[GRASS-user] LS factor for USLE

[Martin Zbinden]

Hi Laura,

I've done some RUSLE3d calculations before, but always based on
projected coordinate systems.

1) With r.watershed you seem to be on the right path, as trying to use
r.flow on a geographic coordinate system gives me this error:
"lat/long projection not supported by r.flow. Please use 'r.watershed'
for calculating flow accumulation."

However, I just tried by reprojecting elevation from north carolina
sampledata (nc_basic_spm_grass7.zip) to geographic and the results are
not the same before and after reprojection. I followed instructions in
[1], page 46f. I don't know anything about reprojections so the error
might lay there. But if your results are also very implausible, then
maybe better try again with projected coordinate system.

2) I never tried slope length based USLE calculations and cannot
answer this question. Page 158 ff in [2] explains many different
approaches to RUSLE in GRASS GIS.

Best regards,
Martin

sources:
[1] http://grass.osgeo.org/sampledata/north_carolina/

[2] Markus Neteler and Helena Mitasova, 2008,
Open Source GIS: A GRASS GIS Approach. Third Edition.

--
Martin Zbinden
Riedacker 523
3154 Rüschegg Heubach
+41 78 628 28 82
--
Martin Zbinden
Riedacker 523
3154 Rüschegg Heubach
+41 78 628 28 82


2014-07-24 9:56 GMT+02:00 Scherer  Laura <scherer at ifu.baug.ethz.ch>:
> Dear all,
>
>
>
> I would like to calculate the slope length and steepness (LS) factor for
> USLE using r.watershed with the parameter length.slope. I have two
> questions:
>
> 1) Is it necessary to reproject my raster into a projected coordinate system
> or can I leave it in latitude-longitude?
>
> 2) It is required to set a value for the threshold. Is it not possible to
> get a value for each cell? If not, do I always have to delineate the
> watersheds as well and the length.slope returned is the value at the outlet?
>
>
>
> Thanks in advance for any help.
>
>
>
> Best regards,
>
> Laura
>
>
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