The geographical mile is an international unit of length determined by 1

minute of arc A minute of arc, arcminute (abbreviated as arcmin), arc minute, or minute arc, denoted by the symbol , is a unit of angular measurement equal to of a degree. Since one degree is of a turn, or complete rotation, one arcminute is of a tu ...

( degree) along the

Earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...

equator The equator is the circle of latitude that divides Earth into the Northern Hemisphere, Northern and Southern Hemisphere, Southern Hemispheres of Earth, hemispheres. It is an imaginary line located at 0 degrees latitude, about in circumferen ...

. For the international ellipsoid 1924 this equalled 1855.4

metre The metre (or meter in US spelling; symbol: m) is the base unit of length in the International System of Units (SI). Since 2019, the metre has been defined as the length of the path travelled by light in vacuum during a time interval of of ...

s. '' The American Practical Navigator'' 2017 defines the geographical mile as . Greater precision depends more on the choice of the

Earth's radius Earth radius (denoted as ''R''🜨 or ''R''E) is the distance from the center of Earth to a point on or near its surface. Approximating the figure of Earth by an Earth spheroid (an oblate ellipsoid), the radius ranges from a maximum (equato ...

of the used

ellipsoid An ellipsoid is a surface that can be obtained from a sphere by deforming it by means of directional Scaling (geometry), scalings, or more generally, of an affine transformation. An ellipsoid is a quadric surface; that is, a Surface (mathemat ...

than on more careful measurement, since the radius of the

geoid The geoid ( ) is the shape that the ocean surface would take under the influence of the gravity of Earth, including gravitational attraction and Earth's rotation, if other influences such as winds and tides were absent. This surface is exte ...

varies more than along the equator. In any ellipsoid, the length of a degree of longitude at the equator is exactly 60 geographical miles. The Earth's radius at the

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 Earth's circumference is the distance around Earth. Measured around the equator, it is . Measured passing through the poles, the circumference is . Treating the Earth as a sphere, its circumference would be its single most important measuremen ...

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 A nautical mile is a unit of length used in air, marine, and space navigation, and for the definition of territorial waters. Historically, it was defined as the meridian arc length corresponding to one minute ( of a degree) of latitude at t ...

(which was historically linked to the circumference measured through both poles); 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 ( or ), 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. (For example, the Danish unit was computed as equivalent to about by the astronomer Ole Rømer.)

Relationship with the nautical mile

The geographical mile is closely related to the

, which was originally determined as 1 minute of arc along a

great circle In mathematics, a great circle or orthodrome is the circular intersection of a sphere and a plane passing through the sphere's center point. Discussion Any arc of a great circle is a geodesic of the sphere, so that great circles in spher ...

of the Earth but is nowadays defined by treaty as exactly 1,852 m. The US

National Institute of Standards and Technology The National Institute of Standards and Technology (NIST) is an agency of the United States Department of Commerce whose mission is to promote American innovation and industrial competitiveness. NIST's activities are organized into Outline of p ...

notes that "The international nautical mile of 1,852 meters (6,076.115 49... feet) 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 (geographical or sea) 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 . Scandinavians used their own version of the geographical mile as their

up to the beginning of the 20th century, causing it to be more well known as the sea mile in Danish (), Norwegian (), and Swedish ().

Use

The unit is not used much in English-speaking countries but is cited in some

United States The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...

laws. For example, Section 1301(a) of the Submerged Lands Act defines state seaward boundaries in terms of geographic miles. While debating what became the

Land Ordinance of 1785 The Land Ordinance of 1785 was adopted by the United States Congress of the Confederation on May 20, 1785. It set up a standardized system whereby settlers could purchase title to farmland in the undeveloped west. Congress at the time did not hav ...

Thomas Jefferson Thomas Jefferson (, 1743July 4, 1826) was an American Founding Fathers of the United States, Founding Father and the third president of the United States from 1801 to 1809. He was the primary author of the United States Declaration of Indepe ...

's committee wanted to divide the public lands in the west into " hundreds of ten geographical miles square, each mile containing 6,086 and 4-10ths of a foot" and "sub-divided into lots of one mile square each, or 850 and 4-10ths of an acre".

References

{{DEFAULTSORT:Geographical Mile Units of length

Historical units

Relationship with the nautical mile

Use

See also

References