[gdal-dev] Gdal and Proj GCS/PCS

[Guillaume Sueur]

Thanks for these precious informations Frank, it seems clearer to me
now. 

Best regards

Guillaume

Le mercredi 18 novembre 2009 à 11:41 -0500, Frank Warmerdam a écrit :
> 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 ? 
> 
> Guillaume,
> 
> 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 ? 
> 
> Yes.
> 
> 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,