Galileo (satellite navigation)
Galileo is the global navigation satellite system that went live in 2016, created by the European Union through the European GNSS Agency, headquartered in Prague in the Czech Republic, with two ground operations centres, Oberpfaffenhofen near Munich in Germany and Fucino in Italy. The €10 billion project is named after the Italian astronomer Galileo Galilei. One of the aims of Galileo is to provide an independent high-precision positioning system so European nations do not have to rely on the U. S. GPS, or the Russian GLONASS systems, which could be disabled or degraded by their operators at any time; the use of basic Galileo services will be open to everyone. The higher-precision capabilities will be available for paying commercial users. Galileo is intended to provide horizontal and vertical position measurements within 1-metre precision, better positioning services at higher latitudes than other positioning systems. Galileo is to provide a new global search and rescue function as part of the MEOSAR system.
The first Galileo test satellite, the GIOVE-A, was launched 28 December 2005, while the first satellite to be part of the operational system was launched on 21 October 2011. As of July 2018, 26 of the planned 30 active satellites are in orbit. Galileo started offering Early Operational Capability on 15 December 2016, providing initial services with a weak signal, is expected to reach Full Operational Capability in 2019; the complete 30-satellite Galileo system is expected by 2020. It is expected that the next generation of satellites will begin to become operational by 2025 to replace older equipment. Older systems can be used for backup capabilities. There are 22 satellites in usable condition, 2 satellites are in "testing" and 2 more are marked as not available. In 1999, the different concepts of the three main contributors of ESA for Galileo were compared and reduced to one by a joint team of engineers from all three countries; the first stage of the Galileo programme was agreed upon on 26 May 2003 by the European Union and the European Space Agency.
The system is intended for civilian use, unlike the more military-oriented systems of the United States and China. The European system will only be subject to shutdown for military purposes in extreme circumstances, it will be available at its full precision to both military users. The countries that contribute most to the Galileo Project are Italy; the European Commission had some difficulty funding the project's next stage, after several "per annum" sales projection graphs for the project were exposed in November 2001 as "cumulative" projections which for each year projected included all previous years of sales. The attention, brought to this multibillion-euro growing error in sales forecasts resulted in a general awareness in the Commission and elsewhere that it was unlikely that the program would yield the return on investment, suggested to investors and decision-makers. On 17 January 2002, a spokesman for the project stated that, as a result of US pressure and economic difficulties, "Galileo is dead."A few months however, the situation changed dramatically.
European Union member states decided it was important to have a satellite-based positioning and timing infrastructure that the US could not turn off in times of political conflict. The European Union and the European Space Agency agreed in March 2002 to fund the project, pending a review in 2003; the starting cost for the period ending in 2005 is estimated at €1.1 billion. The required satellites were to be launched between 2011 and 2014, with the system up and running and under civilian control from 2019; the final cost is estimated at €3 billion, including the infrastructure on Earth, constructed in 2006 and 2007. The plan was for private companies and investors to invest at least two-thirds of the cost of implementation, with the EU and ESA dividing the remaining cost; the base Open Service is to be available without charge to anyone with a Galileo-compatible receiver, with an encrypted higher-bandwidth improved-precision Commercial Service available at a cost. By early 2011 costs for the project had run 50% over initial estimates.
Galileo is intended to be an EU civilian GNSS. GPS reserved the highest quality signal for military use, the signal available for civilian use was intentionally degraded; this changed with President Bill Clinton signing a policy directive in 1996 to turn off Selective Availability. Since May 2000 the same precision signal has been provided to the military. Since Galileo was designed to provide the highest possible precision to anyone, the US was concerned that an enemy could use Galileo signals in military strikes against the US and its allies; the frequency chosen for Galileo would have made it impossible for the US to block the Galileo signals without interfering with its own GPS signals. The US did not want to lose their GNSS capability with GPS while denying enemies the use of GNSS; some US officials became concerned when Chinese interest in Galileo was reported. An anonymous EU official claimed that the US officials implied that they might consider shooting down Galileo satellites in the event of a major conflict in which Galileo was used in attacks against American forces.
