UTM and Datums, ArcUser magazine (2024)

Datums

A datum is a set of parameters defining a coordinate system and a set of control points whose geometric relationships are known either through measurements or calculation (Dewhurst, 1990). All datums are based on a spheroid, which approximates the shape of the earth.

NAD27

The North American Datum of 1927 uses the Clarke spheroid of 1866 to represent the shape of the earth. The origin of this datum is a point on the earth referred to as Meades Ranch in Kansas. Many NAD27 control points were calculated from observations taken in the 1800s. These calculations were done manually and in sections over many years, therefore errors varied from station to station.

NAD83

Many technological advances in surveying and geodesy since the establishment of NAD2--electronic theodolites, GPS satellites, Very Long Baseline Interferometry, and Doppler systems--revealed weaknesses in the existing network of control points. The North American Datum of 1983 is based on earth and satellite observations, using the GRS80 spheroid. The origin of the datum is the earth's center of mass. This affects the surface location of all latitude-longitude values enough to cause locations of previous North American control points to shift, sometimes as much as 500 feet.