Geographic Coordinate System.
The geographic coordinate system (GCS) is a widely used system for measuring and communicating positions on the Earth's surface as latitude and longitude. Unlike Cartesian coordinate systems, which use linear measurements, the GCS uses angular measurements because the Earth is a sphere or ellipsoid.
Longitude lines are perpendicular to the Equator and measure the distance east or west of the Prime Meridian, which is a reference line that passes through the Royal Observatory in Greenwich, England. Latitude lines, on the other hand, are parallel to the Equator and measure the distance north or south of the Equator.
A complete GCS specification, as defined in standards such as EPSG and ISO 19111, includes a choice of geodetic datum, which includes an Earth ellipsoid. Different datums will yield different latitude and longitude values for the same location due to variations in the shape and size of the Earth's surface.
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.[note 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 at 90° N latitude, and the South Pole is at 90° S latitude. The 0° parallel of latitude is designated the Equator, the fundamental plane of all geographic coordinate systems. The Equator divides the globe into the 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 southeast London, England, is the international prime meridian, although some organizations—such as the French Institut national de l'information géographique et forestière—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 to keep the Old World on a single side. The antipodal meridian of Greenwich is both 180°W and 180°E. This should not be confused 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 the Earth, without consideration of altitude or depth. The visual grid on a map formed by lines of latitude and longitude is known as a graticule.
The origin/zero point of this system is located in the Gulf of Guinea about 625 km (390 mi) south of Tema, Ghana, at a location often facetiously referred to as Null Island.
History .
The development of the geographic coordinate system can be attributed to several individuals throughout history. Eratosthenes of Cyrene is credited with inventing the system in the 3rd century BC when he composed his Geography at the Library of Alexandria. Hipparchus of Nicaea later improved the system by using stellar measurements to determine latitude and lunar eclipses to determine longitude.
Marinus of Tyre created an extensive gazetteer and plotted a world map in the 1st or 2nd century, using coordinates measured from a prime meridian located at the westernmost known land, designated the Fortunate Isles. Ptolemy credited Marinus with the full adoption of longitude and latitude.
Ptolemy's 2nd-century Geography used a prime meridian but measured latitude from the Equator instead. After their work was translated into Arabic in the 9th century, Al-Khwārizmī's Book of the Description of the Earth corrected Marinus' and Ptolemy's errors regarding the length of the Mediterranean Sea, causing medieval Arabic cartography to use a prime meridian around 10° east of Ptolemy's line. Mathematical cartography resumed in Europe after Maximus Planudes recovered Ptolemy's text in the 13th century, which was later translated into Latin by Jacobus Angelus.
In 1884, the International Meridian Conference was held in the United States with representatives from twenty-five nations in attendance. Twenty-two of them agreed to adopt the longitude of the Royal Observatory in Greenwich, England as the zero-reference line. France adopted Greenwich Mean Time in place of local determinations by the Paris Observatory in 1911, while the Dominican Republic voted against the motion and Brazil abstained.
Geodetic Datum .
To ensure unambiguous measurements of the "vertical" and "horizontal" surface, map-makers use a reference ellipsoid with a specific origin and orientation that best fits the area to be mapped. This is known as a terrestrial reference system or geodetic datum. Datums can be either global, representing the whole Earth, or local, representing only a portion of the Earth. Points on the Earth's surface move relative to each other due to various factors, including continental plate motion, subsidence, and diurnal Earth tidal movement caused by the Moon and the Sun. The magnitude of daily movement can be as much as a meter, while continental movement can be up to 10 cm per year or 10 m in a century. Weather systems can also cause changes in the surface level of the Earth. These changes are statistically significant when using a global datum, but not when using a local datum.
Global datums include the World Geodetic System (WGS 84), which is the default datum used for the Global Positioning System, and the International Terrestrial Reference System and Frame (ITRF), used for estimating continental drift and crustal deformation. Local datums chosen by national cartographic organizations include the North American Datum, the European ED50, and the British OSGB36. The latitude and longitude obtained from a GPS receiver may not match those obtained from a map made against a local datum, and converting coordinates from one datum to another requires a datum transformation such as a Helmert transformation. In popular GIS software, data projected in latitude/longitude is often represented as a Geographic Coordinate System, denoted by the name of the datum, such as 'GCS North American 1983' for data in latitude/longitude with the North American Datum of 1983.
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