[GRASS-user] Re: r.stream.extract with external flow accumulation maps

[Markus Metz]

On Tue, Jun 28, 2011 at 5:06 AM, jentjr <jentjr at gmail.com> wrote:
> It worked! I also tried it without changing the spatial extent of the map and
> it worked. So, I just needed to set the null values to zero.
>
Hmm, this is not such a good idea, the zeros can have interesting
effects. Rather create a MASK where either accumulation or elevation
or both are NULL.

The accumulation map does indeed not match the elevation map. Another,
modified elevation map was used to create the accumulation map. The
accumulation map is full of artefacts that are not present in the
elevation map. You can compare the accumulation map produced by
r.watershed with the accumulation map supplied on the website. For
easy visual comparison, use r.watershed -a and set the color rules of
"fac" to those of the output of r.watershed, e.g.
r.colors map=fac raster=elev_cm.ac

Markus M