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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.
==First North American Datum==
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.〔Shalowitz, USC&GS Field Engineers Bulletin, December 1938〕〔Craig, Cindy (October 2013) ("Where Theory Meets Practice: United States Standard Datum" ) ''Professional Surveyor Magazine'' Frederick, Maryland〕
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