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 <[email protected]>: >> 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 >> >> >> _______________________________________________ >> grass-user mailing list >> [email protected] >> http://lists.osgeo.org/mailman/listinfo/grass-user > _______________________________________________ > grass-user mailing list > [email protected] > http://lists.osgeo.org/mailman/listinfo/grass-user > -- Stefan Lüdtke Section 5.4- Hydrology Tel.: +49 331 288 2821 Fax: +49 331 288 1570 Email: [email protected] Helmholtz-Zentrum Potsdam Deutsches GeoForschungsZentrum GFZ (GFZ German Research Centre for Geoscience) Stiftung des öff. Rechts Land Brandenburg Telegrafenberg, 14473 Potsdam ------------------- PGP Public Key: http://bit.ly/13d9Sca _______________________________________________ grass-user mailing list [email protected] http://lists.osgeo.org/mailman/listinfo/grass-user
