Geographic coordinate system
A spherical or geodetic coordinate system for measuring positions on Earth as latitude and longitude.
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A geographic coordinate system (GCS) is a spherical or geodetic coordinate system for measuring and communicating positions directly on Earth as latitude and longitude. It is the simplest, oldest, and most widely used type of the various spatial reference systems that are in use, and forms the basis for most others. Although latitude and longitude form a coordinate pair like a Cartesian coordinate system, geographic coordinate systems are not Cartesian because the measurements are angles and are not on a planar surface. The system’s invention is credited to Eratosthenes of Cyrene in the 3rd century BC, with later refinements by Hipparchus, who used stellar measurements for latitude and lunar eclipses for longitude.
History
Marinus of Tyre later compiled a world map using coordinates from a prime meridian at the Fortunate Isles, while Ptolemy’s 2nd-century Geography measured latitude from the Equator. After Arabic translation in the 9th century, Al-Khwārizmī corrected earlier errors in the Mediterranean’s length, shifting the prime meridian. European cartography revived after Ptolemy’s text was recovered around 1300 and translated into Latin in 1407.
Latitude and longitude
The 1884 International Meridian Conference, with twenty-five nations, adopted the Greenwich meridian as the zero-reference line, though France and Brazil abstained and the Dominican Republic voted against. Latitude is defined by the angle between the equatorial plane and a line from the surface point to a second point on that plane, which varies by system: astronomical (plumb bob vertical), geodetic (ellipsoid normal), or geocentric (Earth’s center). Parallels of latitude are circles parallel to the equator, with poles at 90° N and S. Longitude measures the angle east or west of a reference meridian; all meridians are halves of great ellipses converging at the poles.
The Greenwich meridian is the international prime meridian, though some organizations use others. The antipodal meridian is 180°W and 180°E, distinct from the International Date Line. The origin, where the equator and prime meridian intersect, lies in the Gulf of Guinea. A full GCS specification includes a geodetic datum and Earth ellipsoid, as different datums yield different coordinates for the same location.
Lore & Background
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. 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.
Ptolemy's 2nd-century Geography used the same 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 following Maximus Planudes' recovery of Ptolemy's text a little before 1300; the text was translated into Latin at Florence by Jacopo d'Angelo 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. France adopted Greenwich Mean Time in place of local determinations by the Paris Observatory in 1911.
Reader's Guide
Latitude of a point on Earth's surface is defined in one of three ways: in an astronomical coordinate system, the second point is found where the extension of the plumb bob vertical from the surface point intersects the equatorial plane; in a geodetic coordinate system, the second point is found where the normal vector from the surface of the ellipsoid at the surface point intersects the equatorial plane; in a geocentric coordinate system, the second point is the center of Earth. The path that joins all points of the same latitude traces a circle 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 defined to be the equator, the fundamental plane of a geographic coordinate system. The equator divides the globe into Northern and Southern Hemispheres.
Longitude 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, 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—continue to use other meridians for internal purposes.
The antipodal meridian of Greenwich is both 180°W and 180°E. This is not to be conflated with the International Date Line, which partly overlaps with the 180° meridian but 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 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 south of Tema, Ghana, a location often facetiously called Null Island.
Frequently Asked Questions
Who is Geographic coordinate system in Lines & Hemispheres 25-28?
Geographic coordinate system is the foundational spatial reference framework that defines positions on Earth's surface using angular measurements of latitude and longitude. It is the oldest and simplest type of coordinate system in the series, serving as the backbone for nearly every other reference framework.
What is Geographic coordinate system's role or 'power' in the story?
Its core function is to let anyone pinpoint a location on the globe by pairing a latitude angle with a longitude angle. It underpins navigation, mapping, and virtually every other spatial system introduced later in the series.
How did Geographic coordinate system come to be? Was it invented by one person?
It did not spring from a single inventor; rather, it evolved gradually over many centuries. The earliest known contributions are attributed to Eratosthenes of Cyrene, with significant refinements later made by Hipparchus of Nicaea among others.
Is Geographic coordinate system the same thing as a Cartesian coordinate system?
No—although both use a pair of values to locate a point, a GCS measures angles on a spherical (or geodetic) surface rather than distances along flat, perpendicular axes. That spherical, angular nature is what sets it apart from a true Cartesian grid.
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Sources
Compiled from Wikipedia and the sources listed below. Text from Wikipedia is available under CC BY-SA 4.0; this entry is adapted from it.
- Wikipedia: Geographic coordinate system (CC BY-SA 4.0).
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