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| {{Otheruses4|geographic coordinates|the use of coordinates on Wikipedia pages|Wikipedia:WikiProject Geographical coordinates}}
| | #REDIRECT [[wp:Geographic_coordinate_system]] |
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| ''Graticule redirects here and may mean, a wire or cross lines in an optical focusing system''
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| {{pp-move-indef|small=yes}}
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| [[File:WorldMapLongLat-eq-circles-tropics-non.png|thumb|440px|Map of [[Earth]] showing lines of [[latitude]] (horizontally) and [[longitude]] (vertically), Eckert VI projection; [https://www.cia.gov/library/publications/the-world-factbook/graphics/ref_maps/pdf/political_world.pdf large version] (pdf, 3.12MB)]]
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| A '''geographic coordinate system''' is a [[coordinate system]] that enables every location on Earth to be specified in three coordinates, using mainly a [[spherical coordinate system]].
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| The Earth is not a [[sphere]], but an irregular shape approximating an [[ellipsoid]]; the challenge is to define a coordinate system that can accurately state each topographical point as an unambiguous tuple of numbers.<ref name=OSGB>[http://www.ordnancesurvey.co.uk/oswebsite/gps/docs/A_Guide_to_Coordinate_Systems_in_Great_Britain.pdf A Guide to coordinate systems in Great Britain] v1.7 Oct 2007 D00659 accessed 14.4.2008</ref>
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| ==Latitude and longitude==
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| {{Main|Latitude|Longitude}}
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| [[File:Geographic coordinates sphere.svg|thumb|Latitude phi (φ) and Longitude lambda (λ)]]<!-- this image needs to be redrawn -->
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| [[Latitude]] (abbreviation: Lat., [[φ]], or phi) is the angle from a point on the Earth's surface to the [[equator|equatorial plane]], measured from the center of the sphere. Lines joining points of the same latitude are called [[circle of latitude|parallels]], which trace concentric circles on the surface of the Earth, parallel to the equator. The [[north pole]] is 90° N; the [[south pole]] is 90° S. The 0° parallel of latitude is designated the [[equator]], the [[fundamental plane (spherical coordinates)|fundamental plane]] of all geographic coordinate systems. The equator divides the globe into Northern and Southern Hemispheres.
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| [[Longitude]] (abbreviation: Long., [[λ]], or lambda) is the angle east or west of a reference meridian between the two geographical poles to another [[meridian (geography)|meridian]] that passes through an arbitrary point. All meridians are halves of great circles, and are not parallel. They converge at the north and south poles.
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| A line passing to the rear of the [[Royal Observatory, Greenwich]] (near London in the [[United Kingdom|UK]]) has been chosen as the international zero-longitude reference line, the [[Prime Meridian]]. Places to the east are in the eastern hemisphere, and places to the west are in the western hemisphere. The [[Antipodes|antipodal]] meridian of Greenwich is both 180°W and 180°E.
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| In 1884, the United States hosted the [[International Meridian Conference]] and twenty-five nations attended. Twenty-two of them agreed to adopt the location of Greenwich as the zero-reference line. [[Dominican Republic|San Domingo]] voted against the adoption of that motion, while [[France]] and [[Brazil]] abstained.<ref>http://wwp.millennium-dome.com/info/conference.htm</ref> To date, there exist organizations around the world which continue using historical prime meridians before the acceptance of Greenwich and the ill-attended conference became common-place.<ref name=ign>The French Institut Géographique National (IGN) still displays a latitude and longitude on its maps centred on a meridian that passes through Paris</ref>
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| The combination of these two components specifies the position of any location on the planet, but does not consider [[altitude]] nor [[depth]].
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| For example, [[Baltimore, Maryland]] (in the [[United States|USA]]) has a latitude of 39.3° North, and a longitude of 76.6° West. So, a vector drawn from the center of the Earth to a point 39.3° north of the equator and 76.6° west of Greenwich will pass through Baltimore.
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| This latitude/longitude "webbing" is known as the '''''conjugate graticule'''''.
