Geographic Names Information System
The Geographic Names Information System is a database that contains name and locative information about more than two million physical and cultural features located throughout the United States of America and its territories. It is a type of gazetteer. GNIS was developed by the United States Geological Survey in cooperation with the United States Board on Geographic Names to promote the standardization of feature names; the database is part of a system that includes bibliographic references. The names of books and historic maps that confirm the feature or place name are cited. Variant names, alternatives to official federal names for a feature, are recorded; each feature receives a permanent, unique feature record identifier, sometimes called the GNIS identifier. The database never removes an entry, "except in cases of obvious duplication." The GNIS accepts proposals for new or changed names for U. S. geographical features. The general public can make proposals at the GNIS web site and can review the justifications and supporters of the proposals.
The Bureau of the Census defines Census Designated Places as a subset of locations in the National Geographic Names Database. U. S. Postal Service Publication 28 gives standards for addressing mail. In this publication, the postal service defines two-letter state abbreviations, street identifiers such as boulevard and street, secondary identifiers such as suite. Canadian Geographical Names Data Base, a similar, but non-public-domain, database for locations within Canada only GEOnet Names Server, a similar database for locations outside the United States United Nations Conference on the Standardization of Geographical Names U. S. Department of the Interior, U. S. Geological Survey, National Mapping Division, Digital Gazeteer: Users Manual. Least Heat Moon, Blue Highways: A Journey Into America. ISBN 0-316-35329-9 Jouris, All Over The Map, ISBN 0-89815-649-1 Report: "Countries, Areas of Special Sovereignty, Their Principal Administrative Divisions," Federal Information Processing Standards, FIPS 10-4.
Standard was withdrawn in September 2008, See Federal Register Notice: Vol. 73, No. 170, page 51276 Report: "Principles and Procedures: Domestic Geographic Names," U. S. Board on Geographic Names, 1997. U. S. Postal Service Publication 28. U. S. Board on Geographic Names website Geographic Names Information System Proposals from the general public Meeting minutes
San Bruno Mountain
San Bruno Mountain is located in northern San Mateo County, with some slopes of the mountain crossing over into southern San Francisco. Most of the mountain lies within the 2,326-acre San Bruno Mountain State Park. Next to the state park is the 83-acre state San Bruno Mountain Ecological Reserve on the north slope, it is near the southern boundary of San Francisco, surrounded by the cities of South San Francisco, Daly City and Brisbane. San Bruno Mountain is topped by a four mile long ridge. Trails to the summit afford expansive views of the San Francisco Bay Area. Radio Peak is the highest point, hosting several radio broadcast towers, KTSF7 television, ION's KKPX television and NBC's KNTV television, serving a huge area that would otherwise have poor service in the hilly Bay Area region; the mountain provides habitat for several species of butterflies. The endangered San Bruno elfin butterfly inhabits a few other locations; the distinct Franciscan fog zone plants of San Bruno Mountain set it apart from other California coastal areas.
The Portola expedition visited San Francisco Bay in 1769. The expedition is considered the first European presence in the area. Five years Fernando Rivera and four soldiers climbed the mountain and watched sunrise across the bay; the mountain was named by Bruno de Heceta for his patron saint. San Bruno Mountain consists of portions of five Mexican land grants. Jose Antonio Sanchez, who rode by mule as a child from Sonora, was given Rancho Buri Buri in 1827, with confirmation in 1835. Rancho Buri Buri extended from the bay salt flats to San Andreas Valley and from Colma to Burlingame. Rancho Canada de Guadalupe la Visitacion y Rodeo Viejo contained most of the present day San Bruno Mountain. In 1835 this rancho was granted to Jacob P. Leese. In 1884 banker Charles Crocker acquired core holdings of this rancho amounting to 3,997 acres from Leese's successors, that land devolved to the Crocker Estate Company, who are the present day owners of San Bruno Mountain. Three other ranchos held minor portions of the northern flank of San Bruno Mountain.
