[GRASS-user] External Database and projections

[Blumentrath, Stefan]

Hi Dylan,

Yes, r.external and r.external.out works pretty well with TGIS. Here is a discussion in this regards I had mainly with Sören…
https://lists.osgeo.org/pipermail/grass-user/2013-December/069377.html

I have used r.external/r.external.out in TGIS for a while (with compressed GeoTiffs, btw you should not pre-define the predictor for LZW-compression as long as you are not sure that all raster maps are of the same type (CELL/FCELL/DCELL)), but went back to using internal GRASS format, because external links are a bit tricky to use on NFS and from different OS (or if data is moved):
https://trac.osgeo.org/grass/ticket/2660

Another minor (and often neglectable) drawback of using r.external is that file names and map names get out of sync if you rename maps. g.rename renames only the GRASS internal map name (modifies the link) the underlying data file is not touched and stays with the old name…

Cheers
Stefan


From: Dylan Beaudette [mailto:dylan.beaudette at gmail.com]
Sent: 8. oktober 2015 00:17
To: Markus Neteler <neteler at osgeo.org>
Cc: Blumentrath, Stefan <Stefan.Blumentrath at nina.no>; GRASS user list <grass-user at lists.osgeo.org>
Subject: Re: [GRASS-user] External Database and projections

The use of r.external makes a lot of sense when dealing with very large files. Does the use of "external" files work as expected in all of the new t.* modules?

Thanks,
Dylan

On Wed, Oct 7, 2015 at 12:09 PM, Markus Neteler <neteler at osgeo.org<mailto:neteler at osgeo.org>> wrote:
On Wed, Oct 7, 2015 at 12:55 PM, Blumentrath, Stefan
<Stefan.Blumentrath at nina.no<mailto:Stefan.Blumentrath at nina.no>> wrote:
...
> My suggestion is to not use PostGIS for big rasters, unless you have to,
> because you want to use the data in a specific application for example.

Note r.external and r.external.out of GRASS GIS 7 for avoiding data duplication:

https://grass.osgeo.org/grass70/manuals/r.external.html
https://grass.osgeo.org/grass70/manuals/r.external.out.html

Here an example:

# register (rather than import) a GeoTIFF file in GRASS GIS:
r.external input=terra_lst1km20030314.LST_Day.tif output=modis_celsius

# define output directory for files resulting from subsequent calculations:
r.external.out directory=$HOME/gisoutput/ format="GTiff"

# perform calculations (here: extract pixels > 20 deg C)
# store output directly as GeoTIFF file, hence add the .tif extension:
r.mapcalc "warm.tif = if(modis_celsius > 20.0, modis_celsius, null() )"

# cease GDAL output connection and turn back to write standard GRASS
raster files:
r.external.out -r

# use the result elsewhere
qgis $HOME/gisoutput/warm.tif

Best
Markus
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