<P> 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 . </P> <P> As more data were gathered, discrepancies appeared, so the datum was recomputed in 1927, using the same spheroid and origin as its predecessor . </P> <P> The North American Datum of 1927 (NAD 27) 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). NAD 27 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 . </P> <Table> <Tr> <Td> Ellipsoid </Td> <Td> Semimajor axis † </Td> <Td> Semiminor axis † </Td> <Td> Inverse flattening † † </Td> </Tr> <Tr> <Td> Clarke 1866 </Td> <Td> 6,378,206.4 m </Td> <Td> 6,356,583.8 m </Td> <Td> 294.978698214 </Td> </Tr> </Table>

What was the purpose of the nad27 datum
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