Re: [GRASS-user] Question about r.watershed and flow accumulation grid

[Micha Silver]

Hi Thomas: On 02/13/2015 10:28 PM, Thomas Adams wrote: Micha, No, after looking at what's going-on in more detail, I think the DEM is too coarse (even at 90m), so the flow direction and accumulation is mis-directed in one critical area of the watershed. I tried using r.carve, but it is taking forever — after 15 minutes, no advancement of the progress bar… I don't see why carving into a DEM should cause r.watershed to run more slowly, unless you have carved out only a section of some of the streams. If your carving does not continue right to the outlet of the stream (i.e. to an ocean, or to the edge of the region) then r.watershed would actually have to fill in that carved out stream to find a flow path.  That could cause the performance hit. Additionally, are you using GRASS 7.0? As you probably know some substantial improvements to the algorithms were introduced in 7 for several modules, r.watershed among them. And third, why use the D8 flow direction when MFD is available (again in GRASS 7.0)? That could also be causing what you refer to as breaks in the channels. MFD is especailly good, I believe, in flat areas. In any case, Keep up posted on your progress. Thanks, Micha I did not want to have to do this for my testing, but I'll probably try at 3 or 10 meter — lots of pixels for my basin! The problem, in general, for me is that I want to apply my techniques at international locations where I probably won't have the benefit of higher resolution DEMs, so I need to develop something a bit more robust… Cheers! Tom On Fri, Feb 13, 2015 at 1:15 PM, Micha Silver <mi...@arava.co.il> wrote: On 02/13/2015 08:48 PM, Thomas Adams wrote: Stefan, A fair question; since I know the stream topology from personal experience it is clear that there should be no break in the stream network and the flow accumulation grid should reflect that. I am seeing the flow accumulation values break at a point where they should continue to accumulate downstream. Are these breaks possibly caused by null pixels in the DEM? Tom On Fri, Feb 13, 2015 at 11:43 AM, Stefan Lüdtke <slued...@gfz-potsdam.de> wrote: -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Hi Tom, just out of curiosity, what do you mean by "break in the flow accumulation"? Cheers, Stefan On 02/13/2015 07:12 PM, Thomas Adams wrote: > Hello all! > > I'm making use of the flow accumulation grid in GRASS 6.4.5 > generated from r.watershed using the SFD (D8) flow algorithm. The > DEM has a 250m spatial resolution. What I'm getting is a break in > the flow accumulation in a few locations which is causing me > serious problems with subsequent processing (with help from some > here, I have put together some scripting to generate a pixel > connectivity file for a distributed hydrologic model). > > Besides going to a higher resolution DEM, are there any thoughts as > to how I can eliminate these flow accumulation breaks? > > Thank you, Tom > > -- > > > > _______________________________________________ grass-user mailing > list grass-user@lists.osgeo.org > 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: slued...@gfz-potsdam.de 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 -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.11 (GNU/Linux) iQEcBAEBAgAGBQJU3kXYAAoJEB5GAbKcg+D8YqMH/jgJZ70cz69IJal6ICi66wqW lQYnnko592KAOlFDu1lReZhWWVgeA59rDsYA2Gg/rBzoL9Nst3hzIJDWqnhIWl3W YgF/vOJgTRnrNJtOibGpOc8hxrHwElxU7afYlXU8Zk+4tXdZRK4/vrQ4tKEcbY0z MrhAYjR66RRpoNf9/3WJ3s14CpPA+KEkOaysOOoTV6Ni7qZTK8rVxt+svQQoPtW+ 5QwRJLkLeM6bUrqfQifHis91j4k3JSoSp7ZjIInKwi1tvCSfcFQhzvANs4x8e1Lo Q5wDbzHshhJieGKyraUZyT8cn9vszv9cm2Sf49O/0FWVz3Eyc9vsKFxVcvfEb1E= =Sy/3 -----END PGP SIGNATURE----- -- This mail was received via Mail-SeCure System. _______________________________________________ grass-user mailing list grass-user@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/grass-user This mail was received via Mail-SeCure System. This mail was received via Mail-SeCure System.

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