[GRASS-user] LS factor for USLE

[Scherer Laura]

Hi Martin, hi Stefan,

Thanks for your feedback. I already tried with projected coordinate systems and different thresholds before asking here, but always got results that seemed strange to me. I don't have any feeling for typical values, but the r.watershed documentation says it's usually below 1 and that's true for most of the cells, but some are as high as 55 (after division by 100). But these extremes are very much dependent on the threshold. And most values are 0.03 (at a small threshold all were 0.03). That's why I thought that maybe only values at outlets are reliable, but according to the link from Stefan it has nothing to do with outlets and I should get results for all cells (except for streams).
I just tested the approach with r.slope.aspect, r.flow and r.mapcalc as described in the GRASS GIS book, but the output is even more strange. Everything is zero up to the 75% quantile and afterwards much higher (up to 4000).
Maybe interesting to know is that I want to do it for the world and as at 1 km the processing never seemed to end, I lowered the resolution to 10 km.

Best regards,
Laura

-----Original Message-----
From: Stefan Lüdtke [mailto:sluedtke at gfz-potsdam.de] 
Sent: 25 July 2014 11:12
To: Martin Zbinden; Scherer Laura
Cc: grass-user at lists.osgeo.org
Subject: Re: [GRASS-user] LS factor for USLE

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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