# Geographic coordinate system

# Geographic coordinate system

A **geographic coordinate system** is a coordinate system that enables every location on Earth to be specified by a set of numbers, letters or symbols.^{[1]} The coordinates are often chosen such that one of the numbers represents a vertical position and two or three of the numbers represent a horizontal position; alternatively, a geographic position may be expressed in a combined three-dimensional Cartesian vector.
A common choice of coordinates is latitude, longitude and elevation.^{[5]}
To specify a location on a plane requires a map projection.^{[6]}

History

The invention of a geographic coordinate system is generally credited to Eratosthenes of Cyrene, who composed his now-lost Geography at the Library of Alexandria in the 3rd century BC.^{[7]} A century later, Hipparchus of Nicaea improved on this system by determining latitude from stellar measurements rather than solar altitude and determining longitude by timings of lunar eclipses, rather than dead reckoning. In the 1st or 2nd century, Marinus of Tyre compiled an extensive gazetteer and mathematically-plotted world map using coordinates measured east from a prime meridian at the westernmost known land, designated the Fortunate Isles, off the coast of western Africa around the Canary or Cape Verde Islands, and measured north or south of the island of Rhodes off Asia Minor. Ptolemy credited him with the full adoption of longitude and latitude, rather than measuring latitude in terms of the length of the midsummer day.^{[8]}

Ptolemy's 2nd-century *Geography* orrected Marinus' and Ptolemy's errors regarding the length of the Mediterranean Sea,^{[2]} causing medieval Arabic cartography to use a prime meridian around 10° east of Ptolemy's line. Mathematical cartography resumed in Europe following Maximus Planudes' recovery of Ptolemy's text a little before 1300; the text was translated into Latin at Florence by Jacobus Angelus around 1407.

In 1884, the United States hosted the International Meridian Conference, attended by representatives from twenty-five nations. Twenty-two of them agreed to adopt the longitude of the Royal Observatory in Greenwich, England as the zero-reference line. The Dominican Republic voted against the motion, while France and Brazil abstained.^{[9]} France adopted Greenwich Mean Time in place of local determinations by the Paris Observatory in 1911.

Geodetic datum

In order to be unambiguous about the direction of "vertical" and the "horizontal" surface above which they are measuring, map-makers choose a reference ellipsoid with a given origin and orientation that best fits their need for the area to be mapped. They then choose the most appropriate mapping of the spherical coordinate system onto that ellipsoid, called a terrestrial reference system or geodetic datum.

Datums may be global, meaning that they represent the whole Earth, or they may be local, meaning that they represent an ellipsoid best-fit to only a portion of the Earth.

Points on the Earth's surface move relative to each other due to continental plate motion, subsidence, and diurnal Earth tidal movement caused by the Moon and the Sun. This daily movement can be as much as a meter. 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 rising by only 0.2 cm. These changes are insignificant if a local datum is used, but are statistically significant if a global datum is used.^{[5]}

Examples of global datums include World Geodetic System (WGS 84, also known as EPSG:4326 ^{[10]}), the default datum used for the Global Positioning System,^{[3]} and the International Terrestrial Reference Frame (ITRF), used for estimating continental drift and crustal deformation.^{[11]} The distance to Earth's center can be used both for very deep positions and for positions in space.^{[5]}

The latitude and longitude on a map made against a local datum may not be the same as one obtained from a GPS receiver.

Converting coordinates from one datum to another requires a datum transformation such as a Helmert transformation, although in certain situations a simple translation may be sufficient.^{[12]}

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'.

Horizontal coordinates

Latitude and longitude

The "latitude" (abbreviation: Lat., φ, or phi) of a point on Earth's surface is the angle between the equatorial plane and the straight line that passes through that point and through (or close to) the center of the Earth.^{[4]} Lines joining points of the same latitude trace circles on the surface of 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 Earth's surface is the angle east or west of a reference meridian to another meridian that passes through that point. All meridians are halves of great ellipses (often called great circles), which converge at the North and South Poles. The meridian of the British Royal Observatory in Greenwich, in south-east London, England, is the international prime meridian, although some organizations—such as the French Institut Géographique National—continue to use other meridians for internal purposes. The prime meridian determines the proper Eastern and Western Hemispheres, although maps often divide these hemispheres further west in order to keep the Old World on a single side. The antipodal meridian of Greenwich is both 180°W and 180°E. This is not to be conflated with the International Date Line, which diverges from it in several places for political and convenience reasons, including between far eastern Russia and the far western Aleutian Islands.

