Nikos: > >>>> While there is a proper (vector) contour map and a very clean stream > >>>> network map (digitised from scratch upon topographic maps of 1:50.000) > >>>> available, the flow direction map is missing. ... > >>> is the direction map bound to a specific stream network, i.e. the one > >>> that r.watershed or r.stream.extract produces or is it an independent > >>> one, based solely on the input elevation model and its (x-y-z?) > >>> resolution?
Micha: > >> It's not clear if your friend needs a flow direction raster (normally, > >> AFAIK, flow direction refers to a raster, where each cell has a value > >> indicating which is the next downstream cell) or if he want to see the > >> flow direction of streams in his stream network. Nikos: > > A flow direction raster map is required. And my silly question (because I > > am not a hydro-specialist) is whether the flow direction production is > > an independent process from the algorithm that produces the streams. Micha: > The r.watershed process makes several rasters: catchments, streams, flow > direction, flow accumulation, and others if you want. > > Why? Because the self-provided streams are of very high resolution and > > accuracy (so why bother to extract them with potential errors like > > circles in the connection of two segments of a stream, of ladder > > effects). > I've found that Markus Metz's newer r.watershed (part of GRASS 6.5, not > 6.4) does a good job of eliminating these problems when you choose the > MFD (Multi flow direction) algorithm. Good to know! I haven't been following all this hydro-related development but I am impressed (again) to see how powerfull grass-gis is. > In any case, probably the best approach is to use the already digitized > streams as input to the r.carve module (or r.stream.carve , Jarek's new > hydrology extension) to produce an "improved" DEM, then use that > resulting DEM to rerun r.watershed and get everything out as a > consistent set of data layers. OK. I think Jarek and Achim understood the problem quickly and suggested this as the best method. Myself have a clear picture now of why r.carve is really useful here. Thank you Micha, Nikos [...] _______________________________________________ grass-user mailing list grass-user@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/grass-user
