[postgis-users] Error in SPATIAL_REF_SYS table

[Frank Warmerdam]

On 11/9/05, Jan Hartmann <j.l.h.hartmann at uva.nl> wrote:
> A bugreport has been entered in PROJ4's Bugzilla:
>
> http://bugzilla.remotesensing.org/show_bug.cgi?id=980

Jan,

Thanks for pointing this out.  I was meaning to dig up the bug
report for this thread.

> The "towgs84" parameter is also missing in the PROJ4 epsg table
> (although it is in the original EPSG database). You need it for
> overlaying on WGS84-based maps, like Google Earth. This formula gives
> precision from 1 mm to about 25 cm, compared to the "official"
> transformation available at http://www.rdnap.nl. The remaining 25 cm are
> due to corrections needed for the slightly irregular Dutch grid, and are
> almost impossible to incorporate into PROJ (see
> http://lists.maptools.org/pipermail/proj/2005-October/001809.html).

Note that the automated translation of the EPSG database
has several rules for identifying a datum shift (TOWGS84) value
to use.
 o There must only be one.
 o It must be listed going directly from the datum in question
    to WGS84.

Many datums in the EPSG database have multiple conversions
to WGS84 - various approximations appropriate in different local
areas.  In these cases I give up and supply none.

Some datums also provide shifts to some other intermediate
datum which then has shifts to WGS84.  My logic is to weak
to follow these.

I do intend to regenerate the "epsg" translation file for PROJ.4
(and PostGIS) at some point but it likely will come after I
complete my pending contract work, and several months of
that are currently piled up.

Best regards,
--
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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    | Geospatial Programmer for Rent