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| In defining an [[ellipse]], the vertical diameter is known as the '''''conjugate diameter''''', and the horizontal diameter — perpendicular, or "transverse", to the conjugate — is the '''''transverse diameter'''''.<ref>{{cite web | last = Haswell | first = Charles Haynes | url = http://books.google.com/books?id=Uk4wAAAAMAAJ&pg=RA1-PA381&zoom=3&hl=en&sig=3QTM7ZfZARnGnPoqQSDMbx8JeHg | title = Mechanics' and Engineers' Pocket-book of Tables, Rules, and Formulas | publisher = Harper & Brothers | date = 1920 | accessdate = 2007-04-09 }}</ref> With a sphere or ellipsoid, the conjugate diameter is known as the '''''[[Semi-minor axis|polar axis]]''''' and the transverse as the '''''[[Semi-major axis|equatorial axis]]'''''. The graticule [[Perspective (graphical)|perspective]] is based on this designation: As the longitudinal rings — geographically defined, all great circles — converge at the poles, it is the poles that the conjugate graticule is defined. If the polar vertex is "pulled down" 90°, so that the vertex is on the equator, or transverse diameter, then it becomes the '''transverse graticule''', upon which all [[spherical trigonometry]] is ultimately based (if the longitudinal vertex is between the poles and equator, then it is considered an '''''oblique graticule''''').
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| <!-- a whole separate article on "transverse graticule" is planned -->
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| ===Degrees: a measurement of angle===
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| {{Main|Angle}}<!-- Attention: the definition of latitude in this article refers to the centre of the earth-->
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| There are several formats for writing degrees, all of them appearing in the same Lat, Long order.
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| * '''DMS''' Degrees:Minutes:Seconds (49°30'00"N, 123°30'00"W)
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| * '''DM''' Degrees:Decimal Minutes (49°30.0', -123°30.0'), (49d30.0m,-123d30.0')
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| * '''DD''' [[Decimal degrees|Decimal Degrees]] (49.5000°,-123.5000°), generally with 4-6 decimal numbers.
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| <!--To convert from DM or DMS to DD, decimal degrees = whole number of degrees, plus minutes divided by 60, plus seconds divided by 3600.-->
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| ==Geodesic height==
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| To completely specify a location of a topographical feature on, in, or above the Earth, one has to also specify the vertical distance from the centre of the sphere, or from the surface of the sphere. Because of the ambiguity of "surface" and "vertical", it is more commonly expressed relative to a more precisely defined [[datum (geodesy)|vertical datum]] such as [[mean sea level]] at a named point. Each country has defined its own datum. In the [[United Kingdom]], the reference point is [[Newlyn]]. The distance to Earth's centre can be used both for very deep positions and for positions in space.<ref name=OSGB/>
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| ==Cartesian coordinates==
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| Every point that is expressed in spherical coordinates can be expressed as an {{nowrap|x y z}} ([[Cartesian coordinate|Cartesian]]) coordinate. This is not a useful method for recording a position on maps but is used to calculate distances and to perform other mathematical operations. The origin is usually the center of the sphere, a point close to the center of the Earth.
