[postgis-users] Weird behavior transforming from srid 4269 to 2163

[Frank Warmerdam]

Paragon Corporation wrote:
> I'm having some strange discrepancies between two postgis servers in how
> they are transforming the same data from 4269 to 2163.  I'm hoping someone
> could shade some light on what could be the problem.
> 
> The problematic server is running Red Hat 4.1.2-14 and postgis version
> "POSTGIS="1.3.2" GEOS="2.2.3-CAPI-1.1.1" PROJ="Rel. 4.6.0, 21 Dec 2007"
> USE_STATS"
> 
> The one that I think is right is running Windows 2003 server
> "POSTGIS="1.3.1" GEOS="3.0.0rc4-CAPI-1.3.3" PROJ="Rel. 4.5.0, 22 Oct 2006"
> USE_STATS"
> 
> So the bad one is running a newer version of the PROJ library, but I wasn't
> the one who compiled and installed, so not sure if there was a step missing.

Regina,

I suspect your EPSG:2163 is evaluating to:

  +proj=laea +lat_0=45 +lon_0=-100 +x_0=0 +y_0=0 +a=6370997 +b=6370997 +units=m 
+no_defs

In PROJ 4.5 the transformation from EPSG:4269 (NAD83) to EPSG:2163 would be
accomplished by transforming NAD83 coordinates into geocentric coordinates and
then back into lat/long on the EPSG:2163 ellipsoid resulting in a substantial
shift in latitude.

However, after a half a decade of complaints about this "default ellipsoid
conversion" which is usually the wrong thing to do, PROJ 4.6 finally changed
the behavior to doing no datum shift if either the source or the destination
lacks apparent datum shift information.  That means, in this case, that the
input coordinates are being used as if they were already on the spherical
ellipsoid and no datum/spheroid shift operation is done.

I really think this is the appropriate change in your case, but it is certainly
a change in behavior.  It may also not agree with data previously converted
to EPSG:2163.

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    | President OSGeo, http://osgeo.org