Dear Moritz, Thank you for testing and reproducing the strange behavior or r.watershed and D8. At least it is not a problem of a library on my computer!
Any help is highly appreciated! Cheers Jörg -----Ursprüngliche Nachricht----- Von: Moritz Lennert [mailto:[email protected]] Gesendet: Mittwoch, 24. Juni 2015 11:30 An: Robl Jörg Christian Cc: [email protected]; Markus Metz Betreff: Re: [GRASS-user] r.watershed On 23/06/15 10:32, Robl Jörg Christian wrote: > Dear GRASS Gurus, > > During the last days I've discovered a strange behaviour of > r.watershed and D8. The used DEM is the result from a landscape > evolution model and there are no depressions. I cannot use MFD as the > D8-FlowDirections input data of a variety of tools I've written during the > last years. > > I got obviously wrong values for the flow accumulation of r.watershed > and D8. The number of cells in the Flow Accumulation grid at the > outlet point was significantly smaller than the area of catchment > analysed catchment (spatial resolution in north south and east west > direction is > 1) also computed with GRASS using r.water.outlet and a conversion to > an Area. I checked Flow Accumulation at the the outlet with an > alternative code of a colleague and the contributing drainage area was > exactly the value of the catchment size computed with r.water.outlet. > > I further found some deviations in the flow direction grid. In some > cases the flow routing leads to the lowest neighbouring cell which > however might not be the steepest decent. r.watershed chooses > sometimes a diagonal flow direction while the the local channel > gradient would be greater in horizontal direction. > > I started some test runs with very simple geometries. I've defined a > ramp with r.mapcalc that dips in north direction and I expected that > the this will also be shown by the flow direction of r.watershed. > However, this is not the case. The synthetic DEM features a > watershed!!! See attachment. It seems that r.watershed fills the region > (without need). > This strange behavior disappears in by applying the multiple flow > direction algorithm. > > > > I have tested with the latest versions grass70 , grass71 and grass645. > > Here is a test case (grass71). I would be great if somebody could > reproduce the problem and show me a workaround. > > --------------------------------------------------------- > > g.region w=0 e=1000 s=0 n=1000 res=1 -p r.mapcalc "DEM = 1000 - y()" > r.watershed -s elevation=DEM@PERMANENT accumulation=ACC drainage=DIR > After a first rapid look I can confirm the issue, but don't have an explanation, nor a workaround. MarkusM, you know r.watershed best. Could you have a look ? Moritz _______________________________________________ grass-user mailing list [email protected] http://lists.osgeo.org/mailman/listinfo/grass-user