The cities that have grown up around the mountain are San Francisco to the north, Brisbane to the east, South San Francisco to the south and both Daly City and Colma to the west. Nossaman attorney Robert D. Thornton pioneered the Habitat Conservation Plan concept creating the first such plan for the area around San Bruno Mountain. KRON was the first television station to place a transmitter tower on Radio Peak, in 1949, followed by KQED and KTVU, though these tenants moved their transmitters to Sutro Tower in the 1970s. A number of FM stations built transmitter towers on the mountain, in 2005, KNTV moved its transmitter to the mountain, on the former KCSM-TV tower. KTSF occupies the former KRON site. In 1965, Westbay Community Associates announced a plan to level a portion of the mountain to fill 27 square miles of San Francisco Bay north of Sierra Point with landfill; the proposal intended to create housing developments in the "Saddle" just north of Guadalupe Canyon Road and in the landfill zone.
In order to remove 250 million cubic yards of earth from the ridge, Westbay proposed using a conveyor belt system to transport the fill across Bayshore Boulevard and Bayshore Freeway to offshore barges, which would deposit the material along the shores of the Bay. Opposition by organizations such as Save The Bay and the residents of Brisbane led to the defeat of Westbay's conveyor plan in June 1967 and the ceasing of all landfill operations at Sierra Point by December 1972; the Terra Bay project was approved in the mid-1980s for development at the south and southeast base of San Bruno Mountain. Terra Bay was constructed in three phases: the first phase constructed townhomes and detached houses; the original developer, W. W. Dean & Associates, was unable to complete the project, SunChase Holdings acquired the project in 1992, completing site preparation before selling the parcels for Phase I to Centex Homes; the Terra Bay site was known to include habitat for the Mission blue and Callippe silverspot butterflies.
Under the terms established by the 1982 amendment to the Endangered Species Act, the nation's first-ever Habitat Conservation Plan was agreed upon, allowing developers to destroy the habitat of endangered species if substitute lands were made available. SunChase agreed to fund ecological restoration to mitigate the impact of Terra Bay during the development of Phase I under the terms of the San Bruno HCP. SunChase entered a joint venture with Myers Development for the development of Phases II and III. Although the shellmound had been noted as early as 1909, a sample of 22 cubic metres of the shellmound conducted in 1989 by Holman & Associates revealed the massive shellmound contained human remains, further, that the shellmound site is eligibl
Pacific Ocean
The Pacific Ocean is the largest and deepest of Earth's oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean in the south and is bounded by Asia and Australia in the west and the Americas in the east. At 165,250,000 square kilometers in area, this largest division of the World Ocean—and, in turn, the hydrosphere—covers about 46% of Earth's water surface and about one-third of its total surface area, making it larger than all of Earth's land area combined; the centers of both the Water Hemisphere and the Western Hemisphere are in the Pacific Ocean. The equator subdivides it into the North Pacific Ocean and South Pacific Ocean, with two exceptions: the Galápagos and Gilbert Islands, while straddling the equator, are deemed wholly within the South Pacific, its mean depth is 4,000 meters. The Mariana Trench in the western North Pacific is the deepest point in the world, reaching a depth of 10,911 meters; the western Pacific has many peripheral seas. Though the peoples of Asia and Oceania have traveled the Pacific Ocean since prehistoric times, the eastern Pacific was first sighted by Europeans in the early 16th century when Spanish explorer Vasco Núñez de Balboa crossed the Isthmus of Panama in 1513 and discovered the great "southern sea" which he named Mar del Sur.
The ocean's current name was coined by Portuguese explorer Ferdinand Magellan during the Spanish circumnavigation of the world in 1521, as he encountered favorable winds on reaching the ocean. He called it Mar Pacífico, which in both Portuguese and Spanish means "peaceful sea". Important human migrations occurred in the Pacific in prehistoric times. About 3000 BC, the Austronesian peoples on the island of Taiwan mastered the art of long-distance canoe travel and spread themselves and their languages south to the Philippines and maritime Southeast Asia. Long-distance trade developed all along the coast from Mozambique to Japan. Trade, therefore knowledge, extended to the Indonesian islands but not Australia. By at least 878 when there was a significant Islamic settlement in Canton much of this trade was controlled by Arabs or Muslims. In 219 BC Xu Fu sailed out into the Pacific searching for the elixir of immortality. From 1404 to 1433 Zheng He led expeditions into the Indian Ocean; the first contact of European navigators with the western edge of the Pacific Ocean was made by the Portuguese expeditions of António de Abreu and Francisco Serrão, via the Lesser Sunda Islands, to the Maluku Islands, in 1512, with Jorge Álvares's expedition to southern China in 1513, both ordered by Afonso de Albuquerque from Malacca.
