[GRASS-user] Re: r.stream.extract Syntax Problem

[Markus Metz]

Considering the length of this thread and the various issues
mentioned, it seems that I need to update the manual of
r.stream.extract a bit. I will try to address the issues:

r.stream.extract is designed to work with a DEM only or with a DEM and
a flow accumulation map as input. This flow accumulation map can be
created with any method and any software, tested with r.watershed,
r.terraflow, TAS (now Whitebox), and SAGA. The methods tested are D8,
MFD, and D-inf. Some modules/software need a hydrologically
conditioned DEM without sinks as input (have a look at r.hydrodem in
GRASS 7 addons), and not the raw DEM. In this case the modified DEM
used to calculate flow accumulation must be used as input for
r.stream.extract if a flow accumulation map is also provided. In case
of r.terraflow, the filled DEM produced by r.terraflow must be used as
input.

The computational region should be aligned to the DEM in order to get
accurate results. Aligning to the resolution of the DEM is not enough,
i.e. you should use g.region align=DEM instead of g.region res=XXX -a.
r.stream.extract should work if only a subregion is to be analysed by
setting the computational region accordingly and/or creating a MASK
e.g. for a specific basin.

The flow accumulation map used as input to r.stream.extract can be
modified, e.g. by multiplication with a weighing map. In this case
valid accumulation values should not be converted to NULL, but zero is
fine. The weighing map should not be used to mask out certain areas,
instead it should be used to weigh the original accumulation map
according to slope, valley type, rainfall, etc.

The module will exit with a fatal error if for a given cell there is a
NULL accumulation value provided, but the DEM is non-NULL. In this
case a MASK must be created for the subregion covered by the
accumulation map.

HTH,

Markus M


On Sat, Apr 7, 2012 at 11:33 PM, Rich Shepard <rshepard at appl-ecosys.com> wrote:
> On Sat, 7 Apr 2012, Helmut Kudrnovsky wrote:
>
>> if the (sub-)watershed are already given, maybe the easiest way to define
>> the region extent for raster processing of your interest is using g.region
>> [1]:
>
>
> Helmut,
>
>  It's been long enough that I totally forgot about this. I don't need a
> mask if I set the region to the basin of interest.
>
>  Thanks very much for the reminder!
>
> Rich
>
>
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