[Proj] Datum shifts in PROJ's EPSG table

[Oscar van Vlijmen]

> From: Martin Vermeer
> Date: Wed, 25 Oct 2006 09:40:15 +0300
> Subject: Re: [Proj] Datum shifts in PROJ's EPSG table
> 
> 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),

....

> 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?

Also interesting: for the Dutch RD there are two sets for the transformation
between Bessel 1841 and WGS84. One is for the direction Bessel 1841 ->
WGS84, and another, slightly different set, is for WGS84 -> Bessel 1841.
I reported this with references in my posting "Re: [Proj] proj and Dutch
RD", dated 2006-01-12.
Using different sets for the to-from directions enhances the accuracy
somewhat.