[GRASS-user] r.watershed / r.terraflow - huge area (amazon)

[Markus Neteler]

On Tue, Jan 10, 2017 at 12:30 PM, Carlos Grohmann
<carlos.grohmann at gmail.com> wrote:
> Hello all
>
> I'm working on deriving a nice drainage network for the Amazon basin. The
> problem is that the area is too large.
>
> The region settings are these:
> projection: 3 (Latitude-Longitude)
> zone:       0
> datum:      wgs84
> ellipsoid:  wgs84
> north:      6:17N
> south:      21:30S
> west:       80:37W
> east:       44:50W
> nsres:      0:00:03
> ewres:      0:00:03
> rows:       33340
> cols:       42940
> cells:      1431619600
>
> With 14.3 billion cels,

Isn't it 1431619600 = 1.431.619.600 = 1.4 billion cells?

> r.watershed would need about 450 GiB of disk space
> running in seg mode, is that correct? (from the manual, 31 MB for 1 million
> cells).

https://grass.osgeo.org/grass72/manuals/r.watershed.html#large-regions-with-many-cells
" The upper limit of the ram version is 2 billion (2^31 - 1) cells,
whereas the upper limit for the seg version is 9 billion-billion (2^63
- 1 = 9.223372e+18) cells."

So that would be supported by r.watershed.

> I do have that disk space available, but it's in a secondary drive. If I set
> TMPDIR to that drive, will r/watershed use it? I ask because the
> documentation (https://grass.osgeo.org/grass72/manuals/variables.html) only
> says that this environmental variable is used by "[Various GRASS GIS
> commands and wxGUI]".

AFAIK the TMPDIR variable is not relevant here. The tmp dir is

location/mapset/.tmp/

which you could link to the extra drive. This should be simplified for sure:

Involved code:
https://trac.osgeo.org/grass/browser/grass/trunk/lib/gis/file_name.c
https://trac.osgeo.org/grass/browser/grass/trunk/lib/gis/tempfile.c

Please open a ticket with the relevant info in it.

> On a side note, r.terraflow should be an alternative for such a large
> dataset, but from the manual:
> "r.terraflow has a limit on the number of rows and columns (max 32,767
> each)".
> That's the size of a positive short integer. Is this limit still needed?

I have no idea here, did you try it?

Markus


-- 
Markus Neteler
http://www.mundialis.de - free data with free software
http://grass.osgeo.org
http://courses.neteler.org/blog