On Thu, Dec 21, 2023 at 4:45 AM Victor Lundström <victor.lundst...@uib.no> wrote:
> Hi Anna, thank you for responding! > > No I haven't tried running r.walk using negative values, I guess I could > give it a try. But, if I remember correctly, didn't Michael Barton inquired > about the use of negative values and r.walk previously. I can't remember > the outcome of that discussion, and I don't seem to have kept the emails > unfortunately. I'll give the negative values a try, if it works, then that > will at least be less distorted as opposed to removing negative values by > addition. > He was testing negative values in a friction map, but that's a different case. Negative values in elevation (that's your case, right?) should not matter I would hope. > > /Victor > > Skickat från Outlook för iOS <https://aka.ms/o0ukef> > ------------------------------ > *Från:* Anna Petrášová <kratocha...@gmail.com> > *Skickat:* Thursday, December 21, 2023 5:18:03 AM > *Till:* Victor Lundström <victor.lundst...@uib.no> > *Kopia:* grass-user@lists.osgeo.org <grass-user@lists.osgeo.org> > *Ämne:* Re: [GRASS-user] re-setting bathymetry data using the raster > calculator. > > 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 < > grass-user@lists.osgeo.org> 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 > grass-user@lists.osgeo.org > https://lists.osgeo.org/mailman/listinfo/grass-user > > > [External email] Make sure you recognize the sender's email address > before you click links, open attachments, or get involved in financial > transactions. Contact IT-support BRITA if you have any questions. >
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