Mile

A mile is a unit of length in a number of different systems. In contemporary English, mile most commonly refers to the statute mile of 5,280 feet (exactly 1,609.344 meters) or the nautical mile of 1,852 meters (about 6,076.1 ft). There are many other historical miles, and similar units in other systems translated as miles in English, varying between one and fifteen kilometers. It is about a third of the old measurement, the league.

Use of the mile as a unit of measurement is largely confined the United States and the United Kingdom where it remains customary. Elsewhere it has been replaced by the kilometer as a measure of distance.

There have been several abbreviations for mile (with and without trailing period): mi, ml, m, M. In the United States, the National Institute of Standards and Technology now uses and recommends mi but in everyday usage (at least in the United States and in the United Kingdom) usages such as miles per hour and miles per gallon are almost always abbreviated as mph or mpg (rather than mi/h or mi/gal).

The formula "multiply by 8 and divide by 5" to convert miles to kilometers gives a conversion of 1.6. As it is only slightly lower than the actual conversion factors, it is a useful approximation.

Statute mile
The statute mile was defined by English Act of Parliament (hence the name) in 1592, during the reign of Queen Elizabeth I; it is equal to 1760 yards (5280 feet). For surveying, the statute mile is divided into eight furlongs; each furlong is ten chains; each chain is four rods (also known as poles or perches); and each rod is 25 links. This makes the rod equal to 5½ yards or 16½ feet in both Imperial and U.S. usage.

The exact conversion of the mile to SI units depends on which definition of the yard is in use. 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 United States redefined the U.S. yard in 1893, but this resulted in U.S. and Imperial units with the same names having very slightly different values. 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 United Kingdom, and the United States. The "international mile" of 1760 international yards is exactly 1609.344 metres.

The difference from the previous standards was about 2 ppm, or about a tenth of an inch in each mile, the old U.S. standard being slightly longer and the old Imperial standards slightly shorter than the international mile. The older standards for the yard (and hence the foot and the mile) continue in use for some surveying purposes in the United States. and 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.

For most applications, the difference between the two definitions is insignificant — one international foot is exactly 0.999998 of a U.S. survey foot, for a difference of about 3 millimeters per mile — but it affects the definition of the State Plane Coordinate Systems (SPCSs), which can stretch over hundreds of miles. 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 = $1200/3937$ meters: this definition was retained for data derived from NAD27, but renamed the U.S. survey foot to distinguish it from the international foot.

The NAD27 was replaced in the 1980s by The North American Datum of 1983 (NAD83), which is defined in meters, replaced the North American Datum of 1927. The State Plane Coordinate Systems 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 State Plane Coordinate Systems are defined in meters, but seven states also have State Plane Coordinate Systems defined in U.S. survey feet and an eighth state in international feet: the other 42 states use only meter-based State Plane Coordinate Systems. 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.

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). In the U.S., twenty-four states have legislated that surveying measures should be based on the U.S. 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.

Historical miles in Britain and Ireland
The statute of Elizabeth I was not the only definition of the mile in Britain and Ireland. Perhaps the earliest tables of English linear measures, Arnold's Customs of London (c. 1500) indicates a mile consisted of 8 furlongs, each of 625 feet, for a total of 5000 feet (1666⅔ yards, 0.95 statute miles): this is the same definition of the mile in terms of feet as used by the Romans. The "old English" mile of medieval and early modern times appears to have measured approximately 1.3 statute miles.

Scots mile
The Scots mile was longer than the English mile, but varied in length from place to place. It was formally abolished by an Act of the Parliament of Scotland in 1685, and again by the Treaty of Union with England in 1707, but continued in use as a customary unit during the 18th century. It was obsolete by the time of its final abolition by the Weights and Measures Act 1824. An estimate of its length can be made from other Scots units: in Scots, the rod was usually called the fall or faw, and was equal to six ells of 37 inches. As there are 320 rods in a mile, this would make the Scots mile equal to 5920 feet (1973⅓ yards, 1.12 statute miles). Other estimates are similar.

Irish mile
The Irish mile was longer still. In Elizabethan times, four Irish miles was often equated to five English, though whether the statute mile or the "old English" mile is unclear. By the seventeenth century, it was 2240 yards (6720 feet, 1.27 statute miles) Again, the difference arose from a different length of the rod in Ireland (usually called the perch locally): 21 feet as opposed to 16½ feet in England.

From 1774, through the forced 1801 union with Britain, until the 1820s, the grand juries of 25 Irish counties commissioned surveyed maps at scales of one or two inches per Irish mile. Scottish engineer William Bald's County Mayo maps of 1809–30 were drawn in English miles and rescaled to Irish miles for printing. The Howth–Dublin Post Office extension of the London–Holyhead turnpike engineered by Thomas Telford had mileposts in English miles. Although legally abolished by the Weights and Measures Act 1824, the Irish mile was used till 1856 by the Irish Post Office. The Ordnance Survey of Ireland, from its establishment in 1824, used English miles.

In 1894, Alfred Austin complained after visiting Ireland that "the Irish mile is a fine source of confusion when distances are computed. In one county a mile means a statute mile, in another it means an Irish mile". When the Oxford English Dictionary definition of "mile" was published in 1906, it described the Irish mile as "still in rustic use". A 1902 guide says regarding milestones, "Counties Dublin, Waterford, Cork, Antrim, Down, and Armagh use English, but Donegal Irish Miles ; the other counties either have both, or only one or two roads have Irish". Variation in signage persisted till the publication of standardised road traffic regulations by the Irish Free State in 1926. In 1937, a man prosecuted for driving outside the 15-mile limit of his licence offered the unsuccessful defence that, since the state was independent, the limit ought to use Irish miles, "just as no one would ever think of selling land other than as Irish acres". A 1965 proposal by two TDs, to replace statute miles with Irish miles in a clause of the Road Transport Act, was rejected. The term is now obsolete as a specific measure, though an "Irish mile" colloquially is a long but vague distance akin to a "country mile". Outside of its downtown core, but within its newer subdivisions, Toronto' street grid is based on the Irish mile.

