Geographic coordinate system
A geographic coordinate system is a coordinate system that enables every location on the Earth to be specified by a set of numbers or letters. The coordinates are often chosen such that one of the numbers represents vertical position, and two or three of the numbers represent horizontal position. A common choice of coordinates is latitude, longitude and elevation.^{[1]}
Contents
 Geographic latitude and longitude 1
 UTM and UPS systems 2
 Stereographic coordinate system 3
 Geodetic height 4
 Cartesian coordinates 5
 Shape of the Earth 6
 Expressing latitude and longitude as linear units 7
 Datums often encountered 8
 Geostationary coordinates 9
 On other celestial bodies 10
 See also 11
 Notes 12
 References 13
 External links 14
Geographic latitude and longitude
The "latitude" (abbreviation: Lat., φ, or phi) of a point on the Earth's surface is the angle between the equatorial plane and the straight line that passes through that point and is normal to the surface of a reference ellipsoid which approximates the shape of the Earth.^{[n 1]} This line passes a few kilometers away from the center of the Earth except at the poles and the equator where it passes through Earth's center.^{[n 2]} Lines joining points of the same latitude trace circles on the surface of the 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 the Earth's surface is the angle east or west from a reference meridian to another meridian that passes through that point. All meridians are halves of great ellipses (often improperly called great circles), which converge at the north and south poles.
A line, which was intended to pass through the Royal Observatory, Greenwich (a suburb of London, UK), was chosen as the international zerolongitude 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 antipodal meridian of Greenwich is both 180°W and 180°E. The zero/zero point is located in the Gulf of Guinea about 625 km south of Tema, Ghana.
In 1884 the United States hosted the
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 Geographic coordinate systems
 Cartography
 Navigation
 Geodesy
 Geocodes

 Mathematics TopicsCoordinate Systems
 Geographic coordinates of countries (CIA World Factbook)
 Coordinates conversion tool (batch conversions of Decimal, DM, DMS and UTM)
 FCC coordinates conversion tool (DD to DMS/DMS to DD)
 Coordinate converter, formats: DD, DMS, DM
 Latitude and Longitude
External links
 Portions of this article are from Jason Harris' "Astroinfo" which is distributed with KStars, a desktop planetarium for Linux/KDE. See The KDE Education Project  KStars
 ^ ^{a} ^{b} ^{c} ^{d} ^{e} ^{f} A Guide to coordinate systems in Great Britain v1.7 October 2007 D00659 accessed 14.4.2008
 ^ Greenwich 2000 Limited (9 June 2011). "The International Meridian Conference". Wwp.millenniumdome.com. Retrieved 31 October 2012.
 ^ DMA Technical Report Geodesy for the Layman, The Defense Mapping Agency, 1983
 ^ "Making maps compatible with GPS". Government of Ireland 1999. Archived from the original on 21 July 2011. Retrieved 15 April 2008.
References
 ^ The surface of the Earth is closer to an ellipsoid than to a sphere, as its equatorial diameter is larger than its northsouth axis.
 ^ The greatest distance between an ellipsoid normal and the center of the Earth is 21.9 km at a latitude of 45°, using Earth radius#Radius at a given geodetic latitude and Latitude#Numerical comparison of auxiliary latitudes: (6367.5 km)×tan(11.67')=21.9 km.
 ^ The French Institut Géographique National (IGN) maps still use longitude from a meridian passing through Paris, along with longitude from Greenwich.
 ^ WGS 84 is the default datum used in most GPS equipment, but other datums can be selected.
Notes
 Geodetic datum
 Geographic coordinate conversion
 Geographic information system
 Geographical distance
 Linear referencing
 Map projection
See also
 A similarly welldefined system based on the reference ellipsoid for Mars.
 Selenographic coordinates for the Moon
Similar coordinate systems are defined for other celestial bodies such as:
On other celestial bodies
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.
Geostationary coordinates
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'.
Latitude and longitude values can be based on different geodetic systems or mapping system can sometimes be roughly 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.^{[4]} More generally one datum is changed into any other datum using a process called Helmert transformations. This involves converting the spherical coordinates into Cartesian coordinates and applying a seven parameter transformation (translation, threedimensional rotation), and converting back.^{[1]}
Datums often encountered
Latitude  City  Degree  Minute  Second  ±0.0001° 

60°  Saint Petersburg  55.80 km  0.930 km  15.50 m  5.58 m 
51° 28' 38" N  Greenwich  69.47 km  1.158 km  19.30 m  6.95 m 
45°  Bordeaux  78.85 km  1.31 km  21.90 m  7.89 m 
30°  New Orleans  96.49 km  1.61 km  26.80 m  9.65 m 
0°  Quito  111.3 km  1.855 km  30.92 m  11.13 m 
where Earth's equatorial radius a equals 6,378,137 m and \scriptstyle{\tan \beta = \frac{b}{a}\tan\phi}\,\!; for the GRS80 and WGS84 spheroids, b/a calculates to be 0.99664719. (\scriptstyle{\beta}\,\! 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.




