[Proj] Default datum transformation behavior

Frank Warmerdam warmerdam at pobox.com
Tue Nov 25 19:49:42 EST 2008

Brendan Billingsley wrote:
> The last question in the PROJ.4 FAQ explains how to use the
> +nadgrids=@null argument to prevent a datum shift when doing a
> conversion between projections with different projection spheroids.  I
> have been using this trick and telling others about this trick.  However
> I recently started using a newer version of FWTools and realized that
> leaving out the "+nadgrids=@null +wktext" produced the same results as
> having it in.  I then grabbed the newest version of proj (4.6.1),
> compiled it and confirmed that the behavior has changed.  In my old
> version (4.5.0) if I use cs2cs then to prevent the datum transformation
> for my conversion I need the +nadgrids=@null argument, but for the new
> version I can leave that argument off and I will get the same results as
> having the argument on and using the 4.5.0 version.
> So has the default behavior been changed, am I missing something or is
> this a bug?  If the default behavior has been changed then that is great
> news.


Yes, the default behavior changed.  PROJ.4 will now only apply a datum
shift if both the source and destination coordinate system have valid
datum shift information.

 From the PROJ.4 4.6.0 Release Notes (in NEWS):

  o MAJOR: Rework pj_transform() to avoid applying ellipsoid to ellipsoid
    transformations as a datum shift when no datum info is available.

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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