The geographic coordinate system (GCS) is a
spherical
A sphere () is a geometrical object that is a three-dimensional analogue to a two-dimensional circle. A sphere is the set of points that are all at the same distance from a given point in three-dimensional space.. That given point is the ce ...
or
ellipsoidal coordinate system for measuring and communicating
positions directly on the
Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
as
latitude and
longitude
Longitude (, ) is a geographic coordinate that specifies the east– west position of a point on the surface of the Earth, or another celestial body. It is an angular measurement, usually expressed in degrees and denoted by the Greek lette ...
.
It is the simplest, oldest and most widely used of the various
spatial reference system
A spatial reference system (SRS) or coordinate reference system (CRS) is a framework used to precisely measure locations on the surface of the Earth as coordinates. It is thus the application of the abstract mathematics of coordinate systems and ...
s that are in use, and forms the basis for most others. Although latitude and longitude form a coordinate
tuple like a
cartesian coordinate system, the geographic coordinate system is not cartesian because the measurements are angles and are not on a planar surface.
A full GCS specification, such as those listed in the
EPSG and ISO 19111 standards, also includes a choice of
geodetic datum (including an
Earth ellipsoid
An Earth ellipsoid or Earth spheroid is a mathematical figure approximating the Earth's form, used as a reference frame for computations in geodesy, astronomy, and the geosciences. Various different ellipsoids have been used as approximations ...
), as different datums will yield different latitude and longitude values for the same location.
History
The
invention of a geographic coordinate system is generally credited to
Eratosthenes of
Cyrene, who composed his now-lost ''
Geography
Geography (from Greek: , ''geographia''. Combination of Greek words ‘Geo’ (The Earth) and ‘Graphien’ (to describe), literally "earth description") is a field of science devoted to the study of the lands, features, inhabitants, an ...
'' at the
Library of Alexandria in the 3rd century BC. A century later,
Hipparchus of
Nicaea improved on this system by determining latitude from stellar measurements rather than solar altitude and determining longitude by timings of
lunar eclipses, rather than
dead reckoning
In navigation, dead reckoning is the process of calculating current position of some moving object by using a previously determined position, or fix, and then incorporating estimates of speed, heading direction, and course over elapsed time. ...
. In the 1st or 2nd century,
Marinus of Tyre compiled an extensive gazetteer and
mathematically plotted world map using coordinates measured east from a
prime meridian at the westernmost known land, designated the
Fortunate Isles, off the coast of western Africa around the
Canary or
Cape Verde Islands
, national_anthem = ()
, official_languages = Portuguese
, national_languages = Cape Verdean Creole
, capital = Praia
, coordinates =
, largest_city = capital
, demonym ...
, and measured north or south of the island of
Rhodes off
Asia Minor.
Ptolemy credited him with the full adoption of longitude and latitude, rather than measuring latitude in terms of the length of the
midsummer
Midsummer is a celebration of the season of summer usually held at a date around the summer solstice. It has pagan pre-Christian roots in Europe.
The undivided Christian Church designated June 24 as the feast day of the early Christian martyr ...
day.
Ptolemy's 2nd-century
''Geography'' used the same prime meridian but measured latitude from the
Equator instead. After their work was translated into
Arabic in the 9th century,
Al-Khwārizmī's ''
Book of the Description of the Earth'' corrected Marinus' and Ptolemy's errors regarding the length of the
Mediterranean Sea, causing
medieval Arabic cartography to use a prime meridian around 10° east of Ptolemy's line. Mathematical cartography resumed in Europe following
Maximus Planudes' recovery of Ptolemy's text a little before 1300; the text was translated into
Latin
Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through the power of the ...
at
Florence
Florence ( ; it, Firenze ) is a city in Central Italy and the capital city of the Tuscany Regions of Italy, region. It is the most populated city in Tuscany, with 383,083 inhabitants in 2016, and over 1,520,000 in its metropolitan area.Bilan ...
by
Jacobus Angelus around 1407.
In 1884, the
United States
The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territori ...
hosted the
International Meridian Conference
The International Meridian Conference was a conference held in October 1884 in Washington, D.C., in the United States, to determine a prime meridian for international use. The conference was held at the request of U.S. President Chester A. ...