The east side of the ocean was discovered by Spanish explorer Vasco Núñez de Balboa in 1513 after his expedition crossed the Isthmus of Panama and reached a new ocean. He named it Mar del Sur because the ocean was to the south of the coast of the isthmus where he first observed the Pacific. In 1519, Portuguese explorer Ferdinand Magellan sailed the Pacific East to West on a Spanish expedition to the Spice Islands that would result in the first world circumnavigation. Magellan called the ocean Pacífico because, after sailing through the stormy seas off Cape Horn, the expedition found calm waters; the ocean was called the Sea of Magellan in his honor until the eighteenth century. Although Magellan himself died in the Philippines in 1521, Spanish Basque navigator Juan Sebastián Elcano led the remains of the expedition back to Spain across the Indian Ocean and round the Cape of Good Hope, completing the first world circumnavigation in a single expedition in 1522. Sailing around and east of the Moluccas, between 1525 and 1527, Portuguese expeditions discovered the Caroline Islands, the Aru Islands, Papua New Guinea.
In 1542–43 the Portuguese reached Japan. In 1564, five Spanish ships carrying 379 explorers crossed the ocean from Mexico led by Miguel López de Legazpi, sailed to the Philippines and Mariana Islands. For the remainder of the 16th century, Spanish influence was paramount, with ships sailing from Mexico and Peru across the Pacific Ocean to the Philippines via Guam, establishing the Spanish East Indies; the Manila galleons operated for two and a half centuries, linking Manila and Acapulco, in one of the longest trade routes in history. Spanish expeditions discovered Tuvalu, the Marquesas, the Cook Islands, the Solomon Islands, the Admiralty Islands in the South Pacific. In the quest for Terra Australis, Spanish explorations in the 17th century, such as the expedition led by the Portuguese navigator Pedro Fernandes de Queirós, discovered the Pitcairn and Vanuatu archipelagos, sailed the Torres Strait between Australia and New Guinea, named after navigator Luís Vaz de Torres. Dutch explorers, sailing around southern Africa engaged in discovery and trade.
In the 16th and 17th centuries Spain considered the Pacific Ocean a mare clausum—a sea closed to other naval powers. As the only known entrance from the Atlantic, the Strait of Magellan was at times patrolled by fleets sent to prevent entrance of non-Spanish ships. On the western side of the Pacific Ocean the Dutch threatened the Spanish Philippines; the 18th cen
Willow
Willows called sallows and osiers, form the genus Salix, around 400 species of deciduous trees and shrubs, found on moist soils in cold and temperate regions of the Northern Hemisphere. Most species are known as willow, but some narrow-leaved shrub species are called osier, some broader-leaved species are referred to as sallow; some willows are creeping shrubs. Willows all have abundant watery bark sap, charged with salicylic acid, soft pliant, tough wood, slender branches, large, fibrous stoloniferous roots; the roots are remarkable for their toughness and tenacity to live, roots sprout from aerial parts of the plant. The leaves are elongated, but may be round to oval with serrated edges. Most species are deciduous. All the buds are lateral; the buds are covered by a single scale. The bud scale is fused into a cap-like shape, but in some species it wraps around and the edges overlap; the leaves are simple, feather-veined, linear-lanceolate. They are serrate, rounded at base, acute or acuminate; the leaf petioles are short, the stipules very conspicuous, resembling tiny, round leaves, sometimes remaining for half the summer.
On some species, they are small and caducous. In color, the leaves show a great variety of greens. Willows are dioecious, with male and female flowers appearing as catkins on separate plants; the staminate flowers are without either calyx with corolla. This scale is square and hairy; the anthers are orange or purple after the flower opens. The filaments are threadlike pale brown, bald; the pistillate flowers are without calyx or corolla, consist of a single ovary accompanied by a small, flat nectar gland and inserted on the base of a scale, borne on the rachis of a catkin. The ovary is one-celled, the style two-lobed, the ovules numerous. All willows take root readily from cuttings or where broken branches lie on the ground; the few exceptions include the goat peachleaf willow. One famous example of such growth from cuttings involves the poet Alexander Pope, who begged a twig from a parcel tied with twigs sent from Spain to Lady Suffolk; this twig was planted and thrived, legend has it that all of England's weeping willows are descended from this first one.