The EU's stance is that Galileo is a neutra
Cartography is the study and practice of making maps. Combining science and technique, cartography builds on the premise that reality can be modeled in ways that communicate spatial information effectively; the fundamental problems of traditional cartography are to: Set the map's agenda and select traits of the object to be mapped. This is the concern of map editing. Traits may be physical, such as roads or land masses, or may be abstract, such as toponyms or political boundaries. Represent the terrain of the mapped object on flat media; this is the concern of map projections. Eliminate characteristics of the mapped object that are not relevant to the map's purpose; this is the concern of generalization. Reduce the complexity of the characteristics that will be mapped; this is the concern of generalization. Orchestrate the elements of the map to best convey its message to its audience; this is the concern of map design. Modern cartography constitutes many theoretical and practical foundations of geographic information systems.
What is the earliest known map is a matter of some debate, both because the term "map" is not well-defined and because some artifacts that might be maps might be something else. A wall painting that might depict the ancient Anatolian city of Çatalhöyük has been dated to the late 7th millennium BCE. Among the prehistoric alpine rock carvings of Mount Bego and Valcamonica, dated to the 4th millennium BCE, geometric patterns consisting of dotted rectangles and lines are interpreted in archaeological literature as a depiction of cultivated plots. Other known maps of the ancient world include the Minoan "House of the Admiral" wall painting from c. 1600 BCE, showing a seaside community in an oblique perspective, an engraved map of the holy Babylonian city of Nippur, from the Kassite period. The oldest surviving world maps are from 9th century BCE Babylonia. One shows Babylon on the Euphrates, surrounded by Assyria and several cities, all, in turn, surrounded by a "bitter river". Another depicts Babylon as being north of the center of the world.
The ancient Greeks and Romans created maps from the time of Anaximander in the 6th century BCE. In the 2nd century CE, Ptolemy wrote his treatise on Geographia; this contained Ptolemy's world map – the world known to Western society. As early as the 8th century, Arab scholars were translating the works of the Greek geographers into Arabic. In ancient China, geographical literature dates to the 5th century BCE; the oldest extant Chinese maps come from the State of Qin, dated back to the 4th century BCE, during the Warring States period. In the book of the Xin Yi Xiang Fa Yao, published in 1092 by the Chinese scientist Su Song, a star map on the equidistant cylindrical projection. Although this method of charting seems to have existed in China before this publication and scientist, the greatest significance of the star maps by Su Song is that they represent the oldest existent star maps in printed form. Early forms of cartography of India included depictions of the pole star and surrounding constellations.
These charts may have been used for navigation. "Mappae mundi are the medieval European maps of the world. About 1,100 of these are known to have survived: of these, some 900 are found illustrating manuscripts and the remainder exist as stand-alone documents; the Arab geographer Muhammad al-Idrisi produced his medieval atlas Tabula Rogeriana in 1154. By combining the knowledge of Africa, the Indian Ocean and the Far East with the information he inherited from the classical geographers, he was able to write detailed descriptions of a multitude of countries. Along with the substantial text he had written, he created a world map influenced by the Ptolemaic conception of the world, but with significant influence from multiple Arab geographers, it remained the most accurate world map for the next three centuries. The map was divided with detailed descriptions of each zone; as part of this work, a smaller, circular map was made depicting the south on top and Arabia in the center. Al-Idrisi made an estimate of the circumference of the world, accurate to within 10%.