Metric mile
The term metric mile is used in sports such as athletics and speed skating to denote a distance of 1500 meters. In United States high school competition, the term is sometimes used for a race of 1600 meters.

Nautical mile


The nautical mile was originally defined as one minute of arc along a meridian of the Earth. It is a convenient reference since it is fairly constant at all latitudes, in contrast with degrees of longitude which vary from 60 NM at the equator to zero at the poles.

Navigators use dividers to step off the distance between two points on the navigational chart, then place the open dividers against the minutes-of-latitude scale at the edge of the chart, and read off the distance in nautical miles. Since it is now known that the Earth is not perfectly spherical but an oblate spheroid, the length derived from this method varies slightly from the equator to the poles. For instance, using the WGS84 Ellipsoid, the commonly accepted Earth model for many purposes today, one minute of latitude at the WGS84 equator is 6,087 feet and at the poles is 6,067 feet. On average it is about 6,076 feet (about 1852 meters or 1.15 statute miles).

In the United States of America, the nautical mile was defined in the nineteenth century as 6,080.2 feet (1,853.249 m), whereas in the United Kingdom the Admiralty nautical mile was defined as 6,080 feet (1,853.184 m) and was approximately one minute of latitude in the latitudes of the south of the UK. Other nations had different definitions of the nautical mile, but it is now internationally defined to be exactly 1,852 meters.

Related nautical units
The nautical mile per hour is known as the knot. Nautical miles and knots are almost universally used for aeronautical and maritime navigation because of their relationship with degrees and minutes of latitude and the convenience of using the latitude scale on a map for distance measuring.

The data mile is used in radar-related subjects and is equal to 6,000 feet (1.8288 kilometers). The radar mile is a unit of time (in the same way that the light year is a unit of distance), equal to the time required for a radar pulse to travel a distance of two miles (one mile each way). Thus, the radar statute mile is 10.8 μs and the radar nautical mile is 12.4 μs.

Roman mile
In Roman times, the unit of long distance mille passuum (literally "a thousand paces" in Latin, with one pace being equal to two steps) was first used by the Romans and denoted a distance of 1,000 paces or 5,000 Roman feet, and is estimated to correspond to about 1,479 meters (1,617 yards). This unit is now known as the Roman mile. This unit spread throughout the Roman empire, often with modifications to fit local systems of measurements.

Grid system
Cities in the continental United States often have streets laid out by miles. Phoenix, Arizona ,Philadelphia, Las Vegas and Miami are several examples. Typically the largest streets are about one mile apart, with others at half-mile, quarter-mile intervals. Also in Manhattan (New York) "streets" are almost exactly 20 per mile, while "avenues" are about six per mile.

Other miles

 * The Arab mile (or Arabic mile) was a unit of length used by medieval Muslim geographers. Its precise length is uncertain, but believed to be around 1925 meters.
 * The Danish mil (traditional) was 24,000 Danish feet or 7532.5 meters. Sometimes it was interpreted as exactly 7.5 kilometers. It is the same as the north German Meile (below).
 * The Meile was a traditional unit in German speaking countries, much longer than a western European mile. It was 24,000 German feet; the SI equivalent was 7586 meters in Austria or 7532.5 meters in northern Germany. There was a version known as the geographische Meile which was 4 Admiralty nautical miles, 7,412.7 meters, or 1/15 degree.
 * In Norway and Sweden a mil is a unit of length which is equal to 10 kilometers and commonly used in everyday language. However in more formal situations, like on road signs and when there is risk of confusion with English miles, kilometers are used instead. The traditional Swedish mil spanned the range from 6000-14,485 meters, depending on province. It was however standardized in 1649 to 36,000 Swedish feet, or 10.687 kilometers . The Norwegian mil was 11.298 kilometers. When the metric system was introduced in the Norwegian-Swedish union in 1889, one standardized the mil to exactly 10 kilometers
 * The Russian milya (руѝѝкаѝ милѝ) was a traditional Russian unit of distance, equal to 7 verst, or 7.468 km
 * The hrvatska milja (Croatian mile) is 11,130 meters = 11.13 kilometers = 1/10 of equator's degree., first time used by Jesuit Stjepan Glavaĝ on map from 1673.
 * The banska milja (also called hrvatska milja) (mile of Croatian Ban, Croatian mile) was 7586 meters = 7.586 kilometers, or 24,000 feet..

Idioms
Even in countries that have moved from the Imperial to the Metric system (for example, Australia and New Zealand), the mile is still used in a variety of idioms. These include:
 * A country mile is used colloquially to denote a very long distance.
 * "An inch is as good as a mile" (failure by a narrow margin is no better than any other failure)
 * "Give him an inch and he'll take a mile" (the person in question will become greedy if shown generosity)
 * "Missed by a mile" (missed by a wide margin)
 * "Talk a mile a minute" (speak at a rapid rate)
 * "To go the extra mile" (to put in extra effort)
 * "Miles away" (lost in thought, or daydreaming)
 * "Milestone" (an event indicating significant progress)