[gdal-dev] Re: Datum Shifts with OSR.CoordinateTransform

[Frank Warmerdam]

On 11-05-10 07:47 PM, srweal wrote:
> Armin,
>
> Thanks for the reply. That seems like a logical approach, but can you or any
> of the GDAL developers tell me whether these +TOWGS84 parameters are
> applied/supported or not.

Steve,

3/7 parameter datum shifts may be specified on the source and
destination srs using the TOWGS84[] notation.

Note that PROJ.4 will not apply a datum shift unless both the source
and target srs are considered to be properly defined in terms of their
datum.

> Also, I have seen some references to using a +NADGRID option for specifying
> a transformation grid (rather than just the 7 parameters).
>
> Are these parameters supported? And if so, can they be defined for the
> CoordinateTransform as a whole without importing from WKT?

PROJ.4 supports +nadgrid notation, but it is often difficult to capture
this in a WKT srs definition.  If you need to do so, appending +wktext to
a PROJ.4 definition before importing it to WKT (with ImportFromWkt())
will ensure that the original PROJ.4 definition is preserved in the WKT.

There are lots of caveats and limitations.  It might be best if you
were more specific about what you want to do.

Best regards,
-- 
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