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| ==Shape of the Earth==
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| The Earth is not a sphere, but an irregular shape approximating a [[Earth ellipsoid|biaxial ellipsoid]]. It is nearly spherical, but has an equatorial bulge making the radius at the equator about 0.3% larger than the radius measured through the poles. The shorter axis approximately coincides with axis of rotation. Map-makers choose the true ellipsoid that best fits their need for the area they are mapping. They then choose the most appropriate mapping of the spherical coordinate system onto that ellipsoid. In the United Kingdom there are three common latitude, longitude, height systems in use. The system used by GPS, [[World Geodetic System|WGS84]], differs at Greenwich from the one used on published maps [[OSGB36]] by approximately 112m. The military system [[ED50]], used by [[NATO]], differs by about 120m to 180m.<ref name=OSGB/>
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| Though early navigators thought of the sea as a flat surface that could be used as a vertical datum, this is far from reality. The Earth can be thought to have a series of layers of equal [[potential energy]] within its [[gravitational field]]. Height is a measurement at right angles to this surface, and although gravity pulls mainly toward the centre of Earth, the geocentre, there are local variations. The shape of these layers is irregular but essentially ellipsoidal. The choice of which layer to use for defining height is arbitrary. The reference height we have chosen is the one closest to the average height of the world's oceans. This is called the [[geoid]].<ref name=OSGB/><ref name=USDOD>[http://www.ngs.noaa.gov/PUBS_LIB/Geodesy4Layman/geo4lay.pdf DMA Technical Report] Geodesy for the Layman, The Defense Mapping Agency, 1983</ref>
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| The Earth is not static as points move relative to each other due to continental plate motion, subsidence, and diurnal movement caused by the [[Moon]] and the [[tide]]s. The daily movement can be as much as a metre. Continental movement can be up to {{nowrap|10 cm}} a year, or {{nowrap|10 m}} in a century. A [[weather system]] high-pressure area can cause a sinking of {{nowrap|5 mm}}. [[Scandinavia]] is rising by {{nowrap|1 cm}} a year as a result of the melting of the ice sheets of the last [[ice age]], but neighbouring [[Scotland]] is only rising by {{nowrap|0.2 cm}}. These changes are insignificant if a local datum is used, but are significant if the global GPS datum is used.<ref name=OSGB/>{{Why|date=July 2009}}
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| == Expressing latitude and longitude as linear units==
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| On a spherical surface at [[sea level]], one latitudinal second measures ''30.82 [[metre]]s'' and one latitudinal minute ''1849 metres'', and one latitudinal degree is ''110.9 kilometres''. The circles of longitude, meridians, meet at the geographical poles, with the west-east width of a second being dependent on the latitude. On the [[equator]] at sea level, one longitudinal second measures ''30.92 metres'', a longitudinal minute ''1855 metres'', and a longitudinal degree ''111.3 kilometres''. At 30° a longitudinal second is ''26.76 metres'', at Greenwich (51° 28' 38" N) is ''19.22 metres'', and at 60° it is ''15.42 metres''.
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| The width of one longitudinal degree on latitude <math>\scriptstyle{\phi}\,\!</math> can be calculated by this formula (to get the width per minute and second, divide by 60 and 3600, respectively):
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| :::::<math>\frac{\pi}{180^{\circ}}\cos(\phi)M_r,\,\!</math>
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| where [[Earth radius#Meridional Earth radius|Earth's average meridional radius]] <math>\scriptstyle{M_r}\,\!</math> approximately equals {{nowrap|6,367,449 m}}. Due to the average radius value used, this formula is of course not precise. You can get a better approximation of a longitudinal degree at latitude <math>\scriptstyle{\phi}\,\!</math> by:
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| ::<math>\frac{\pi}{180^{\circ}}\cos(\phi)\sqrt{\frac{a^4\cos(\phi)^2+b^4\sin(\phi)^2}{(a\cos(\phi))^2+(b\sin(\phi))^2}},\,\!</math>
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| where Earth's equatorial and polar radii, <math>\scriptstyle{a,b}\,\!</math> equal ''6,378,137 m'', ''6,356,752.3 m'', respectively.
