[OSRS-PROJ] epsg code 21781 towgs values gone

Frank Warmerdam warmerdam at pobox.com
Mon Nov 10 10:27:42 EST 2003

Markus Neteler wrote:
> Frank,
> the current EPSG 6.4 is implemented in SQL (so a PostgreSQL database
> is available). Do you generate the 'epsg' file with some big
> SQL query?
> I would like to update our version here as now the Italian datums
> are available. Before starting from scratch, maybe you are willing
> to publish how the file is generated.


I have taken the liberty of cc:ing the proj list since this may be of
general interest.

My methodology is rather multi-step.

  o I dump the EPSG Microsoft Access database to .csv files (those distributed
    with libgeotiff).

  o I then derive the pcs.csv, gcs.csv and gdal_datum.csv files from those
    dumped files using a few python scripts.  These are the scripts that make
    decisions such as to avoid emitting any "towgs84" information if there is
    more than one "path" available in EPSG.   These scripts can be found in the
    libgeotiff/csv directory (csv_tools.py, and build_pcs.py).

  o Then the proj.4 epsg file is created by running the epsg_tr.py script
    from "gdal/pymod" on the full set of epsg GCS and PCS values.  The result
    is the epsg file.

The build_pcs.py script is really the hairy part and encapsulates the logic
that likely needs improvement. The other part particularly pertenant to
PROJ.4 is the C++ logic in GDAL for converting it's internal representation
of a coordinate system into PROJ.4 format which is (I think) working pretty
well within the  limits of the source data.

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    | Geospatial Programmer for Rent

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