In the Age of Exploration, from the 15th century to the 17th century, European cartographers both copied earlier maps and drew their own, based on explorers' observations and new surveying techniques. The invention of the magnetic compass and sextant enabled increasing accuracy. In 1492, Martin Behaim, a German cartographer, made the oldest extant globe of the Earth. In 1507, Martin Waldseemüller produced a globular world map and a large 12-panel world wall map bearing the first use of the name "America". Portuguese cartographer Diego Ribero was the author of the first known planisphere with a graduated Equator. Italian cartographer Battista Agnese produced at least 71 manuscript atlases of sea charts. Johannes Werner promoted the Werner projection; this was an equal-area, heart-shaped world map projection, used in the 16th and 17th centuries. Over time, other iterations of this map type arose; the Werner projection places its standard parallel at the North Pole. In 1569, mapmaker Gerardus Mercato
Royal Observatory, Greenwich
The Royal Observatory, Greenwich is an observatory situated on a hill in Greenwich Park, overlooking the River Thames. It played a major role in the history of astronomy and navigation, is best known for the fact that the prime meridian passes through it, thereby gave its name to Greenwich Mean Time; the ROG has the IAU observatory code of the first in the list. ROG, the National Maritime Museum, the Queen's House and Cutty Sark are collectively designated Royal Museums Greenwich; the observatory was commissioned in 1675 by King Charles II, with the foundation stone being laid on 10 August. The site was chosen by Sir Christopher Wren. At that time the king created the position of Astronomer Royal, to serve as the director of the observatory and to "apply himself with the most exact care and diligence to the rectifying of the tables of the motions of the heavens, the places of the fixed stars, so as to find out the so much desired longitude of places for the perfecting of the art of navigation."
He appointed John Flamsteed as the first Astronomer Royal. The building was completed in the summer of 1676; the building was called "Flamsteed House", in reference to its first occupant. The scientific work of the observatory was relocated elsewhere in stages in the first half of the 20th century, the Greenwich site is now maintained exclusively as a museum, although the AMAT telescope became operational for astronomical research in 2018. 1675 – 22 June, Royal Observatory founded. 1675 – 10 August, construction began. 1714 Longitude Act established the Board of Longitude rewards. The Astronomer Royal was, until the Board was dissolved in 1828, always an ex officio Commissioner of Longitude. 1767 Astronomer Royal Nevil Maskelyne began publication of the Nautical Almanac, based on observations made at the Observatory. 1818 Oversight of the Royal Observatory was transferred from the Board of Ordnance to the Board of Admiralty. 1833 Daily time signals began. 1899 The New Physical Observatory was completed.
1924 Hourly time signals from the Royal Observatory were first broadcast on 5 February. 1948 Office of the Astronomer Royal was moved to Herstmonceux. 1957 Royal Observatory completed its move to Herstmonceux. The Greenwich site is renamed the Old Royal Observatory. 1990 RGO moved to Cambridge. 1998 RGO closed. Greenwich site is returned to its original name, the Royal Observatory, is made part of the National Maritime Museum. 2011 The Greenwich museums, including the ROG, become collectively the Royal Museums Greenwich. There had been significant buildings on this land since the reign of William I. Greenwich Palace, on the site of the present-day Maritime Museum, was the birthplace of both Henry VIII and his daughters Mary I and Elizabeth I. Greenwich Castle was a favourite place for Henry VIII to house his mistresses, so that he could travel from the Palace to see them; the establishment of a Royal Observatory was proposed in 1674 by Sir Jonas Moore who, in his role as Surveyor General at the Ordnance Office, persuaded King Charles II to create the observatory, with John Flamsteed installed as its director.
The Ordnance Office was given responsibility for building the Observatory, with Moore providing the key instruments and equipment for the observatory at his own personal cost. Flamsteed House, the original part of the Observatory, was designed by Sir Christopher Wren assisted by Robert Hooke, was the first purpose-built scientific research facility in Britain, it was built for a cost of £520 out of recycled materials on the foundations of Duke Humphrey's Tower, the forerunner of Greenwich Castle, which resulted in the alignment being 13 degrees away from true North, somewhat to Flamsteed's chagrin. The original observatory at first housed the scientific instruments to be used by Flamsteed in his work on stellar tables, over time incorporated additional responsibilities such as marking the official time of day, housing Her Majesty's Nautical Almanac Office. Moore donated two clocks, built by Thomas Tompion, which were installed in the 20 foot high Octagon Room, the principal room of the building.
