[OSRS-PROJ] problem with nad_init and initializing ntv1_can.dat

Frank Warmerdam warmerdam at pobox.com
Thu Nov 20 09:42:48 EST 2003

Charles Seaton wrote:
> I am wondering if the functions for handling the initialization of the 
> Canadian datums NTv1 and NTv2 have been moved out of nad_init into some 
> other function, or whether these functions have somehow been lost. I 
> have looked through the mailing list archives for any mention of this 
> problem, and I have not found anything.


The short answer is "yes".  The datum shifting code was substantially
restructured when NTv2 support was incorporated last winter.  The nad_init
is just used for old style PROJ.4 datum shift files, and not for
NTv1 or NTv2 files.

I would strongly encourage you to use the pj_init() and pj_transform()
functions to do datum shifts if possible.  So, for instance, to apply
a datum shift from NAD27 to NAD83 create a projPJ handle for
"+proj=latlong +nadgrids=ntv1_can.dat" and "+proj=latlong +datum=NAD83" using
pj_init_plus(), then use pj_transform() to convert between them.

If you have to go in deeper review the pj_datum_transform(),
pj_apply_gridshift() and various gridinfo functions to better understand
how things work.

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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