Mile - Statute Mile

The statute mile was so-named because it was defined by an English Act of Parliament in 1593, during the reign of Queen Elizabeth I. The statute states: "A Mile ſhall contain eight Furlongs, every Furlong forty Poles, and every Pole ſixteen Foot and a half." (35 Eliz. cap. 6.) It was thus 1760 yards (5280 feet, about 1609 metres). For surveying, the statute mile is divided into eight furlongs; each furlong into ten chains; each chain into four rods (also known as poles or perches); and each rod into 25 links. This makes the rod equal to 5½ yards or 16½ feet in both Imperial and US usage.

The exact conversion of the mile to SI units depends on which definition of the yard is used. Different English-speaking countries maintained independent physical standards for the yard that were found to differ by small, but measurable, amounts and even to slowly shorten in length. The US redefined the US yard in 1893, but this resulted in US and Imperial measures of distance having very slightly different lengths. The difference was resolved in 1959 with the definition of the international yard in terms of the metre by Australia, Canada, New Zealand, South Africa, the UK and the US. The "international mile" of 1760 international yards is exactly 1609.344 metres.

The difference from the previous standards was 2 ppm, or about 3.2 millimetres (⅛ inch) per mile. The US standard was slightly longer and the old Imperial standards had been slightly shorter than the international mile. When the international mile was introduced in English-speaking countries, the basic geodetic datum in North America was the North American Datum of 1927 (NAD27). This had been constructed by triangulation based on the definition of the foot in the Mendenhall Order of 1893, with 1 foot = 1200⁄₃₉₃₇ metres and the definition was retained for data derived from NAD27, but renamed the US survey foot to distinguish it from the international foot.

The US survey mile is 5280 survey feet, or about 1609.347 218 694 metres. In the US, statute mile formally refers to the survey mile, but for most purposes, the difference between the survey mile and the international mile is insignificant—one international mile is exactly 0.999 998 of a US survey mile—so statute mile can be used for either. But in some cases, such as in the US State Plane Coordinate Systems (SPCSs), which can stretch over hundreds of miles, the accumulated difference can be significant, so it is important to note that the reference is to the US survey mile.

The North American Datum of 1983 (NAD83), which replaced the NAD27, is defined in metres. State Plane Coordinate Systems were then updated, but the National Geodetic Survey left individual states to decide which (if any) definition of the foot they would use. All State Plane Coordinate Systems are defined in metres, and 42 of the 50 states only use the metre-based State Plane Coordinate Systems. However, eight states also have State Plane Coordinate Systems defined in feet, seven of them in US Survey feet and one in international feet. State legislation in the US is important for determining which conversion factor from the metric datum is to be used for land surveying and real estate transactions, even though the difference (2 ppm) is hardly significant, given the precision of normal surveying measurements over short distances (usually much less than a mile). Twenty-four states have legislated that surveying measures be based on the US survey foot, eight have legislated that they be based on the international foot, and eighteen have not specified which conversion factor to use.

The old Imperial value of the yard was used in converting measurements to metric values in India in a 1976 Act of the Indian Parliament. However, The current National Topographic Database of the Survey of India is based on the metric WGS-84 datum, which is also used by the Global Positioning System.