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At the time of the agreement, 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 ft = 1200 ⁄ 3937 m: this definition was retained for data derived from NAD27, but renamed the US survey ...
≡ 404.685 642 24 m 2: square chain (US Survey) sq ch ≡ 66 ft (US) × 66 ft (US) = 1 ⁄ 10 US survey acre ≈ 404.6873 m 2: square foot: sq ft ≡ 1 ft × 1 ft: ≡ 9.290 304 × 10 −2 m 2: square foot (US Survey) sq ft ≡ 1 ft (US) × 1 ft (US) ≈ 9.290 341 161 3275 × 10 −2 m 2: square inch: sq in ≡ 1 in × 1 in: ≡ 6.4516 × 10 ...
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.
If the railway ran predominantly east and west, a 10-mile (16 km) range of one square mile sections was allotted on each side of the 400-foot (120 m) right-of-way. If the railway ran predominantly north and south, a 10-mile (16 km) township of one square mile sections was allotted on each side of the 400-foot (120 m) right-of-way.
In the United States, the term statute mile formally refers to the survey mile, [3] but for most purposes, the difference of less than 1 ⁄ 8 inch (3.2 mm) between the survey mile and the international mile (1609.344 metres exactly) is insignificant—one international mile is 0.999 998 US survey miles—so statute mile can be used for either.
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historical definitions of the units and their derivatives used in old measurements; e.g., international foot vs. US survey foot. For some purposes, conversions from one system of units to another are needed to be exact, without increasing or decreasing the precision of the expressed quantity.
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.