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

[Anna Petrášová]

On Thu, Dec 21, 2023 at 4:45 AM Victor Lundström <Victor.Lundstrom at 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 <kratochanna at gmail.com>
> *Skickat:* Thursday, December 21, 2023 5:18:03 AM
> *Till:* Victor Lundström <Victor.Lundstrom at uib.no>
> *Kopia:* grass-user at lists.osgeo.org <grass-user at 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 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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