Geographic coordinate system

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This article is about longitude and latitude; see also UTM coordinate system

Image:WorldMapLongLat-eq-circles-tropics-non.png
Map of Earth showing lines of latitude (horizontally) and longitude (vertically); large version (pdf)

The geographic (earth-mapping) coordinate system expresses every horizontal position on Earth by two of the three coordinates of a spherical coordinate system which is aligned with the spin axis of the Earth. Borrowing from theories of ancient Babylonians, later expanded by the famous Greek thinker and geographer Ptolemy, a full circle is assigned 360 degrees (360°). This is also called absolute location

It defines two angles measured from the center of the Earth:

  • the latitude (Lat.) is the vertical data and measures the angle between any point and the equator. Lines of constant latitude are called parallels. They trace circles on the surface of the Earth, but the only parallel that is a great circle is the equator (latitude=0 degrees),with each pole being 90 degrees (north pole +90°; south pole −90° ).
  • the longitude (Long.) is the horizontal data and measures the angle east-wards from an arbitrary point on the Earth: Greenwich in London (U.K.) is the accepted zero-longitude point internationally in most modern societies (longitude=0 or 360 degrees). Longitude is measured form 0 to +360 deg, going east from the zeropoint (Greenwich or 0°), being 180 degrees the opposite point on the globe to Greenwich. Lines of constant longitude are called meridians . The meridian passing through Greenwich is the Prime Meridian. Unlike parallels, all meridians are great circles, and meridians are not parallel: they intersect at the north and south poles.

By combining these two angles, the plane position of any location on Earth can be specified.

For example, Baltimore, Maryland (in the USA) has a latitude of 39.3 degrees North, and a longitude of 76.6 degrees West (39.3° N 76.6° W). So, a vector drawn from the center of the Earth to a point 39.3 degrees north of the equator and 76.6 degrees west of Greenwich will pass through Baltimore.

This latitude/longitude "webbing" is known as the common graticule. There is also a complementary transverse graticule (meaning the graticule is shifted 90°, so that the poles are on the horizontal equator), upon which all spherical trigonometry is ultimately based on.

Traditionally, degrees have been divided into minutes ( ' ) and seconds ( " ). There are formats for degrees, all of them appearing in a Lat.-Long. order :

  • DM Degree:Minute (49:30.0-123:30.0)
  • DMS Degree:Minute:Second (49:30:00-123:30:00)
  • DD Decimal Degree (49.5000-123.5000), generally with 4 decimal numbers.

To change from DM or DMS to DD, Decimal degrees = whole number of degrees, plus minutes divided by 60, plus seconds divided by 3600. Decimal division is now the most common and standard.


The equator is obviously an important part of this coordinate system, it represents the zeropoint of the latitude angle, and the halfway point between the poles. The equator is the fundamental plane of the geographic coordinate system. All spherical coordinate systems define such a fundamental plane.

This article originates from Jason Harris' Astroinfo which comes along with KStars, a Desktop Planetarium for Linux/KDE. See http://edu.kde.org/kstars/index.phtml

In popular GIS software, data projected in latitude/longitude is often represented as 'Geographic Coordinate System'. For example, data in latitude/longitude with datum as the North American Datum of 1983 is denoted by 'GCS_North_American_1983'.

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Notation

You can use the cardinal or the numerical only system:

  • Cardinal system includes North, South, East, West (i.e., 60° E).
  • Numerical system uses only numbers. i.e., 60°E is 60° and 60°W is 360°−60° = 300º.

PoLat uses instead of Equator the North Pole (North pole = 0°, Equator = 90° and South Pole = 180°). In this notation, there are no negative numbers.

The UGN (Unified Geographic Notation) uses:

  • Numerical system in Decimal degrees.
  • Latitude appears first, followed by a -, the latitude and UGN.
  • PoLat is used for latitude.

I.e., 90.00-18.01 UGN is in the Equator (90°) and 18.01° in the east

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Geostationary coordinates

Geostationary satellites (i.e., television satellites ) are over the Equator. So, their position related to Earth is expressed in longitude degrees. Latitude does not change, and is always zero over the Equator.

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Third dimension : altitude

To completely specify a location on, in, or above the Earth, one has to specify also the elevation / height position. This can e.g., be expressed relative to a datum such as mean sea level (above mean sea level) or the geoid. The distance to the Earth's center is a practical coordinate both for very deep positions and for positions in space.

The elevation specifies the vertical position of the Earth surface.

Various elevation / height coordinates either with respect to the surface or some other datum are altitude, height, and depth.


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See also

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External links

de:Geografische Koordinaten et:Geograafilised koordinaadid el:Γεωγραφικές συντεταγμένες es:Coordenadas geográficas ko:지리 좌표계 id:Sistem koordinat geografi it:Coordinate geografiche nl:Geografische coördinaten ja:測地系 pt:Sistema de coordenadas geográficas ru:Географические координаты sl:Geografski koordinatni sistem sv:Jordens koordinatsystem