[gdal-dev] Gdal and Proj GCS/PCS

Frank Warmerdam warmerdam at pobox.com
Wed Nov 18 11:41:29 EST 2009

Guillaume Sueur wrote:
> Having just realised that gdal doesn't use proj EPSG file for
> reprojection (see my previous post on "Unsupported SRS but available in
> epsg file"), I have a few questions to clarify the reprojection
> processes both in gdal and mapserver :
> - where do the gdal gcs/pcs definitions come from ? Is it a separate
> repository or proj source files ? 


Yes, the gdal/data/{pcs,gcs}.csv files.

> - Doesn't ogr2ogr use proj definition files at all ? 

There are special circumstances when the proj.4 epsg init file is
consulted (if no definition is found in the pcs/gcs csv files).
But normally ogr2ogr does not use the proj.4 init files at all.

 > Does this mean that
> an ogr2ogr reprojected file can be different than a mapserver
> reprojection result if proj/epsg definition and gdal pcs defs are
> slightly different ? 

Yes, this is possible, particularly if someone has hand modified
one or the other.

> - If I say "GDAL/OGR uses its own definition files to retrieve proj4
> definitions and lets proj4 engine do the reprojection job", am I true ? 


The last point of note is that the PROJ.4 epsg init file is periodically
generated from the GDAL pcs/gcs.csv files and so they do normally
match quite exactly.

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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