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| {| class="wikitable" border="1"
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| |+ '''Length equivalent at selected latitudes in km'''
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| |-
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| !width="100"|Latitude
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| !width="150"|Town
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| !width="100"|Degree
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| !width="100"|Minute
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| !width="100"|Second
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| !width="100"|±0.0001°
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| |-
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| |60°||[[Saint Petersburg]]||align="center" | 55.65 km||align="center" | 0.927 km||align="center" | 15.42m ||align="center" | 5.56m
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| |-
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| |51° 28' 38" N||[[Greenwich]]||align="center" | 69.29 km||align="center" | 1.155 km||align="center" | 19.24m||align="center" | 6.93m
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| |-
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| |45°||[[Bordeaux]]||align="center" | 78.7 km||align="center" | 1.31 km||align="center" | 21.86m||align="center" | 7.87m
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| |-
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| |30°||[[New Orleans]]||align="center" | 96.39 km||align="center" | 1.61 km||align="center" | 26.77m||align="center" | 9.63m
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| |-
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| |0°||[[Quito]]||align="center" | 111.3 km ||align="center" | 1.855 km ||align="center" | 30.92m || align="center" | 11.13m
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| |}
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| <!--The equator is the [[fundamental plane (spherical coordinates)|fundamental plane]] of all geographic coordinate systems. All spherical coordinate systems define such a fundamental plane.-->
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| == Datums often encountered ==
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| Latitude and longitude values can be based on several different [[geodetic system]]s or [[datum (geodesy)|datum]]s, the most common being [[World Geodetic System|WGS 84]] used by all GPS equipment.<ref>WGS 84 is the ''default'' datum used in most GPS equipment, but other datums can be selected.</ref> Other datums however are significant because they were chosen by a national cartographical organisation as the best method for representing their region, and these are the datums used on printed maps. Using the latitude and longitude found on a map may not give the same reference as on a GPS receiver. Coordinates from the [[Figure of the Earth#Historical Earth ellipsoids|mapping system]] can sometimes be changed into another datum using a simple [[translation]]. For example, to convert from ETRF89 (GPS) to the Irish Grid add 49 metres to the east, and subtract 23.4 metres from the north.<ref name=irish>[http://www.osi.ie/GetAttachment.aspx?id=25113681-c086-485a-b113-bab7c75de6fa Making maps compatible with GPS] Government of Ireland 1999. Accessed 15.4.2008 </ref> More generally one datum is changed into any other datum using a process called [[Helmert transformation]]s. This involves converting the spherical coordinates into Cartesian coordinates and applying a seven parameter transformation (translation, three-dimensional [[rotation]]), and converting back.<ref name=OSGB/>
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| 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'.
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| ==Geostationary coordinates==
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| [[Geostationary]] satellites (e.g., television satellites) are over the equator at a specific point on Earth, so their position related to Earth is expressed in longitude degrees only. Their latitude is always zero, that is, over the equator.
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| ==See also==
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| {{portalpar|Atlas|BlankMap-World6.svg|65}}
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| * [[Automotive navigation system]]
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| * [[Geographic coordinate conversion]]
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| * [[Geocoding]]
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| * [[Geographical distance]]
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| * [[Geotagging]]
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| * [[Great-circle distance]] the shortest distance between any two points on the surface of a sphere.
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| * [[Lambert conformal conic projection|Lambert coordinate system]]
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| * [[Map projection]]
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| * [[Tropic of Cancer]]
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| * [[Tropic of Capricorn]]
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| * [[Universal Transverse Mercator coordinate system]]
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| * [[Utility pole#Coordinates on pole labels|Utility pole label coordinates]]
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| ==Notes==
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| {{reflist}}
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| ==References==
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| {{More footnotes|date=February 2009}}
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| <div class="references-small">
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| * ''Portions of this article are from Jason Harris' "Astroinfo" which is distributed with [[KStars]], a desktop planetarium for [[Linux]]/[[KDE]]. See [http://edu.kde.org/kstars/index.phtml]''
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| </div>
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| ==External links==
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| *[http://math.rice.edu/~lanius/pres/map/mapcoo.html Mathematics Topics-Coordinate Systems]
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| *[https://www.cia.gov/library/publications/the-world-factbook/index.html Geographic coordinates of countries (CIA World Factbook)]
