Is this MFD or SFD? If MFD, flow may split and re-join. I also noticed that r.stream.extract finds different flow-paths than r.watershed, which complicates comparisons between the two. The work-around for this is to use a mask to force r.stream.extract to find the flow-path you want.
-k. On 2016-08-25 at 13:35, Micha Silver <[email protected]> wrote: > I noticed something today that I don't understand. When I run > r.watershed and calculate the flow accumulation raster, I expect that > along each stream, in the direction of flow, accumulation values will > always be higher from one cell to the next. But I see this is not the > case. Sometimes, along a stream, the accumulation will drop for one > cell, then "jump up" again a few cells downstream. > > See the attached image. The green squares are clipped from a flow > accum grid. And the numbers are flow accum values. The arrow is > general flow direction, and the circles show examples of a sudden drop > in flow accum. > > I'm aware of the "edge of map" behavior where out of region accum gets > a negative value. That's not the issue here, since the sample in the > attached image is from right in the center of the region, no off map > flow is involved. > > This problem surfaced in a script I've prepared to calculate total > flow accumulation for each stream reach. After running the addon > r.stream.order I add columns to the streams vector map for X-Y of the > end points and total flow for each reach, then I use v.what.rast to > get the flow accum at each stream reach end-point. But the values I'm > getting are strange. Sometimes a downstream reach shows lower total > accum than the previous, upstream segment. > > Maybe someone can shed some light? > > Thanks, Micha _______________________________________________ grass-user mailing list [email protected] http://lists.osgeo.org/mailman/listinfo/grass-user
