min = 0. However, > while doing this I have now also added > "866" to every other cell in the raster, and not only will this be > incorrect for places inland where a cell that originally > was 5 m.a.s.l. now is 871m, but it will probably affect the cells close to > the sea in the same way (which in most cases should probalby be close to, > or slightly above, 0m). More importantly, if I would just stick to this > approach, I can't help but imagine that it won't produce inaccurate results > for r.walk further down the line as well. > > I can't help but think that there is some clever work-around to this using > the mapcalculator, but I'm simply stuck > and don't know how to proceed. Hope any of you can provide some advice! > > Best, > Victor > > > _______________________________________________ > grass-user mailing list > [email protected] > https://lists.osgeo.org/mailman/listinfo/grass-user >
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