[Proj] How to convert a sphere to ellipsoid with correct datum?

Frank Warmerdam warmerdam at pobox.com
Thu Sep 2 09:19:32 EST 2010

Heiko Klein wrote:
> The geodetic<->geocentric code depends only on the spheroid/ellipsoid, 
> not of further datum information, and it is strange that this code is 
> not run without both datum information, even if the ellipsoid changes.


Note that it used to just convert to geocentric from one ellipsoid and
the back to geodetic on the second ellipsoid.  But very often it turned
out this was not appropriate and caused lots of problems for users.  So
after most of a decade I bowed to pressure and stopped doing this as it is
rarely the right thing to do apparently.

> I'm still very confused on what to use to convert from a spherical earth 
> model to one or another projection.

I also don't know.  It really "depends".  What I have seen in the past
is that when data was apparently processed on a sphere was really intended
to be treated as if the geographic coordinates were in WGS 84.  So doing
an automatic sphere to ellipsoid conversion resulted in huge offsets in
the north south direction.

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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