Willows are planted on the borders of streams so their interlacing roots may protect the bank against the action of the water. The roots are much larger than the stem which grows from them. Willows have a wide natural distribution from the tropics to the arctic zones and are extensively cultivated around the world. Willows are cross-compatible, numerous hybrids occur, both and in cultivation. A well-known ornamental example is the weeping willow, a hybrid of Peking willow from China and white willow from Europe; the hybrid cultivar'Boydii' has gained the Royal Horticultural Society's Award of Garden Merit. Numerous cultivars of Salix L. have been named over the centuries. New selections of cultivars with superior technical and ornamental characteristics have been chosen deliberately and applied to various purposes. Most Salix has become an important source for bioenergy production and for various ecosystem services; the first edition of the Checklist for Cultivars of Salix L. was compiled in 2015, which includes 854 cultivar epithets with accompanying information.
The International Poplar Commission of the FAO UN holds the International Cultivar Registration Authority for the genus Salix. The ICRA for Salix produces and maintains The International Register of Cultivars of Salix L.. Willows are used as food plants by the larvae of some Lepidoptera species, such as the mourning cloak butterfly. Ants, such as wood ants, are common on willows inhabited by aphids, coming to collect aphid honeydew, as sometimes do wasps. A small number of willow species were planted in Australia, notably as erosion-control measures along watercourses, they are now regarded as invasive weeds which occupy extensive areas across southern Australia and are considered'Weeds of National Significance'. Many catchment management authorities are replacing them with native trees. Substantial research undertaken from 2006 has identified that willows inhabit an unoccupied niche when they spread across the bed of shallow creeks and streams and if removed, there is a potential water saving of up to 500 ML/per year per hectare of willow canopy area, depending on willow species and climate zone.
This water could benefit the environment or provision of local water resources during dry periods. To aid management of willows, a remote sensing method has been developed to map willow area along and in streams a
United States Geological Survey
The United States Geological Survey is a scientific agency of the United States government. The scientists of the USGS study the landscape of the United States, its natural resources, the natural hazards that threaten it; the organization has four major science disciplines, concerning biology, geography and hydrology. The USGS is a fact-finding research organization with no regulatory responsibility; the USGS is a bureau of the United States Department of the Interior. The USGS employs 8,670 people and is headquartered in Reston, Virginia; the USGS has major offices near Lakewood, Colorado, at the Denver Federal Center, Menlo Park, California. The current motto of the USGS, in use since August 1997, is "science for a changing world." The agency's previous slogan, adopted on the occasion of its hundredth anniversary, was "Earth Science in the Public Service." Since 2012, the USGS science focus is directed at six topical "Mission Areas", namely Climate and Land Use Change, Core Science Systems, Ecosystems and Minerals and Environmental Health, Natural Hazards, Water.
In December 2012, the USGS split the Energy and Minerals and Environmental Health Mission Area resulting in seven topical Mission Areas, with the two new areas being: Energy and Minerals and Environmental Health. Administratively, it is divided into six Regional Units. Other specific programs include: Earthquake Hazards Program monitors earthquake activity worldwide; the National Earthquake Information Center in Golden, Colorado on the campus of the Colorado School of Mines detects the location and magnitude of global earthquakes. The USGS runs or supports several regional monitoring networks in the United States under the umbrella of the Advanced National Seismic System; the USGS informs authorities, emergency responders, the media, the public, both domestic and worldwide, about significant earthquakes. It maintains long-term archives of earthquake data for scientific and engineering research, it conducts and supports research on long-term seismic hazards. USGS has released the UCERF California earthquake forecast.
As of 2005, the agency is working to create a National Volcano Early Warning System by improving the instrumentation monitoring the 169 volcanoes in U. S. territory and by establishing methods for measuring the relative threats posed at each site. The USGS National Geomagnetism Program monitors the magnetic field at magnetic observatories and distributes magnetometer data in real time; the USGS collaborates with Canadian and Mexican government scientists, along with the Commission for Environmental Cooperation, to produce the North American Environmental Atlas, used to depict and track environmental issues for a continental perspective. The USGS operates the streamgaging network for the United States, with over 7400 streamgages. Real-time streamflow data are available online. National Climate Change and Wildlife Science Center implements partner-driven science to improve understanding of past and present land use change, develops relevant climate and land use forecasts, identifies lands and communities that are most vulnerable to adverse impacts of change from the local to global scale.
