[GRASS-user] unable to reproduce r.water.outlet example of GRASS book

Markus Metz markus.metz.giswork at googlemail.com
Wed Sep 7 02:22:55 EDT 2011

Moritz Lennert wrote:
> Hello,
> I'm trying to reproduce an exercise of the GRASS book, third edition, pp.
> 145-146, which demonstrates the usage of r.water.outlet. Using exactly the
> same commands as listed in the book, I get a basin which is hardly visible
> unless zooming very closely to it. r.report reports a size of 0.0002 ha. Is
> this normal ?
> The relevant commands in the book are (for the NC dataset):
> g.region rural_1m -p (or g.region rast=elev_lid792_1m)
> r.watershed elev_lid792_1m thresh=5000 accum=accum_5K \
>            drain=draindir_5K basin=basin_5K
> r.water.outlet drainage=draindir_5K basin=basin_A30 \
>               east=638872.6 north=220042.6
The version of r.watershed used for the example in the book had some
bugs, in particular a bias towards diagonal drainage direction which
is mosz notable with high-res LiDAR DEMs like elev_lid792_1m. One
consequence is that accumulation patterns shifted a bit and
east=638872.6 north=220042.6 are most probably no longer falling onto
a main channel leading to minute basins. Try plotting east=638872.6
north=220042.6 on top of accum_5K and adjusting the coords such that
they fall onto the nearest main channel.



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