 \frac{\pi}{180}a \cos \beta \,\!



where Earth's average meridional radius \scriptstyle{M_r}\,\! is 6,367,449 m. Since the Earth is not spherical that result can be off by several tenths of a percent; a better approximation of a longitudinal degree at latitude \scriptstyle{\phi}\,\! is




 \frac{\pi}{180}M_r\cos \phi \!



To estimate the length of a longitudinal degree at latitude \scriptstyle{\phi}\,\! we can assume a spherical Earth (to get the width per minute and second, divide by 60 and 3600, respectively):
(Those coefficients can be improved, but as they stand the distance they give is correct within a centimeter.)




 111132.954  559.822\, \cos 2\varphi + 1.175\, \cos 4\varphi



On the WGS84 spheroid, the length in meters of a degree of latitude at latitude φ (that is, the distance along a northsouth line from latitude (φ  0.5) degrees to (φ + 0.5) degrees) is about
On the GRS80 or WGS84 spheroid at sea level at the equator, one latitudinal second measures 30.715 metres, one latitudinal minute is 1843 metres and one latitudinal degree is 110.6 kilometres. The circles of longitude, meridians, meet at the geographical poles, with the westeast width of a second naturally decreasing as latitude increases. On the equator at sea level, one longitudinal second measures 30.92 metres, a longitudinal minute is 1855 metres and a longitudinal degree is 111.3 kilometres. At 30° a longitudinal second is 26.76 metres, at Greenwich (51° 28' 38" N) 19.22 metres, and at 60° it is 15.42 metres.
Expressing latitude and longitude as linear units
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 tides. The daily movement can be as much as a metre. Continental movement can be up to 10 cm a year, or 10 m in a century. A weather system highpressure area can cause a sinking of 5 mm. Scandinavia is rising by 1 cm a year as a result of the melting of the ice sheets of the last ice age, but neighbouring Scotland is rising by only 0.2 cm. These changes are insignificant if a local datum is used, but are statistically significant if the global GPS datum is used.^{[1]}
Though early navigators thought of the sea as a flat surface that could be used as a vertical datum, this is not actually the case. The Earth has a series of layers of equal potential energy within its gravitational field. Height is a measurement at right angles to this surface, roughly toward the centre of the Earth, but local variations make the equipotential layers irregular (though roughly ellipsoidal). The choice of which layer to use for defining height is arbitrary. The reference height that has been chosen is the one closest to the average height of the world's oceans. This is called the geoid.^{[1]}^{[3]}
The Earth is not a sphere, but an irregular shape approximating a 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. Mapmakers 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, 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.^{[1]}
Shape of the Earth
An example is the NGS data for a brass disk near Donner Summit, in California. Given the dimensions of the ellipsoid, the conversion from lat/lon/heightaboveellipsoid coordinates to XYZ is straightforward—calculate the XYZ for the given latlon on the surface of the ellipsoid and add the XYZ vector that is perpendicular to the ellipsoid there and has length equal to the point's height above the ellipsoid. The reverse conversion is harder: given XYZ we can immediately get longitude, but no closed formula for latitude and height exists. See "Geodetic system." Using Bowring's formula in 1976 Survey Review the first iteration gives latitude correct within 10^{11} degree as long as the point is within 10000 meters above or 5000 meters below the ellipsoid.
Zaxis along the axis of the ellipsoid, positive northward
X and Yaxis in the plane of the equator, Xaxis positive toward 0 degrees longitude and Yaxis positive toward 90 degrees east longitude.
With the origin at the center of the ellipsoid, the conventional setup is the expected righthand:
Every point that is expressed in ellipsoidal coordinates can be expressed as an x y z (Cartesian) coordinate. Cartesian coordinates simplify many mathematical calculations. The origin is usually the center of mass of the earth, a point close to the Earth's center of figure.
Cartesian coordinates
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 Earth, or from the surface of the Earth. Because of the ambiguity of "surface" and "vertical", it is more commonly expressed relative to a precisely defined vertical datum which holds fixed some known point. Each country has defined its own datum. For example, in the United Kingdom the reference point is Newlyn, while in Canada, Mexico and the United States, the point is near Rimouski, Quebec, Canada. The distance to Earth's centre can be used both for very deep positions and for positions in space.^{[1]}
Geodetic height
Although no longer used in navigation, the stereographic coordinate system is still used in modern times to describe crystallographic orientations in the fields of crystallography, mineralogy and materials science.
During medieval times, the stereographic coordinate system was used for navigation purposes. The stereographic coordinate system was superseded by the latitudelongitude system.
Stereographic coordinate system
The Universal Transverse Mercator (UTM) and Universal Polar Stereographic (UPS) coordinate systems both use a metricbased cartesian grid laid out on a conformally projected surface to locate positions on the surface of the Earth. The UTM system is not a single map projection but a series of map projections, one for each of sixty 6degree bands of longitude. The UPS system is used for the polar regions, which are not covered by the UTM system.
UTM and UPS systems
The combination of these two components specifies the position of any location on the planet, but does not consider altitude nor depth. This latitude/longitude "webbing" is known as the graticule. A graticule representing latitude and longitude of the Earth does not constitute a hierarchy of geographical areas. This is to say, it is not an arrangement of related information or data.
[n 3]