Since 1962, the Astrogeology Research Program has been involved in global and planetary exploration and mapping. In collaboration with Stanford University, the USGS operates the USGS-Stanford Ion Microprobe Laboratory, a world-class analytical facility for U--Pb geochronology and trace element analyses of minerals and other earth materials. USGS operates a number of water related programs, notably the National Streamflow Information Program and National Water-Quality Assessment Program. USGS Water data is publicly available from their National Water Information System database; the USGS operates the National Wildlife Health Center, whose mission is "to serve the nation and its natural resources by providing sound science and technical support, to disseminate information to promote science-based decisions affecting wildlife and ecosystem health. The NWHC provides information, technical assistance, research and leadership on national and international wildlife health issues." It is the agency responsible for surveillance of H5N1 avian influenza outbreaks in the United States.
The USGS runs 17 biological research centers in the United States, including the Patuxent Wildlife Research Center. The USGS is investigating collaboration with the social networking site Twitter to allow for more rapid construction of ShakeMaps; the USGS produces several national series of topographic maps which vary in scale and extent, with some wide gaps in coverage, notably the complete absence of 1:50,000 scale topographic maps or their equivalent. The largest and best-known topographic series is the 7.5-minute, 1:24,000 scale, quadrangle, a non-metric scale unique to the United States. Each of these maps covers an area bounded by two lines of latitude and two lines of longitude spaced 7.5 minutes apart. Nearly 57,000 individual maps in this series cover the 48 contiguous states, Hawaii, U. S. territories, areas of Alaska near Anchorage and Prudhoe Bay. The area covered by each map varies with the latitude of its represented location due to convergence of the meridians. At lower latitudes, near 30° north, a 7.5-minute quadrangle contains an area of about 64 square miles.
At 49° north latitude, 49 square miles are contained within a quadrangle of that size. As a unique non-metric map scale, the 1:24,000 scale requires a separate and specialized romer scale for pl
Eucalyptus
Eucalyptus is a genus of over seven hundred species of flowering trees, shrubs or mallees in the myrtle family, Myrtaceae known as eucalypts. Plants in the genus Eucalyptus have bark, smooth, fibrous or stringy, leaves with oil glands, sepals and petals that are fused to form a "cap" or operculum over the stamens; the fruit is a woody capsule referred to as a "gumnut". Australia is covered by 92,000,000 hectares of eucalypt forest, comprising three quarters of the area covered by native forest. There most are native to Australia. One species, Eucalyptus deglupta, ranges as far north as the Philippines. Of the 15 species found outside Australia, just nine are non-Australian. Species of eucalyptus are cultivated in the tropical and temperate world, including the Americas, Africa, the Mediterranean Basin, the Middle East and the Indian subcontinent. However, the range over which many eucalypts can be planted in the temperate zone is constrained by their limited cold tolerance. On warm days, eucalyptus forests are sometimes shrouded in a smog-like mist of vaporised volatile organic compounds.
Eucalypts vary in habit from shrubs to tall trees. Trees have a single main stem or trunk but many eucalypts are mallees that are multistemmed from ground level and taller than 10 metres. There is no clear distinction between a mallee and a shrub but in eucalypts, a shrub is a mature plant less than 1 metre tall and growing in an extreme environment. E. vernicosa in the Tasmanian highlands, E. yalatensis on the Nullarbor and E. surgens growing on coastal cliffs in Western Australia are examples of eucalypt shrubs. The terms "mallet" and "marlock" are only applied to Western Australian eucalypts. A mallet is a tree with a single thin trunk with a steeply branching habit but lacks both a lignotuber and epicormic buds. E. astringens is an example of a mallet. A marlock is a shrub or small tree with a single, short trunk, that lacks a lignotuber and has spreading, densely leafy branches that reach to the ground. E. platypus is an example of a marlock. Eucalyptus trees, including mallees and marlocks, are single-stemmed and include Eucalyptus regnans, the tallest known flowering plant on Earth.