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| *[http://www.fcc.gov/mb/audio/bickel/DDDMMSS-decimal.html FCC coordinates conversion tool (DD to DMS/DMS to DD)]
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| {{DEFAULTSORT:Geographic Coordinate System}}
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| [[Category:Geographic coordinate systems|*]]
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| [[Category:Cartography]]
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| [[Category:Navigation]]
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| [[Category:Geocodes]]
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| [[af:Geografiese koördinatestelsel]]
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| [[als:Geografische Lage]]
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| [[am:የምድር መጋጠሚያ ውቅር]]
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| [[ar:نظام الإحداثيات الجغرافية]]
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| [[ast:Sistema de Coordenaes Xeográfiques]]
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| [[az:Coğrafi koordinat sistemi]]
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| [[bn:ভৌগলিক স্থানাঙ্ক ব্যবস্থা]]
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| [[be:Геаграфічныя каардынаты]]
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| [[be-x-old:Геаграфічныя каардынаты]]
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| [[bar:Geografische Koordinatn]]
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| [[bg:Географски координати]]
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| [[ca:Coordenades geogràfiques]]
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| [[cs:Zeměpisné souřadnice]]
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| [[de:Geografische Koordinaten]]
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| [[et:Geograafilised koordinaadid]]
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| [[el:Γεωγραφικές συντεταγμένες]]
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| [[es:Coordenadas geográficas]]
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| [[eo:Geografia koordinata sistemo]]
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| [[eu:Sare geografiko]]
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| [[fr:Coordonnées géographiques]]
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| [[gu:અક્ષાંશ-રેખાંશ]]
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| [[ko:지리 좌표계]]
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| [[hr:Zemljopisne koordinate]]
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| [[ilo:Heograpikal a nagsasabtan]]
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| [[id:Sistem koordinat geografi]]
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| [[ia:Systema geographic de coordinatas]]
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| [[is:Hnit (landafræði)]]
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| [[it:Coordinate geografiche]]
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| [[he:קואורדינטות גאוגרפיות]]
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| [[ka:გეოგრაფიული კოორდინატები]]
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| [[kk:Географиялық координаттар]]
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| [[sw:Anwani ya kijiografia]]
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| [[lo:ພິກັດພູມສາດ]]
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| [[la:Coordinata geographica]]
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| [[lv:Ģeogrāfiskā koordinātu sistēma]]
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| [[lmo:Cuurdinat geugrafich]]
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| [[hu:Földrajzi koordináta-rendszer]]
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| [[mk:Географски координатен систем]]
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| [[ml:ജ്യോഗ്രഫിക് കോഓര്ഡിനേറ്റ് സിസ്റ്റം]]
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| [[mr:भौगोलिक गुणक पद्धती]]
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| [[ms:Sistem koordinat geografi]]
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| [[nah:Cemonocāyōtl cemānāhuacāyōcopa]]
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| [[nl:Geografische coördinaten]]
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| [[nap:Sëštém d'u cördënazjôn gjögrafëxë]]
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| [[no:Jordens koordinatsystem]]
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| [[pms:Coordinà geogràfiche]]
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| [[nds:Geograafsche Laag]]
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| [[pl:Współrzędne geograficzne]]
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| [[pt:Coordenadas geográficas]]
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| [[ro:Coordonate geografice]]
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| [[qu:Tinkurachina siwi]]
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| [[ru:Географические координаты]]
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| [[sq:Koordinatat gjeografike]]
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| [[simple:Geographic coordinate system]]
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| [[sk:Geografický súradnicový systém]]
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| [[sl:Geografski koordinatni sistem]]
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| [[szl:Geograficzne wspůłrzyndne]]
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| [[sr:Географске координате]]
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| [[sh:Geografski koordinatni sistem]]
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| [[su:Sistim koordinat géografi]]
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| [[fi:Koordinaattijärjestelmä]]
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| [[sv:Geografiska koordinatsystem]]
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| [[ta:புவியியல் ஆள்கூற்று முறை]]
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| [[th:พิกัดภูมิศาสตร์]]
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| [[tr:Coğrafi koordinat sistemi]]
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| [[tk:Koordinatlar]]
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| [[uk:Географічні координати]]
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| [[vec:Cordinade giogràfeghe]]
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| [[vi:Hệ tọa độ địa lý]]
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| [[vo:Koordinatasit taledavik]]
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| [[zh-yue:地理座標系統]]
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