[postgis-users] transform() does not do work for NGO1948 zone III -> WGS84 longlat (epsg:27393->epsg:4326)

[Frank Warmerdam]

On 12/20/05, strk at refractions.net <strk at refractions.net> wrote:
> On Tue, Dec 20, 2005 at 04:46:16PM -0000, Mark Cave-Ayland wrote:
> ...
> > Actually, looking through the source lwgeom_transform.c I see the following
> > comment around line 500:
> >
> > //this is *exactly* the same as PROJ.4's pj_transform(), but it doesn't
> > //do the datum shift.
> >
> > So does this mean that the PostGIS transform() function won't do datum
> > shifts? Can anyone with a bit more PROJ4 knowledge say if this is correct,
> > and if so, why PostGIS can't support it?
>
> I'm not responsible for that code, anyway with a closer look, that
> function is only invoked as a second attempt if pj_transform()
> returns -38. Line 930:
>
>                 if (pj_errno == -38)  //2nd chance
>
> Finding meaning of -38 would be the next thing to do ...

Strk,

Error -38 is:
 	"failed to load NAD27-83 correction file",  	/* -38 */

So basically, if the datum shift files are not found, then the
code fallback to trying a conversion without the datum shift.

Best regards,
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