[GRASS-user] LS factor for USLE

[Stefan Lüdtke]

Hi Laura,

for question 2)

I had the same question some time ago, see here for an answer why a
threshold is required.

http://lists.osgeo.org/pipermail/grass-user/2012-December/066491.html

HTH,

Stefan


On 07/25/2014 10:43 AM, Martin Zbinden wrote:
> 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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-- 
Stefan Lüdtke

Section 5.4-  Hydrology
Tel.: +49 331 288 2821
Fax: +49 331 288 1570
Email: sluedtke at gfz-potsdam.de

Helmholtz-Zentrum Potsdam
Deutsches GeoForschungsZentrum GFZ
(GFZ German Research Centre for Geoscience)
Stiftung des öff. Rechts Land Brandenburg
Telegrafenberg, 14473 Potsdam
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