On Thu, Aug 25, 2016 at 4:07 PM, Ken Mankoff <[email protected]> wrote: > > Is this MFD or SFD? If MFD, flow may split and re-join.
Exactly. With MFD, flow may be distributed to several downstream cells and the accumulation value can drop when following the extracted path. This is is most prominent when a river flows into an ocean. Paths, however, follow Horton's stream network logic and can not split, only 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. r.stream.extract is the preferred stream extraction tool. Markus M > > -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 _______________________________________________ grass-user mailing list [email protected] http://lists.osgeo.org/mailman/listinfo/grass-user
