[gdal-dev] Datum transformation method when projecting from a sphere to NAD83 shapefile

Frank Warmerdam warmerdam at pobox.com
Tue Sep 9 13:23:57 EDT 2008

Limei Ran wrote:
> Hi Frank:
> Thank you for your information.  Is any reason that PROJ4.6. or newer 
> does not do any datum transformation when going from LCC Sphere to NAD83 
> Albers?
> We are using many shapefiles generated in arcGIS, GDAL or by other 
> tools.  We want to have the consistent shapefiles.  Could you tell me 
> which approach is correct or proper?
> You are right about the difference.  We have GDAL1.5.0 compiled with 
> PROJ4.5 and GDAL1.5.2  compiled with PROJ4.6.  The projected shapefiles 
> using ogr2ogr from the two versions of GDAL do have quite big position 
> shiftings.


Generally speaking, when going between "real" datums and these spherical
based projections, the right thing to do is to apply no shift which is why
the default behavior was changed in PROJ 4.6.

But to get consistent overlay, you really need to know quite a bit about
your 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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