The combination of these two components specifies the position of any location on the surface of Earth, without consideration of altitude or depth. The grid formed by lines of latitude and longitude is known as a "graticule".^{[13]} The origin/zero point of this system is located in the Gulf of Guinea about 625 km (390 mi) south of Tema, Ghana.

Length of a degree

On the GRS80 or WGS84 spheroid at sea level at the Equator, one latitudinal second measures 30.715 meters, one latitudinal minute is 1843 meters 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 meters, a longitudinal minute is 1855 meters and a longitudinal degree is *111.3 kilometres. At 30° a longitudinal second is 26.76 meters, at Greenwich (51°28′38″N) 19.22 meters, and at 60° it is 15.42 meters.*

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

^{[14]}

Similarly, the length in meters of a degree of longitude can be calculated as

^{[14]}

(Those coefficients can be improved, but as they stand the distance they give is correct within a centimetre.)

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 |

Map projection

To establish the position of a geographic location on a map, a map projection is used to convert geodetic coordinates to plane coordinates on a map; it projects the datum ellipsoidal coordinates and height onto a flat surface of a map. The datum, along with a map projection applied to a grid of reference locations, establishes a *grid system* for plotting locations. Common map projections in current use include the Universal Transverse Mercator (UTM), the Military Grid Reference System (MGRS), the United States National Grid (USNG), the Global Area Reference System (GARS) and the World Geographic Reference System (GEOREF).^{[15]}
Coordinates on a map are usually in terms northing N and easting E offsets relative to a specified origin.

Map projection formulas depend in the geometry of the projection as well as parameters dependent on the particular location at which the map is projected.

The set of parameters can vary based on type of project and the conventions chosen for the projection.

For the transverse Mercator projection used in UTM, the parameters associated are the latitude and longitude of the natural origin, the false northing and false easting, and an overall scale factor.^{[16]} Given the parameters associated with particular location or grin, the projection formulas for the transverse Mercator are a complex mix of algebraic and trigonometric functions.^{[16]}

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 sixty, each covering 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 navigation, the stereographic coordinate system is still used in modern times to describe crystallographic orientations in the fields of crystallography, mineralogy and materials science.

Vertical coordinates

Vertical coordinates include height and depth.

3D Cartesian coordinates

Every point that is expressed in ellipsoidal coordinates can be expressed as an rectilinear x y z (Cartesian) coordinate. Cartesian coordinates simplify many mathematical calculations. The Cartesian systems of different datums are not equivalent.^{[6]}

Earth-centered, Earth-fixed

The Earth-centered Earth-fixed (also known as the ECEF, ECF, or conventional terrestrial coordinate system) rotates with the Earth and has its origin at the center of the Earth.

The conventional right-handed coordinate system puts:

The origin at the center of mass of the Earth, a point close to the Earth's center of figure

The Z axis on the line between the North and South Poles, with positive values increasing northward (but does not exactly coincide with the Earth's rotational axis)

^{[17]}The X and Y axes in the plane of the Equator

The X axis passing through extending from 180 degrees longitude at the Equator (negative) to 0 degrees longitude (prime meridian) at the Equator (positive)

The Y axis passing through extending from 90 degrees west longitude at the Equator (negative) to 90 degrees east longitude at the Equator (positive)

An example is the NGS data ^{[20]} 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.

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.

Local tangent plane

A local tangent plane can be defined based on the vertical and horizontal dimensions. The vertical coordinate can point either up or down. There are two kinds of conventions for the frames:

East, North, up (ENU), used in geography

North, East, down (NED), used specially in aerospace

On other celestial bodies

Similar coordinate systems are defined for other celestial bodies such as:

A similarly well-defined system based on the reference ellipsoid for Mars.

Selenographic coordinates for the Moon

See also

Decimal degrees

Geodetic datum

Geographical distance

Geographic coordinate conversion

Geographic information system

Geo URI scheme

Linear referencing

Map projection

Spatial reference systems

## References

*Explanatory Supplement to the Astronomical Almanac, 3rd ed., (Mill Valley CA: University Science Books, 2013) pp. 20–23.*

*A guide to coordinate systems in Great Britain*(PDF), D00659 v2.3, Ordnance Survey, March 2015, retrieved 22 June 2015

*Reconstructing Eratosthenes' Map of the World*(PDF), Dunedin: University of Otago, pp. 20–24.

*The History and Practice of Ancient Astronomy*, Oxford, England: Oxford University Press, pp. 102–103, ISBN 9780199874453.

*GIS Fundamentals*(PDF) (5th ed.). Atlas books. p. 102. ISBN 978-0-9717647-3-6.

*Glossary of the Mapping Sciences*. ASCE Publications. p. 224. ISBN 9780784475706.