[Proj] Datum shifts in PROJ's EPSG table

Frank Warmerdam warmerdam at pobox.com
Tue Oct 24 11:45:11 EDT 2006

Jan Hartmann wrote:
> Hi,
> I was wondering whether Datum shift parameters (+towgs84=..) should be 
> officially part of the epsg file that comes with PROJ. I computed the 
> values for the Dutch and Belgian national grids (EPSG 28992 and 31370), 
> see:
> http://postgis.refractions.net/pipermail/postgis-users/2005-November/009894.html 
> and
> http://postgis.refractions.net/pipermail/postgis-users/2006-October/013570.html 
> At the moment, these have to be put manually in the epsg-file and in the 
> spatial_ref_sys table of PostGIS.


The scripts I use to translate the EPSG database into PROJ.4 format already
attempts to identify datum shift values if they are present in the EPSG
database.  However, if there is more than one choice (which is quite common
for important datums) the script gives up and uses none of them since any
selection would be arbitrary.

So, one thing we could do is try and capture the EPSG datum shifts better.

The other thing I have been doing is that I have inserted an "override"
step in my translation from EPSG to PROJ.4 format that allows forcing a
datum shift solution for particular datums.  So when folks submit requests
to use a particular datum shift for a given datum, I apply it via this
override mechanism.  Of course, soon I'll no doubt get into the problem of
different people wanting different overrides for the same datum (applicable
to different regions) and then I will be back in the same problem of not
knowing which to choose.

Best regards,
I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | President OSGeo, http://osgeo.org

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