[GRASS-user] need help computing all flow routing for large DEM

Markus Metz markus.metz.giswork at gmail.com
Tue Nov 20 00:56:57 PST 2018


On Tue, Nov 20, 2018 at 9:50 AM Ken Mankoff <mankoff at gmail.com> wrote:
>
> Hi GRASS list,
>
> I'm seeking some advice about how to determine the eventual edge-cell
destination of every pixel in a DEM. I have an algorithm that works on
rasters up to 0.5 billion pixels but am now trying to do it on a region
about 8x as large and it is failing.
>
> I've spent a week exploring r.watershed and r.terraflow and can't figure
out how to make it work.
>
> What has worked for the 0.5 B pixel region is running r.watershed and
then for every negative pixel, I run r.water.outlet and find the upstream
contributors. But I can't run r.watershed on the entire large region. I can
run it quickly on sub-regions, but some very large interior basins cause
this method to fail.

How does it fail? What is the error output? You did use the -m flag for
r.watershed, right?
>
> I've run r.terraflow, but the direction raster from that has a massive
number of internal gaps and I don't see how to get it to work.

You don't need r.watershed or r.terraflow for this purpose., You can run
r.stream.extract with a DEM as the only input, then you get the outlets and
can feed them to r.stream.basins.

>
> Does anyone have any advice how to process large rasters with
r.watershed, or r.fill.dir, or some other method that can trace all pixels
to their edge destination?

r.watershed should process very large rasters, granted that there is enough
free disk space.

Markus M

>
> Thanks,
>
>   -k.
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