Geocentric Coordinate System 

Release 9.2
Last modified August 3, 2007 
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The geocentric coordinate system is not a planar coordinate system based on a map projection. It is a geographic coordinate system in which the earth is modeled as a sphere or spheroid in a righthanded X,Y,Z system measured from the center of the earth.
The Xaxis points to the prime meridian, the Yaxis points 90° away in the equatorial plane, and the Zaxis points in the direction of the North Pole.
A geocentric coordinate system is similar to a geographic coordinate system based on latitude and longitude, which measures angles from the earth's center
The geocentric coordinate system is used internally as an transient system. It is used as a framework for calculations as part of several geographic (datum) transformation methods.
Learn more about geographic transformation methods