[OSRS-PROJ] Geocentric systems and ellipsoids

Frank Warmerdam warmerdam at pobox.com
Tue Jan 13 09:03:49 EST 2004

czaht at ocag.ch wrote:
> Hi,
> I have a question regarding the handling of geocentric system libproj. It
> seems that I need to define an ellipsoid together with a geocentric system,
> in my case the WGS84 Cartesian
> system. Why? 
> In my opinion, a geocentric system is defined by the centre of the earth and
> a rotation relative to the WGS84 Cartesian system.
> Could someone shed a bit of light on this matter?


The geocentric support in PROJ.4 (as opposed to Gerald's newer libproj4)
is quite new and somewhat experimental.  The need for an ellipse is really
an artifact of internal assumptions of the library before geocentric support
was added.

Just select the same ellipsoid/datum as the output coordinate system you
will be transforming to (or the input system you are transforming from).
You are correct that the geocentric coordinates define the location without
any real need for an ellipse.

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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