Map of Earth showing lines of latitude (horizontally) and longitude (vertically), Eckert VI projection; large version (pdf, 3.12MB)

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. An example is the WGS84 coordinate system used in the Global Positioning System.

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.

Contents 1 Latitude and longitude 2 UTM and UPS systems 3 Stereographic coordinate system 4 Geodetic height 5 Cartesian coordinates 6 Shape of the Earth 7 Expressing latitude and longitude as linear units 8 Datums often encountered 9 Geostationary coordinates 10 See also 11 Notes

Latitude and longitude

Latitude (abbreviation: Lat., φ, or phi) is the angle from a point on the Earth's surface to the equatorial plane, measured from the center of the sphere. Lines joining points of the same latitude are called 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 of all geographic coordinate systems. The equator divides the globe into Northern and Southern Hemispheres.

Longitude (abbreviation: Long., λ, or lambda) is the angle east or west of a reference meridian between the two geographical poles to another 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.

A line passing to the rear of 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.

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. San Domingo voted against the adoption of that motion, while France and Brazil abstained. 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.

The combination of these two components specifies the position of any location on the planet, but does not consider altitude nor depth.

For example, Baltimore, Maryland (in the 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.

This latitude/longitude "webbing" is known as the .

In defining an ellipse, the short (vertical) diameter is known as the , and the long (horizontal) diameter — perpendicular, or "transverse", to the conjugate — is the . With a sphere or ellipsoid, the conjugate diameter is known as the and the transverse as the . 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 ).

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 zones. 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 navigation, the stereographic coordinate system is still used in modern times to describe crystallographic orientations in the field of 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 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 vertical datum such as mean sea level at a named point. Each country has defined its own datum. For example, 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.

Cartesian coordinates

Every point that is expressed in spherical coordinates can be expressed as an x y z (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.

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 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.

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 high-pressure 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 only rising by 0.2 cm. 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 a spherical surface at sea level, one latitudinal second measures and one latitudinal minute , and one latitudinal degree is . 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 , a longitudinal minute , and a longitudinal degree . At 30° a longitudinal second is , at Greenwich (51° 28' 38" N) is , and at 60° it is .

The width of one longitudinal degree on latitude 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 approximately equals 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 by:

where Earth's equatorial and polar radii, equal , , respectively.

Length equivalent at selected latitudes in km

Latitude	Town	Degree	Minute	Second	±0.0001°
60°	Saint Petersburg	55.65 km	0.927 km	15.42m	5.56m
51° 28' 38" N	Greenwich	69.29 km	1.155 km	19.24m	6.93m
45°	Bordeaux	78.7 km	1.31 km	21.86m	7.87m
30°	New Orleans	96.39 km	1.61 km	26.77m	9.63m
0°	Quito	111.3 km	1.855 km	30.92m	11.13m

Datums often encountered

Latitude and longitude values can be based on several different geodetic systems or datums, the most common being WGS 84 used by all GPS equipment. 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 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. 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, 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.

See also

• Automotive navigation system
• Geographic coordinate conversion
• Geocoding
• Geodetic system
• Geographical distance
• Geotagging
• Great-circle distance the shortest distance between any two points on the surface of a sphere.
• Lambert coordinate system
• Map projection
• Tropic of Cancer
• Tropic of Capricorn

Notes