[GRASS-user] Questions concerning r.mapcalc, r.cost and high resolution DEMs

[Daniel Becker]

Hello,

i just joined this mailing list and i hope i'm doing everything right.

I'm in a project where i try to provide an easy way to do a cost-distance
analysis, based on a given raster DEM, i did this with ArcGIS earlier but i
want more people to be able to use this simple (and faster) approach, but i
ran into some problems.



The workflow looks like this:

DEM -> slope -> r.mapcalc(tobler hiking function) -> cost raster -> r.cost
-> cost distance raster -> contours that represent walking distance from a
given point.

The function i use in r.mapcalc is: h/m=(0.000166666*(exp(3.5*(abs(tan(
"slope_in_degree" )+0.05)))))*cellsize


The problems i ran into:


   1. This works with SRTM-data, but if i use a higher resolution DEM
   (10m), the resulting cost raster contains some infinite values and the
   r.cost tool doesn't really work anymore
   - Is there a way to adress "inf" in r.mapcalc? I can just filter
      everything > 500, but that isn't really satisfying. :)
      - I don't really know what the underlying problem is, exp() leads to
      the infinite values, but maybe there is a way to filter those extreme
      slopes earlier or in the DEM (although the slope raster contains only
      values <90°)?

      2. ERROR: G_malloc: unable to allocate 524288 bytes of memory at
   setup.c:64 - happens at 144.024.001 pixels (merged SRTM-DEM of Spain)
   and above if i don't set "Percent of map to keep in memory" to quite low
   values of below 20-30%, although neither my RAM nor the harddrive with the
   .tmp-folder is fully utilized. The problem is: What happens at even higher
   pixelcounts?

   3. This may be the wrong place for this but: Usability-wise it would be
   even nicer to use QGIS and its modelbuilder for this approach. The problem
   is that the parameters for stop_point or stop_coordinate can't stay empty
   in the QGIS-GUI (which it has to be, since the whole raster should be
   processed). Maybe there is a way to work around this?


So, i'd be happy if someone could help me. Here is also an example (
https://www.dropbox.com/s/i6rsm9v9p7gwfx2/01_isochronen_arroyotobler_ardales.png
) of what i'm trying to do, so that it's less abstract. :-)


Thanks in advance and have a nice week

Daniel
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