, attended by representatives from twenty-five nations. Twenty-two of them agreed to adopt the longitude of the
Royal Observatory in
Greenwich, England
Greenwich ( , ,) is a town in south-east London, England, within the ceremonial county of Greater London. It is situated east-southeast of Charing Cross.
Greenwich is notable for its maritime history and for giving its name to the Greenwich ...
as the zero-reference line. The
Dominican Republic
The Dominican Republic ( ; es, República Dominicana, ) is a country located on the island of Hispaniola in the Greater Antilles archipelago of the Caribbean region. It occupies the eastern five-eighths of the island, which it shares with ...
voted against the motion, while France and
Brazil
Brazil ( pt, Brasil; ), officially the Federative Republic of Brazil (Portuguese: ), is the largest country in both South America and Latin America. At and with over 217 million people, Brazil is the world's fifth-largest country by area ...
abstained. France adopted
Greenwich Mean Time
Greenwich Mean Time (GMT) is the mean solar time at the Royal Observatory in Greenwich, London, counted from midnight. At different times in the past, it has been calculated in different ways, including being calculated from noon; as a con ...
in place of local determinations by the
Paris Observatory in 1911.
Latitude and longitude
The "latitude" (abbreviation: Lat.,
φ, or phi) of a point on Earth's surface is the angle between the equatorial plane and the straight line that passes through that point and through (or close to) the center of the Earth. Lines joining points of the same latitude trace circles on the surface of Earth called
parallels, as they are parallel to the Equator and to each other. The
North Pole is 90° N; the
South Pole is 90° S. The 0° parallel of latitude is designated the
Equator, the
fundamental plane of all geographic coordinate systems. The Equator divides the globe into
Northern and
Southern Hemispheres.
The "longitude" (abbreviation: Long.,
λ, or lambda) of a point on Earth's surface is the angle east or west of a reference
meridian to another meridian that passes through that point. All meridians are halves of great
ellipses (often called
great circles), which converge at the North and South Poles. The meridian of the
British
British may refer to:
Peoples, culture, and language
* British people, nationals or natives of the United Kingdom, British Overseas Territories, and Crown Dependencies.
** Britishness, the British identity and common culture
* British English, ...
Royal Observatory in
Greenwich
Greenwich ( , ,) is a town in south-east London, England, within the ceremonial county of Greater London. It is situated east-southeast of Charing Cross.
Greenwich is notable for its maritime history and for giving its name to the Greenwich ...
, in southeast London, England, is the international
prime meridian, although some organizations—such as the French
Institut national de l'information géographique et forestière
An institute is an organisational body created for a certain purpose. They are often research organisations (research institutes) created to do research on specific topics, or can also be a professional body.
In some countries, institutes can ...
—continue to use other meridians for internal purposes. The prime meridian determines the proper
Eastern
Eastern may refer to:
Transportation
*China Eastern Airlines, a current Chinese airline based in Shanghai
*Eastern Air, former name of Zambia Skyways
*Eastern Air Lines, a defunct American airline that operated from 1926 to 1991
*Eastern Air Li ...
and
Western Hemispheres, although maps often divide these hemispheres further west in order to keep the
Old World on a single side. The
antipodal meridian of Greenwich is both 180°W and 180°E. This is not to be conflated with the
International Date Line, which diverges from it in several places for political and convenience reasons, including between far eastern Russia and the far western
Aleutian Islands
The Aleutian Islands (; ; ale, Unangam Tanangin,”Land of the Aleuts", possibly from Chukchi ''aliat'', "island"), also called the Aleut Islands or Aleutic Islands and known before 1867 as the Catherine Archipelago, are a chain of 14 large v ...
.
The combination of these two components specifies the position of any location on the surface of Earth, without consideration of
altitude or depth. The visual grid on a map formed by lines of latitude and longitude is known as a ''
graticule''. The origin/zero point of this system is located in the
Gulf of Guinea about south of
Tema
Tema is a city on the Bight of Benin and Atlantic coast of Ghana. It is located east of the capital city; Accra, in the region of Greater Accra, and is the capital of the Tema Metropolitan District. As of 2013, Tema is the eleventh most populo ...
,
Ghana
Ghana (; tw, Gaana, ee, Gana), officially the Republic of Ghana, is a country in West Africa. It abuts the Gulf of Guinea and the Atlantic Ocean to the south, sharing borders with Ivory Coast in the west, Burkina Faso in the north, and To ...
