[Gdal-dev] OGR, PROJ.4, and handling of Datum
ben at vterrain.org
Wed Apr 16 22:57:27 EDT 2003
> -----Original Message-----
> From: Ed McNierney
> While it might seem "quite reasonable" to think 7 degrees North, 81
> degrees West is covered by NAD27, it's not there. So the first issue is
> that your datum shift effort should not ever succeed, because the input
> value is undefined (outside valid domain).
Hmmmmm... i have this message from the person who provided the Panama data:
"I have two sources of data.. The basemaps down there are 1960's vintage US
Army, and are clearly labeled NAD27 / Clark 1866 spheroid, UTM Zone 17."
So perhaps there is such as a thing as NAD27 for Panama, and we (users of
PROJ) simply lack a Datum shift file for it.
> You didn't describe exactly what OGR does, other than give "bogus"
> results. Can you give us a bit more detail? Thanks!
Correct output for the UTM point is around: (-81.8, 7.7)
The resulting OGRCoordinateTransformation is producing: (-1.42, 0.13)
Eeek, i just realized that it's incorrect to point the finger at OGR's
successful creation of a transformation - the failure isn't until the
Transform, and Transform is indeed return 0 to indicate an error for the
point lying outside its known Datum conversion space. What was throwing me
off is that although it returns 0 to indicate error, it *also* modifies the
values passed in by pointer, as if it was succeeding :)
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