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The US survey foot is defined so that 1 metre is exactly 39.37 inches, making the international foot of 0.3048 metres exactly two parts per million shorter. This is a difference of just over 3.2 mm, or a little more than one-eighth of an inch per mile.
The small difference between the survey foot and the international foot would not be detectable on a survey of a small parcel, but becomes significant for mapping, or when the state plane coordinate system (SPCS) is used in the US, because the origin of the system may be hundreds of thousands of feet (hundreds of miles) from the point of interest.
The National Institute of Standards and Technology formerly contended that customary area units are defined in terms of the square survey foot, not the square international foot, [17] but from 2023 it states that "although historically defined using the U.S. survey foot, the statute mile can be defined using either definition of the foot, as is ...
The US survey acre is about 4,046.872 square metres; its exact value (4046 + 13,525,426 / 15,499,969 m 2) is based on an inch defined by 1 metre = 39.37 inches exactly, as established by the Mendenhall Order of 1893. [6] Surveyors in the United States use both international and survey feet, and consequently, both varieties of acre. [7]
In 2020, the National Institute of Standards and Technology announced that the U.S. survey foot would "be phased out" on 1 January 2023 and be superseded by the international foot (also known as the foot) equal to 0.3048 metres exactly, for all further applications. [48] This implies that the survey inch was replaced by the international inch.
In 1959 United States kept the US survey foot as definition for the fathom. In October 2019, the U.S. National Geodetic Survey and the National Institute of Standards and Technology announced their joint intent to retire the U.S. survey foot, with effect from the end of 2022. The fathom in U.S. Customary units is thereafter defined based on the ...
The international foot is exactly 2 parts per million shorter than the US survey foot (as the article says) but the US survey foot is not exactly 2 parts per million longer than the international foot (for comparison 4 is 20% smaller than 5 but 5 is 25% bigger than 4). How about a proof in a footnote? JIMp talk·cont 01:46, 3 April 2013 (UTC)
These efforts are known as "High Accuracy Reference Network" (HARN) or "High Precision GPS Network" (HPGN). In addition, the basic unit of distance used is sometimes feet and sometimes meters. Thus a fully described coordinate system often looks something like: "Washington State Plane North, NAD83 HARN, US Survey feet".