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 . The coordinates are often chosen such that one of the numbers represent vertical position , and two or three of the numbers represent horizontal position . A common choice of coordinates is latitude , longitude and elevation . Geographic latitude and longitude . The geographic latitude ( abbreviation : Lat . , φ , or phi ) of a point on the Earth ' s surface is the angle between the equatorial plane and a line that passes through that point and is normal to the surface of a reference ellipsoid which approximates the shape of the Earth . 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 . 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 ellipse s ( often improperly called great circle s ) , which converge at the north and south poles . A line passing near the Royal Observatory , Greenwich ( near London in the 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 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 International Meridian Conference and twenty - five nations attended . Twenty - two of them agreed to adopt the location of Greenwich as the zero - reference line . The Dominican Republic voted against the adoption of that motion , while France and Brazil abstained . To date , there exist organizations around the world which continue to use historical prime meridians which existed before the acceptance of Greenwich became common - place . 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 conjugate graticule . In defining an ellipse , the short ( vertical ) diameter is known as the conjugate diameter , and the long ( horizontal ) diameter — perpendicular , or " transverse " , to the conjugate — is the transverse diameter . With a sphere or ellipsoid , the conjugate diameter is known as the polar axis and the transverse as the equatorial axis . The graticule 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 ) . UTM and UPS systems . The Universal Transverse Mercator ( UTM ) and Universal Polar Stereographic ( UPS ) coordinate systems both use a metric - based 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 6 - degree bands of longitude . The UPS system is used for the polar regions , which are not covered by the UTM system . Stereographic coordinate system . During medieval times , the stereographic coordinate system was used for navigation purposes . The stereographic coordinate system was superseded by the latitude - longitude system . Although no longer used in navigations , the stereographic coordinate system is still used in modern times to describe crystallographic orientations in the fields of crystallography , mineralogy and materials science . Geodetic height . 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 . Cartesian coordinates . Every point that is expressed in ellipsoidal coordinates can be expressed as an ( 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 . With the origin at the center of the ellipsoid , the conventional setup is the expected right - hand : Z - axis along the axis of the ellipsoid , positive northward < br > X - and Y - axis in the plane of the equator , X - axis positive toward 0 degrees longitude and Y - axis positive toward 90 degrees east longitude 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 / height - above - ellipsoid coordinates to X - Y - Z is straightforward — calculate the X - Y - Z for the given lat - lon on the surface of the ellipsoid and add the X - Y - Z 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 X - Y - Z we can immediately get longitude , but no closed formula for latitude and height exists . However , using Bowring ' s formula in 1976 " Survey Review " the first iteration gives latitude correct within formula _ 1 degree as long as the point is within 10000 meters above or 5000 meters below the ellipsoid . Shape of the Earth . 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 . 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 , 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 . 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 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 we have chosen is the one closest to the average height of the world ' s oceans . This is called the geoid . 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 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 neighbouring Scotland is rising by only . These changes are insignificant if a local datum is used , but are significant if the global GPS datum is used . Expressing latitude and longitude as linear units . On the GRS80 or WGS84 spheroid at sea level at the equator , one latitudinal second measures " 30 . 715 metre s " , 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 west - east 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 " . On the WGS84 spheroid , the length in meters of a degree of latitude at latitude φ ( that is , the distance along a north - south line from latitude ( φ - 0 . 5 ) degrees to ( φ + 0 . 5 ) degrees ) is about 111132 . 954 - 559 . 822 ( cos 2φ ) + 1 . 175 ( cos 4φ ) The width of one longitudinal degree at latitude formula _ 2 can be calculated by this formula ( to get the width per minute and second , divide by 60 and 3600 , respectively ) : where Earth ' s average meridional radius formula _ 4 approximately equals . Due to the average radius value used , this formula is of course not precise . A better approximation of a longitudinal degree at latitude formula _ 2 is where Earth ' s equatorial radius formula _ 7 equals " 6 , 378 , 137 m " and formula _ 8 ; for the GRS80 and WGS84 spheroids , b / a calculates to be 0 . 99664719 . ( formula _ 9 is known as the parametric or reduced 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 . Datums often encountered . Latitude and longitude values can be based on different geodetic system s or datums , the most common being WGS 84 , a global datum used by all GPS equipment . Other datums 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 . The latitude and longitude on a map may not be the same as on a GPS receiver . Coordinates from the 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 . 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 . 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 ' . Geostationary coordinates . 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 .