Ivan - you may talk to Yann on this list before you buy more memory -
he is trying to do the same as you , but with a bigger DEM and 8GB of
memory (Yann I hope it is OK with you that I am revealing this here).
My experience with large DEMs (up to 10,000x10,000) has been that I
had to split the area into sections that were about
2000x2000 on 1GB memory computer) to get it done (I was able to do
that for Panama because of its shape - many small watersheds rather
than a single big one) - it took me several days to do that.
Then I ran r.terraflow and I got it in 3 hours.
Yann says that r.terraflow did not work for him - now I remember what
the problem was when I tried to run it recently -
it needs a LOT of hard drive space which is not a problem these days,
BUT the default has been changed
to /tmp which for my linux box is only 2GB or so. But when running
r.terraflow you can define where
you want the temporary files to be written - so give it something
with a lot of space (tens of gigabytes at least)
and it should run. I think that the default should be changed to
where it was - I think it is the regular grass tmp where people
usually have a lot of space for the data.
If even that does not work you can give a try to brand new
TerraSTREAM - see the link below
(and let me know whether it works for you),
Helena
TerraSTREAM provides a series of components that
perform flow modeling and terrain analysis tasks on very large digital
elevation models and works equally well on TIN and grid DEMs. The
algorithms
used in the libraries have provable efficient performance in the
worst case,
even on very large terrains that do not fit in the main memory of the
computer.
TerraSTREAM 0.2 comes with direct GRASS and ArcGIS support as well as
a simple
standalone graphical user interface and powerful command line tools
that can be
used alone or integrated into most GIS environments by scripting. For
more
information about this release and for contact information, visit
http://madalgo.au.dk/Trac-TerraSTREAM/.
The TerraSTREAM 0.2 users guide is available here:
http://madalgo.au.dk/Trac-TerraSTREAM/wiki/UsersGuide .
On Mar 18, 2008, at 5:32 AM, ivan marchesini wrote:
Dear Grass Users and Developers, sorry for cross posting but we
hope the
argument can be of interest for all and we hope someone can give us a
solution to this problem..
We have this kind of problem:
* a large DEM (150000000 cells)
* an ordinary computer (2 GB ram)
* we must obtain the drainage map using r.watershed (without changing
resolution), because then we need to be able to calculate the upstream
basin for each cell (r.water.outlet).
* we have tried r.watershed straigth (but after few seconds a memory
allocation problem crashed the program)
* we have tried the "-m" option but after 4 days of work it is
still at
0%.
* giving up the last option, because it takes too long, we have
monitored the ram usage by means of "free -m" and we have seen that
r.watershed rapidly saturate the ram and then, after a little usage of
swap (20 mb) crashes.. so it seems that r.watershed doesn't use swap
memory... (and is then unuseful, as we did, to increase the swap
memory)
* we have tried to modify
"Swappiness" (http://www.gentoo.it/doc/memory.html#doc_chap5) but
without success... the error is still the same
so at this point:
* is adding ram to the computer the only solution?
* if yes, how can we estimate the ram to buy!
* can we obtain some better results compiling grass with option
--enable-largefile
* does someone solved a similar problem in some other way?
Thank you for your suggestions!!!
we hope really in your help
Luca & Ivan
--
Ti prego di cercare di non inviarmi files .dwg, .doc, .xls, .ppt.
Preferisco formati liberi.
Please try to avoid to send me .dwg, .doc, .xls, .ppt files.
I prefer free formats.
http://it.wikipedia.org/wiki/Formato_aperto
http://en.wikipedia.org/wiki/Open_format
Ivan Marchesini
Department of Civil and Environmental Engineering
University of Perugia
Via G. Duranti 93/a
06125
Perugia (Italy)
Socio fondatore GFOSS "Geospatial Free and Open Source Software"
http://www.gfoss.it
e-mail: [EMAIL PROTECTED]
[EMAIL PROTECTED]
tel: +39(0)755853760
fax (university): +39(0)755853756
fax (home): +39(0)5782830887
jabber: [EMAIL PROTECTED]
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