What is a datum – Google Earth User Guide User Manual
Page 102
What Is a Datum?
While a projection is used in mapping to define the earth on a flat surface, a datum is used to describe the
actual shape of the earth in mathematical terms. This is because the earth's surface is not perfectly round, but
ellipsoid in shape. A datum also defines the association of latitude and longitude coordinates to points on the
surface of the earth, and defines the basis for elevation measurements.
As with projections, there is more than one mathematical interpretation of the earth's shape. Google Earth
uses WGS84 datum.
1.
North Pole
2.
Equator
3.
South Pole
4.
Semi-minor axis or polar
radius
5.
Semi-major axis or
equatorial radius
Semi-major axis
Semi-minor axis
NAD83
6,378,137.0
6,356,752.3141
WGS84
6,378,137.0
6,356,752.3142
Clark 1866
6,378,206.4
6,356,583.8
Airy 1830
6,377,563.4
6,356,256.9
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