, a location often facetiously called
Null Island
Null Island is the point on Earth's surface at zero degrees latitude and zero degrees longitude (), i.e., where the prime meridian and the Equator intersect. Null Island is located in international waters in the Atlantic Ocean, roughly 600 km of ...
.
Geodetic datum
In order to be unambiguous about the direction of "vertical" and the "horizontal" surface above which they are measuring, map-makers choose a
reference ellipsoid with a given origin and orientation that best fits their need for the area to be mapped. They then choose the most appropriate mapping of the
spherical coordinate system onto that ellipsoid, called a terrestrial reference system or
geodetic datum.
Datums may be global, meaning that they represent the whole Earth, or they may be local, meaning that they represent an ellipsoid best-fit to only a portion of the Earth. Points on the Earth's surface move relative to each other due to continental plate motion, subsidence, and diurnal
Earth tidal movement caused by the
Moon
The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...
and the Sun. This daily movement can be as much as a meter. Continental movement can be up to a year, or in a century. A
weather system high-pressure area can cause a sinking of .
Scandinavia is rising by a year as a result of the melting of the ice sheets of the
last ice age, but neighboring
Scotland
Scotland (, ) is a Countries of the United Kingdom, country that is part of the United Kingdom. Covering the northern third of the island of Great Britain, mainland Scotland has a Anglo-Scottish border, border with England to the southeast ...
is rising by only . These changes are insignificant if a local datum is used, but are statistically significant if a global datum is used.
Examples of global datums include
World Geodetic System (WGS84, also known as EPSG:4326
), the default datum used for the
Global Positioning System
The Global Positioning System (GPS), originally Navstar GPS, is a satellite-based radionavigation system owned by the United States government and operated by the United States Space Force. It is one of the global navigation satellite sy ...
, and the
International Terrestrial Reference System and Frame
The International Terrestrial Reference System (ITRS) describes procedures for creating reference frames suitable for use with measurements on or near the Earth's surface. This is done in much the same way that a physical standard might be descri ...
(ITRF), used for estimating
continental drift
Continental drift is the hypothesis that the Earth's continents have moved over geologic time relative to each other, thus appearing to have "drifted" across the ocean bed. The idea of continental drift has been subsumed into the science of pl ...
and
crustal deformation.
The distance to Earth's center can be used both for very deep positions and for positions in space.
Local datums chosen by a national cartographical organization include the
North American Datum, the European
ED50
ED50 ("European Datum 1950", EPSG:4230) is a geodetic datum which was defined after World War II for the international connection of geodetic networks.
Background
Some of the important battles of World War II were fought on the borders of Ger ...
, and the British
OSGB36. Given a location, the datum provides the latitude
and longitude
. In the United Kingdom there are three common latitude, longitude, and height systems in use. WGS84 differs at Greenwich from the one used on published maps
OSGB36 by approximately 112m. The military system
ED50
ED50 ("European Datum 1950", EPSG:4230) is a geodetic datum which was defined after World War II for the international connection of geodetic networks.
Background
Some of the important battles of World War II were fought on the borders of Ger ...
, used by
NATO
The North Atlantic Treaty Organization (NATO, ; french: Organisation du traité de l'Atlantique nord, ), also called the North Atlantic Alliance, is an intergovernmental military alliance between 30 member states – 28 European and two No ...
, differs from about 120m to 180m.
[
The latitude and longitude on a map made against a local datum may not be the same as one obtained from a GPS receiver. Converting coordinates from one datum to another requires a datum transformation such as a Helmert transformation, although in certain situations a simple translation may be sufficient.]
In popular GIS software, data projected in latitude/longitude is often represented as a ''Geographic Coordinate System''. For example, data in latitude/longitude if the datum is the North American Datum of 1983 is denoted by 'GCS North American 1983'.
Length of a degree
On the GRS80 or WGS84 spheroid at sea level
Mean sea level (MSL, often shortened to sea level) is an average surface level of one or more among Earth's coastal bodies of water from which heights such as elevation may be measured. The global MSL is a type of vertical datuma standardise ...
at the Equator, one latitudinal second measures 30.715 meters, one latitudinal minute is 1843 meters and one latitudinal degree is 110.6 kilometers. The circles of longitude, meridians, meet at the geographical poles, with the west–east width of a second naturally decreasing as latitude increases. On the Equator at sea level, one longitudinal second measures 30.92 meters, a longitudinal minute is 1855 meters and a longitudinal degree is 111.3 kilometers. At 30° a longitudinal second is 26.76 meters, at Greenwich (51°28′38″N) 19.22 meters, and at 60° it is 15.42 meters.
