[GRASS-user] Datum transformations

[Jose Gomez-Dans]

Hi!
This is a GDAL-related question, but since I am also using GRASS, here
it goes. I have a number of GeoTIFF images in UTM30N/WGS-84 format. I
want to conver the datum to European 1950 and viceversa (in EPSG
parlance, this is a transformation from 32630 to 23030, if memory
serves). This is easily accomplished with gdalwarp, but I am unable to
specify the transformation parameters for my geographic area. For
example, if I define a region in GRASS from EPSG codes, I am asked for
the EPSG code, as well as for which datum transformation I want to
use. I believe this can be set in the -towgs84 paramter in GDAL, but I
am unsure on what numbers to use for Southern Spain (in GRASS, I think
it's either datum transform option 4 or 5 for a location defined as
EPSG 23030). Can anyone point me to the file where these parameters
are stored, or some other relevant info?

Many thanks!
jose