An XYZ Cartesian Earth Centered Earth Fixed (ECEF) Coordinate System is a coordinate system with the origin at the center of the Earth (as defined by a reference ellipsoid). The Z-axis coincides with the minor axis of the reference ellipsoid. The X-axis runs from the origin through a point on the equatorial plane at the zero meridian. The Y-axis is perpendicular to the X-axis on the equatorial plane.
A geodetic datum is the only required defining parameter for an XYZ Cartesian ECEF Coordinate System in GeoCalc. A geodetic datum defines constants that relate a Geodetic Coordinate System to the physical Earth, to the dimensions of the reference ellipsoid, to the location of the origin of the system and to the orientation of the system.
An XYZ Cartesian ECEF coordinate is specified by X, Y, and Z values in any of the defined distance units of measure.