[GRASS-user] re-setting bathymetry data using the raster calculator.
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
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 > ___ grass-user mailing list grass-user@lists.osgeo.org https://lists.osgeo.org/mailman/listinfo/grass-user
[GRASS-user] GRASS annual report 2023
Dear all, We are preparing a report on all GRASS-related achievements in the year 2023 and we would like your input! For example, if you presented at a conference about GRASS, we would like to hear about this and include it. The report will be posted as a news item on grass.osgeo.org. See the current state of the report in this PR: https://github.com/OSGeo/grass-website/pull/405 Feel free to comment on the PR directly and I will incorporate your suggestions there. Thank you for your input! Anna ___ grass-user mailing list grass-user@lists.osgeo.org https://lists.osgeo.org/mailman/listinfo/grass-user
