[postgis-users] Re: Datum Change

[Patrick]

"Armin Burger" <armin.burger at gmx.net> wrote in message 
news:43F656AD.3070100 at gmx.net...
>
> And the EPSG database (at www.epsg.org) has also a bigger set of those 
> parameters. There was once a posting of somebody on this list providing 
> additional tables and views to extract the information from the EPSG 
> tables.
>
Jaime, Armin,

I (presumably the "somebody" above) developed a set of functions that take 
the original EPSG tables and create the spatial_ref_sys view from it 
on-the-fly. It can list all the available datum shift parameters for a 
particular region and datum, but unfortunately PostGIS can not yet use that 
information in coordinate transformations. The reason is that PostGIS relies 
on the proj4text string, rather than on raw EPSG data and PROJ.4 design 
states that if multiple datum shift parameter sets (from the datum to WGS84) 
exist, then no information is given at all. Consequently (since many datums 
actually have multiple sets of parameters), datum shift information is not 
given.

IMHO this is something that the Refractions team should take a hard look at. 
(As well as the issue of adding new CRSs that use existing "building blocks" 
(UOMs, spheroids, datums, etc) and parsing a WKT to produce the 
corresponding srid.) Paul, just send me an email if you want to discuss.

Jaime, as an intermediate solution you could use my functions to select the 
set of parameters that is suitable for your study area and use that to patch 
the proj4text field as Armin suggested. You can download the SQL script with 
instructions from 
http://prdownloads.sourceforge.net/dspatial/PostgreSQL-CRS.zip?download.

Good luck,
Patrick