North American Datum

The North American Datum (NAD) is the datum now used to define the geodetic network in North America. A datum is a formal description of the shape of the Earth along with an "anchor" point for the coordinate system. In surveying, cartography, and land-use planning, two North American Datums are in use: the North American Datum of 1927 (NAD27) and the North American Datum of 1983 (NAD83). Both are geodetic reference systems based on slightly different assumptions and measurements.

In 1901 the United States Coast and Geodetic Survey adopted a national horizontal datum called the United States Standard Datum, based on the Clarke Ellipsoid of 1866. It was fitted to data previously collected for regional datums, which by that time had begun to overlap. In 1913, Canada and Mexico adopted that datum, so it was also renamed the North American Datum.

As more data were gathered, discrepancies appeared, so the datum was recomputed in 1927, using the same spheroid and origin as its predecessor.

The North American Datum of 1927 (NAD27) was based on surveys of the entire continent from a common reference point that was chosen in 1901, because it was as near the center of the contiguous United States as could be calculated: It was based on a triangulation station at the junction of the transcontinental triangulation arc of 1899 on the 39th parallel north and the triangulation arc along the 98th meridian west that was near the geographic center of the contiguous United States. The datum declares the Meades Ranch Triangulation Station to be 39°13′26.686″ north latitude, 98°32′30.506″ west longitude (NGS data). NAD27 is oriented by declaring the azimuth from Meades Ranch to Waldo to be 255°28′14.52″ from north. The latitude and longitude of every other point in North America is then based on its distance and direction from Meades Ranch: If a point was X meters in azimuth Y degrees from Meades Ranch, measured on the Clarke Ellipsoid of 1866, then its latitude and longitude on that ellipsoid were defined and could be calculated.

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