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- [coordinate systems] A Cartesian coordinate system that is valid for the entire earth, with its origin at the center of mass for the earth and its axes fixed relative to earth's rotation. The x-axis is in the plane of the equator, passing through the origin and extending from 180 degrees 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 degrees west longitude (negative) to 90 degrees east longitude (positive); the z-axis is the line between the north and south poles, with positive values increasing northward, and negative values increasing southward.