IMO the esri term of a filled DEM being "hydrologically correct" is a
misnomer.
Natural terrain has real depressions that impact surface water flow.
Big depressional wetlands can retain water and release via groundwater
or evapotranspiration.
I like how r.watershed acomodates known depressions and handles the
flow as interception. Also, then one can use other tools to find
problematic areas in a raw DEM. I modeled an internally drained basin
using known depressions in GRASS, and it worked fantastic.
One problematic example for esri is modeling an internally drained or
sink-watershed. These have no surface water outlet. If one filled it
to get flow out of a 9 sq. mile watershed, the esri analyses are then
meaningless. Which is one reason why filling a dem just to get an
esri module to work and calling the DEM "hydrologically correct" is a
misnomer an a limitation.
Mark
On Feb 13, 2009, at 11:09 AM, Markus Metz <markus.metz.gisw...@googlemail.com
> wrote:
Christian Schwartze wrote:
Dear GRASS users,
with r.watershed I get strange basin boundaries for some areas und
I'm not able
to give account of it. Attached you can find that part of the basin
map which
looks curiously. I means the sharp-edged regions...
Whats the reason?
This is most probably a flat area (no slope). Flow direction, flow
accumulation, stream segments and basins can not reasonably be
calculated for flat areas, these are regarded as missing information
and some assumption has been made by the algorithm.
What could help is to use a raster DEM as input that is *not*
filled, some would say not hydrologically correct, but r.watershed
works better with the raw, not filled DEM.
What could also help, if this does not work or it really is a flat
area, is r.watershed of grass7 with multiple flow direction. Note
that the result may look nicer, but it still holds true that
drainage direction (and therefore all other output) has to be
estimated for flat areas. The A * Search of r.watershed is doing a
pretty good job, and multiple flow accumulation can improve it a bit
more, within limits.
Basis is an Arc Info .adf raster file for DEM data.
I think Arc Info wants a depressionless, filled, hydrologically
correct DEM. r.watershed does explicitely not want such a DEM, it
wants a raw DEM with depressions, not filled, and not hydrologically
correct.
I hope that helps,
Markus M
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