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Universal Transverse Mercator System Projection
For the Universal Transverse Mercator System, the globe is divided into 60 zones, each spanning six degrees of longitude.
Each zone has its own central meridian from which it spans 3 degrees west and 3 degrees east. X and Y coordinates are
recorded in meters. The origin of each zone is the equator and its central meridian. The value given to the central meridian
is a false easting of 500,000. In the continental United States, the North American Datum of 1927 (NAD27) and the Clarke
spheroid are most commonly used.
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.
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