Thayer, Great suggestions! I'll give it a try...
Thank you, Tom On Fri, Feb 13, 2015 at 4:56 PM, Thayer Young <[email protected]> wrote: > Tom, > > Hopefully the higher spatial resolution data will also have higher > vertical resolution. > > That said, if you have any idea where the stream channels are, for example > a vector file, in the flat areas you can "burn" them into the raster using > r.mapcalc. So you can rasterize vector streams (v.to.rast) and then thin > the raster streams (r.thin) and use r.mapcalc to subtract a constant value > where the raster stream map is. If you do not thin the raster you will get > an error, as there will be ambiguous flow paths (endless loops). > > QGIS has a nice feature in its vector to raster tool. It will do the burn > for you directly from the vector file into your DEM. You should make a > backup of your DEM though first, since it edits the original file. You > just add a field to the vector streams for a desired elevation. Then you > select that elevation field in the vector to raster dialog and select your > DEM as the output raster. After saying that yes it is OK to overwrite your > DEM, it will burn the vector elevation. I have only used this to edit small > areas on a raster, so I can not tell you if the burnt channel is > sufficiently thinned or not. One of the big problems with using flow > algorithms is that bridges and roads will frequently act like dams, causing > the flow to be rerouted outside of the actual channel. To prevent this you > can digitize a vector map of places where you want to cut through these > "dams" and set a field with desired elevations as described above. > > -Thayer > > > ------------------------------ > *From:* Thomas Adams <[email protected]> > *To:* Thayer Young <[email protected]> > *Cc:* "[email protected]" <[email protected]> > *Sent:* Friday, February 13, 2015 5:48 PM > *Subject:* Re: grass-user Digest, Vol 106, Issue 34 > > Thayer, > > Thank you for the suggestion; I just tried it with the same result. I > think the area just happens to be a difficult area, that is pretty flat… > > Cheers! > Tom > > > > On Fri, Feb 13, 2015 at 1:54 PM, Thayer Young <[email protected]> wrote: > > I don't know if you have tried this yet, but you may also want to look at > r.terraflow. It does both D8 (single flow direction) and multiple flow > direction (flow is partitioned, according to the steepness of slope, to all > directions that are lower than the central cell). Supposedly you can also > switch from MFD to SFD once flow exceeds a threshold, but I have not been > successful at doing this. It would be a simple simulation of > channelization though. > > Just set the D8 flag in the options tab, otherwise it will give you MFD by > default. > > -Thayer > > > ------------------------------ > Date: Fri, 13 Feb 2015 11:12:30 -0700 > From: Thomas Adams <[email protected]> > To: "[email protected]" <[email protected]> > Subject: [GRASS-user] Question about r.watershed and flow accumulation > grid > Message-ID: > <cagxgkwgefettcjzwki3jp8wj79eb0j3fzsf6hgzt8m9-dgc...@mail.gmail.com> > Content-Type: text/plain; charset="utf-8" > > 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 > > -- > -------------- next part -------------- > An HTML attachment was scrubbed... > URL: < > http://lists.osgeo.org/pipermail/grass-user/attachments/20150213/9306343f/attachment-0001.html > > > > > > > -- > Thomas E Adams, III > 718 McBurney Drive > Lebanon, OH 45036 > > 1 (513) 739-9512 (cell) > > > >
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