What is a Datum?

A datum or geodetic datum) references data to the Earth. Once we had measurements for the entire globe, datums became descriptions of the shape of the earth.

A datum specifies (modern versions):

• Shape of the ellipsoid
  • Axis lengths, or "flattening"
• Starting Latitude or Prime Meridian
  • Almost always Greenwhich England
• Units
  • Always degrees

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