[Proj] Datum shifts in PROJ's EPSG table

[Martin Vermeer]

On Tue, 2006-10-24 at 11:45 -0400, Frank Warmerdam wrote:
> 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.
> 
> Jan,
> 
> 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.

Both of these mechanisms, while better than their absence, seem
arbitrary. 

Has there been thinking on how to support more than one datum shift if
available? What would that require? Could the EPSG labels be modified
inside Proj.4, e.g. <4123:1>, <4123:2> etc? Or is this issue bigger than
Proj.4?

(If we are talking about slightly different +towgs84 for different parts
of one datum target area, the use of a shift grid could be an
alternative solution)

- Martin

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