On the WGS84 spheroid, the length in meters of a degree of latitude at latitude φ (that is, the number of meters you would have to travel along a north–south line to move 1 degree in latitude, when at latitude φ), is about
The returned measure of meters per degree latitude varies continuously with latitude.
Similarly, the length in meters of a degree of longitude can be calculated as
(Those coefficients can be improved, but as they stand the distance they give is correct within a centimeter.)
The formulae both return units of meters per degree.
An alternative method to estimate the length of a longitudinal degree at latitude is to assume a spherical Earth (to get the width per minute and second, divide by 60 and 3600, respectively):
where Earth's average meridional radius is . Since the Earth is an oblate spheroid, not spherical, that result can be off by several tenths of a percent; a better approximation of a longitudinal degree at latitude is
where Earth's equatorial radius equals ''6,378,137 m'' and ; for the GRS80 and WGS84 spheroids, b/a calculates to be 0.99664719. ( is known as the reduced (or parametric) latitude). Aside from rounding, this is the exact distance along a parallel of latitude; getting the distance along the shortest route will be more work, but those two distances are always within 0.6 meter of each other if the two points are one degree of longitude apart.
Alternate encodings
Like any series of multiple-digit numbers, latitude-longitude pairs can be challenging to communicate and remember. Therefore, alternative schemes have been developed for encoding GCS coordinates into alphanumeric strings or words:
* the Maidenhead Locator System
The Maidenhead Locator System (a.k.a. QTH Locator and IARU Locator) is a geocode system used by amateur radio operators to succinctly describe their geographic coordinates, which replaced the deprecated QRA locator, which was limited to Europea ...
, popular with radio operators.
* the World Geographic Reference System
The World Geographic Reference System (GEOREF) is a geocode, a grid-based method of specifying locations on the surface of the Earth. GEOREF is essentially based on the geographic system of latitude and longitude, but using a simpler and more fle ...
(GEOREF), developed for global military operations, replaced by the current Global Area Reference System (GARS).
* Open Location Code
The Open Location Code (OLC) is a geocode based in a system of regular grids for identifying an area anywhere on the Earth.
It was developed at Google's Zürich engineering office, and released late October 2014. Location codes created by the OL ...
or "Plus Codes", developed by Google and released into the public domain.
* Geohash
Geohash is a public domain geocode system invented in 2008 by Gustavo NiemeyerEvidences at the Wayback Machine:
labix.org in 2008, the G. Niemeyer's blog announcing Geohash
*an article about Geohash witnessing and citing G. Niemeyer works, befor ...
, a public domain system based on the Morton Z-order curve.
* What3words, a proprietary system that encodes GCS coordinates as pseudorandom sets of words by dividing the coordinates into three numbers and looking up words in an indexed dictionary.
* Military Grid Reference System is the geocoordinate standard used by NATO militaries for locating points on Earth.
See also
*
*
*
*
* ISO 6709, standard representation of geographic point location by coordinates
*
*
* Planetary coordinate system
** Selenographic coordinate system
The selenographic coordinate system is used to refer to locations on the surface of Earth's moon. Any position on the lunar surface can be referenced by specifying two numerical values, which are comparable to the latitude and longitude of Earth. ...
*
* Jan Smits (2015)
Mathematical data for bibliographic descriptions of cartographic materials and spatial data
''Geographical co-ordinates''. ICA Commission on Map Projections.
Notes
References
Sources
* ''Portions of this article are from Jason Harris' "Astroinfo" which is distributed with KStars
KStars is a freely licensed planetarium program using the KDE Platform. It is available for Linux, BSD, macOS, and Microsoft Windows. A light version of KStars is available for Android devices. It provides an accurate graphical representation ...
, a desktop planetarium for Linux
Linux ( or ) is a family of open-source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically packaged as a Linux distribution, w ...
/ KDE. Se
The KDE Education Project - KStars
'
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