[GRASS-user] re-setting bathymetry data using the raster calculator.

[Anna Petrášová]

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 at 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
>
>
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