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

[Markus Metz]

[Answering to your latest post in a new thread, but I prefer to keep
it all in one thread.]


On Tue, Apr 10, 2012 at 1:54 AM, Rich Shepard <rshepard at appl-ecosys.com> wrote:
> On Mon, 9 Apr 2012, Markus Metz wrote:
>
>> 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.
>
>
> Markus, madi, et al.:
>
>  The region does need to be set with a mask (in my case, and based on the
> v.to.rast output of the individual subbasins).

v.to.rast? Basins as created by r.watershed, r.stream.basins and
r.water.outlet are already raster maps.
>
>  The last advice in the example on the manual page is to compare the
> weighted and unweighted extracted stream maps and "see which looks better."
> As a newcomer to this module I have no experience on what "better" looks
> like.
>
>  Attached are two .pdf files. One shows the weighted output streams, the
> other the unweighted output streams. Both have the vector streams overlaid
> on them. The weighted output has a solid yellow background and I don't know
> what that represents.

In this case there is indeed no answer to "what looks better" because
both look terrible, to be honest. Something terrible must have
happened to the DEM, or there are indeed completely flat areas in the
west. If the aim is to extract streams that resemble the digitized
vector lines, the DEM you used seems to be not suitable. You could try
to run r.stream.extract with the DEM as the only input and a larger
threshold. The region should be aligned to the DEM first with g.region
-p align=dem. If that produces reasonable results, you can try to
fine-tune by adjusting the threshold and supplying a (weighed)
accumulation map as additional input.

Markus M

PS: Please do not change the thread all the time. With regard to this
subject, you have started now the 7th or so thread, I lost count,
impossible for anyone to follow.