Geographical mile
The geographical mile is an international unit of length determined by 1 minute of arc along the Earth's equator. For the [Hayford ellipsoid|international ellipsoid 1924] this equalled 1855.4 metres. The American Practical Navigator 2017 defines the geographical mile as. Greater precision depends more on the choice of the Earth's radius of the used ellipsoid than on more careful measurement, since the radius of the geoid varies more than along the equator. In any ellipsoid, the degree of longitude">degree (angle)">degree of longitude at the equator is exactly 60 geographical miles. The Earth's radius at the equator in the GRS80 ellipsoid is, which makes the geographical mile 1,855.3248 m. The rounding of the Earth's radius to metres in GRS80 has an effect of 0.0001 m.
The shape of the Earth is a slightly flattened sphere, which results in the Earth's circumference being 0.168% larger when measured around the equator as compared to through the poles. The geographical mile is slightly larger than the nautical mile ; one geographic mile is equivalent to approximately.
Historical units
Historically, certain nations used slightly different divisions to create their geographical miles.The Portuguese system derived their miles as one third of their league of three separate values. When each equatorial degree was divided into 18 leagues, the geographical mile was equal to degree or about ; when divided into 20 leagues, the geographical mile was equal to degree, approximating the values provided above; and when divided into 25 leagues, the geographical mile was equal to degree or about.
The geographical miles of the traditional Dutch, German, and Danish systems all approximated their much longer milesequivalent to English leaguesby using a larger division of the equatorial degree. Instead of using one minute of arc, they all used four degreeto produce a distance now notionally equal to but actually differing slightly depending on official measurements and computations.
Relationship with the nautical mile
The geographical mile is closely related to the nautical mile, which was originally determined as 1 minute of arc along a great circle of the Earth but is nowadays defined by treaty as exactly 1,852 m. The US National Institute of Standards and Technology notes that "The international nautical mile of 1,852 meters was adopted effective July 1, 1954, for use in the United States. The value formerly used in the United States was 6,080.20 feet = 1 nautical mile." This deprecated value of 6,080.2 feet is equivalent to. A separate reference identifies the geographic mile as being identical to the international nautical mile of 1,852 m and slightly shorter than the British nautical mile of.Scandinavian nautical mile
Scandinavians used their own version of the geographical mile as their nautical mile up to the beginning of the 20th century, indigenously known as a sea mile. It was defined as of an equatorial degree, equivalent to approximately four modern nautical miles or "medium meridian minutes" – a nautical mile is approximately one sixtieth of a degree along a meridian.During metrification in 1875, this brought it down to about from its former equivalence of 3950 fathoms or about.