[OSRS-PROJ] Geocentric systems and ellipsoids

[czaht at ocag.ch]

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?

Thanks!

/Thomas Aho
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