[coordinate systems]
A cartesian coordinate system that is valid for the entire earth, with the origin at the center of mass for the earth. The X axis is in the plane of the equator, passing through the origin and extending from 180� longitude (negative) to the prime meridian (positive); the Y axis is also in the plane of the equator, passing through the origin and extending from 90�W longitude (negative) to 90�E longitude (positive); the Z axis is the line between the North and South Poles, with positive values increasing northward, and negative values increasing southward.
