What are the basic gravitational parameters for the Earth?.What is the rotation rate of the Earth?.Where is the reference meridian (for determining longitudes)?.What is the orientation of the spin axis, and hence where is the equatorial plane (for determining latitudes)?.Where is the center of the Earth, which is the origin of the reference system (co-ordinate system) that you use for locating everything?.The datum provides the following information: The ellipsoid used is its own, but it is so close to the GRS80 ellipsoid that it makes no practical difference. WGS (these days usually WGS84, but there were also WGS72, WGS66, and WGS60, and WGS84 is currently on its 5th revision/update) is a datum and ellipsoid that model the Earth. These are two different but related concepts.
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