<P> When international measure was introduced in the English - speaking countries, the basic geodetic datum in North America was the North American Datum of 1927 (NAD27), which had been constructed by triangulation based on the definition of the foot in the Mendenhall Order of 1893, that is 1 foot = ⁄ meters: this definition was retained for data derived from NAD27, but renamed the US survey foot to distinguish it from the international foot . For most applications, the difference between the two definitions is insignificant--one international foot is exactly 0.999998 of a US survey foot, for a difference of about ⁄ inch (3 mm) per mile--but it affects the definition of the State Plane Coordinate Systems (SPCSs), which can stretch over hundreds of miles . </P> <P> The NAD27 was replaced in the 1980s by the North American Datum of 1983 (NAD83), which is defined in meters . The SPCSs were also updated, but the National Geodetic Survey left the decision of which (if any) definition of the foot to use to the individual states . All SPCSs are defined in meters, but seven states also have SPCSs defined in US survey feet and an eighth state in international feet: the other 42 states use only meter - based SPCSs . </P> <P> State legislation is also important for determining the conversion factor to be used for everyday land surveying and real estate transactions, although the difference (2 ppm) is of no practical significance given the precision of normal surveying measurements over short distances (usually much less than a mile). Twenty - four states have legislated that surveying measures should be based on the US survey foot, eight have legislated that they be made on the basis of the international foot, and eighteen have not specified the conversion factor from metric units . </P> <Table> <Tr> <Th> Unit </Th> <Th> Divisions </Th> <Th> SI equivalent </Th> </Tr> <Tr> <Td_colspan="3"> Exact relationships shown in boldface </Td> </Tr> <Tr> <Td> 1 square survey foot (sq ft or ft) </Td> <Td> 144 square inches </Td> <Td> 6998929034100000000 ♠ 0.092 903 41 m </Td> </Tr> <Tr> <Td> 1 square chain (sq ch or ch) </Td> <Td> 7003435600000000000 ♠ 4356 sq ft (survey) or 16 sq rods </Td> <Td> 7002404687300000000 ♠ 404.6873 m </Td> </Tr> <Tr> <Td> 1 acre </Td> <Td> 7004435600000000000 ♠ 43 560 sq ft (survey) or 10 sq ch </Td> <Td> 7003404687300000000 ♠ 4046. 873 m </Td> </Tr> <Tr> <Td> 1 section </Td> <Td> 640 acres or 1 sq mile (survey) </Td> <Td> 7006258999800000000 ♠ 2.589 998 km </Td> </Tr> <Tr> <Td> 1 survey township (twp) </Td> <Td> 36 sections or 4 sq leagues </Td> <Td> 7007932399300000000 ♠ 93.239 93 km </Td> </Tr> </Table>

The system of measurement that uses inches feet and gallons is the