Daniel
For the first problem I would sugest you to filter high slope values
r.mapcalc h_m=if(slope<70, (0.000166666*(exp(3.5*(abs(tan(
"slope_in_degree" )+0.05)))))*cellsize, null())
For the second problem, did you try to use a smaller percentage (say 10%)?
best regards
El 24/03/14 10:15, Daniel Becker escribió:
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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