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Horizontal Datum

A horizontal datum forms the basis for computations of horizontal positions. A horizontal datum may be defined at an origin point on the ellipsoid such that the center of the ellipsoid coincides with the earth’s center of mass. NOAA uses the North American Datum of 1983 (NAD 83) for all of its charts. This datum is navigationally equivalent to the World Geodetic System of 1984 (WGS 84).

It is important to specify the horizontal datum when giving geographic coordinates. For example, the geographic coordinates given relative to the NAD 27 datum might represent a position hundreds of meters different than those same coordinates given relative to the NAD 83 datum.

This is a note that appears on NOAA nautical chart.  It is entitled Horizontal Datum.  It says, 'The horizontal reference datum of this chart is North American Datum of 1983 (NAD 83) which for charting purposes is considered equivalent to the World Geodetic System 1984 (WGS 84).  Geographic positions referred to the North American Datum of 1927 must be corrected an average of 0.349' northward and 1.822' eastward to agree with this chart.'
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