[GRASS-user] Re: GRASS script to export raster

Kim Besson kimbesson1981 at gmail.com
Wed Mar 9 11:29:19 EST 2011

Hi Glynn, Martin,  Hamish and rest of GRASS mailing list
Thanks for your feedback. Unfortunely, I was "analyzing" my problem in the
wrong way...
WHat I need is to:
- Export raster values in an ASCII file written from left to right and
bottom to top and each line only includes Raster value (not X neither Y).
Is this possible in GRASS with a Python Script?

Also I will need to import raster values written in ASCII file written from
left to right and bottom to top where each line is a single raster value
without X and Y.

Is there any easy-to-implement solution?

Best regards and sorry for my previous mistake,

2011/3/9 Glynn Clements <glynn at gclements.plus.com>

> Martin Landa wrote:
> > > so just flipping all the rows in the file you can get now?
> > >
> > > don't know how to do that in python, but if unix shell powertools are
> > > available:
> > >   r.out.ascii | tac
> > print os.linesep.join(reversed(grass.read_command('r.out.ascii',
> FWIW, both of these are sub-optimal.
> The Python approach reads everything into memory then generates a
> reversed copy, so you need enough memory to hold two copies of the
> output from r.out.ascii.
> When given a file, tac reads the input file backwards a block at a
> time, so it only needs enough memory to hold a complete line, rounded
> up to a whole number of blocks.
> However, it can't do this when reading from a pipe; it has to store
> everything in memory.
> If you want to reverse a large file without storing everything in
> memory, the simplest solution is e.g.:
>        r.out.ascii ... > tmp.txt
>        tac tmp.txt > reversed.txt
>        rm tmp.txt
> If this was likely to be a common operation, r.out.ascii could easily
> be modified to flip the row number passed to Rast_get_row(). However,
> while that will be optimal for native GRASS rasters, it may be
> sub-optimal (or even fail) for "linked" (r.external) rasters; many
> raster formats have to be read top-to-bottom (I don't know how GDAL
> handles out-of-order reads for such formats).
> --
> Glynn Clements <glynn at gclements.plus.com>
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