The term "morrell" is somewhat obscure in origin and appears to apply to trees of the western Australian wheatbelt and goldfields which have a long, straight trunk rough-barked. It is now used for E. longicornis and E. melanoxylon. Tree sizes follow the convention of: Small: to 10 m in height Medium-sized: 10–30 m Tall: 30–60 m Very tall: over 60 m All eucalypts add a layer of bark every year and the outermost layer dies. In about half of the species, the dead bark is shed exposing a new layer of living bark; the dead bark may be shed in ribbons or in small flakes. These species are known as "smooth barks" and include E. sheathiana, E. diversicolor, E. cosmophylla and E. cladocalyx. The remaining species retain the dead bark which accumulates. In some of these species, the fibres in the bark are loosely intertwined or more adherent. In some species the rough bark is infused with gum resin. Many species are ‘half-barks’ or ‘blackbutts’ in which the dead bark is retained in the lower half of the trunks or stems — for example, E. brachycalyx, E. ochrophloia, E. occidentalis — or only in a thick, black accumulation at the base, as in E. clelandii.
In some species in this category, for example E. youngiana and E. viminalis, the rough basal bark is ribbony at the top, where it gives way to the smooth upper stems. The smooth upper bark of the half-barks and that of the smooth-barked trees and mallees can produce remarkable colour and interest, for example E. deglupta. E. Globulus bark cells are able to photosynthesize in the absence of foliage, conferring an "increased capacity to re-fix internal CO2 following partial defoliation"; this allows the tree to grow in less-than-ideal climates, in addition to providing a better chance of recovery from damage sustained to its leaves in an event such as a fire. Different recognised types of bark include: Stringybark — consists of long fibres and can be pulled off in long pieces, it is thick with a spongy texture. Ironbark — is hard and furrowed, it is impregnated with dried kino which gives a dark red or black colour. Tessellated — bark is broken up into many distinct flakes, they can flake off. Box — has short fibres.
Some show tessellation. Ribbon -- is still loosely attached in some places, they can be firmer strips, or twisted curls. Nearly all eucalyptus are evergreen, but some tropical species lose their leaves at the end of the dry season; as in other members of the myrtle family, eucalyptus leaves are covered with oil glands. The copious oils produced are an important feature of the genus. Although mature eucalyptus trees may be towering and leafed, their shade is characteristically patchy because the leaves hang downwards; the leaves on a mature eucalyptus plant are lanceolate, petiolate alternate and waxy or glossy green. In contrast, the leaves of seedlings are opposite and glaucous, but many exceptions to this pattern exist. Many species
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. The coordinates are chosen such that one of the numbers represents a vertical position and two or three of the numbers represent a horizontal position. A common choice of coordinates is latitude and elevation. To specify a location on a plane requires a map projection; the invention of a geographic coordinate system is credited to Eratosthenes of Cyrene, who composed his now-lost Geography at the Library of Alexandria in the 3rd century BC. A century 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, 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. 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 Brazil abstained. France adopted Greenwich Mean Time in place of local determinations by the Paris Observatory in 1911.
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 they are mapping. They 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 and diurnal Earth tidal movement caused by the Moon and the Sun; this daily movement can be as much as a metre. Continental movement can be up to 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. Examples of global datums include World Geodetic System, the default datum used for the Global Positioning System, the International Terrestrial Reference Frame, used for estimating continental drift and crustal deformation; the distance to Earth's center can be used both for deep positions and for positions in space. Local datums chosen by a national cartographical organisation include the North American Datum, the European ED50, the British OSGB36. Given a location, the datum provides the latitude ϕ and longitude λ. In the United Kingdom there are three common latitude and height systems in use. WGS 84 differs at Greenwich from the one used on published maps OSGB36 by 112 m; the military system ED50, used by NATO, differs from about 120 m to 180 m. The latitude and longitude on a map made against a local datum may not be the same as one obtained from 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 to the Irish Grid add 49 metres to the east, subtract 23.4 metres from the north. More 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, converting back. In popular GIS software, data projected in latitude/longitude is 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'; the "latitude" 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 the center of the Earth. 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 0° parallel of latitude is designated the Equator, the fun
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