They were of unusual design, each with a pendulum 13 feet in length mounted above the clock face, giving a period of four seconds and an accuracy unparalleled, of seven seconds per day. British astronomers have long used the Royal Observatory as a basis for measurement. Four separate meridians have passed through the buildings, defined by successive instruments; the basis of longitude, the meridian that passes through the Airy transit circle, first used in 1851, was adopted as the world's Prime Meridian at the International Meridian Conference on 22 October 1884. Subsequently, nations across the world used it as their standard for timekeeping; the Prime Meridian was marked by a brass strip in the Observatory's courtyard once the buildings became a museum in 1960, since 16 December 1999, has been marked by a powerful green laser shining north across the London night sky. Since the first triangulation of Great Britain in the period 1783–1853, Ordnance Survey maps have been based on an earlier version of the Greenwich meridian, defined by the transit instrument of James
Global Positioning System
The Global Positioning System Navstar GPS, is a satellite-based radionavigation system owned by the United States government and operated by the United States Air Force. It is a global navigation satellite system that provides geolocation and time information to a GPS receiver anywhere on or near the Earth where there is an unobstructed line of sight to four or more GPS satellites. Obstacles such as mountains and buildings block the weak GPS signals; the GPS does not require the user to transmit any data, it operates independently of any telephonic or internet reception, though these technologies can enhance the usefulness of the GPS positioning information. The GPS provides critical positioning capabilities to military and commercial users around the world; the United States government created the system, maintains it, makes it accessible to anyone with a GPS receiver. The GPS project was launched by the U. S. Department of Defense in 1973 for use by the United States military and became operational in 1995.
It was allowed for civilian use in the 1980s. Advances in technology and new demands on the existing system have now led to efforts to modernize the GPS and implement the next generation of GPS Block IIIA satellites and Next Generation Operational Control System. Announcements from Vice President Al Gore and the White House in 1998 initiated these changes. In 2000, the U. S. Congress authorized the modernization effort, GPS III. During the 1990s, GPS quality was degraded by the United States government in a program called "Selective Availability"; the GPS system is provided by the United States government, which can selectively deny access to the system, as happened to the Indian military in 1999 during the Kargil War, or degrade the service at any time. As a result, several countries have developed or are in the process of setting up other global or regional satellite navigation systems; the Russian Global Navigation Satellite System was developed contemporaneously with GPS, but suffered from incomplete coverage of the globe until the mid-2000s.
GLONASS can be added to GPS devices, making more satellites available and enabling positions to be fixed more and to within two meters. China's BeiDou Navigation Satellite System is due to achieve global reach in 2020. There are the European Union Galileo positioning system, India's NAVIC. Japan's Quasi-Zenith Satellite System is a GPS satellite-based augmentation system to enhance GPS's accuracy; when selective availability was lifted in 2000, GPS had about a five-meter accuracy. The latest stage of accuracy enhancement uses the L5 band and is now deployed. GPS receivers released in 2018 that use the L5 band can have much higher accuracy, pinpointing to within 30 centimetres or 11.8 inches. The GPS project was launched in the United States in 1973 to overcome the limitations of previous navigation systems, integrating ideas from several predecessors, including classified engineering design studies from the 1960s; the U. S. Department of Defense developed the system, which used 24 satellites, it was developed for use by the United States military and became operational in 1995.
Civilian use was allowed from the 1980s. Roger L. Easton of the Naval Research Laboratory, Ivan A. Getting of The Aerospace Corporation, Bradford Parkinson of the Applied Physics Laboratory are credited with inventing it; the work of Gladys West is credited as instrumental in the development of computational techniques for detecting satellite positions with the precision needed for GPS. The design of GPS is based on similar ground-based radio-navigation systems, such as LORAN and the Decca Navigator, developed in the early 1940s. Friedwardt Winterberg proposed a test of general relativity – detecting time slowing in a strong gravitational field using accurate atomic clocks placed in orbit inside artificial satellites. Special and general relativity predict that the clocks on the GPS satellites would be seen by the Earth's observers to run 38 microseconds faster per day than the clocks on the Earth; the GPS calculated positions would drift into error, accumulating to 10 kilometers per day. This was corrected for in the design of GPS.
Winterberg, Friedwardt. “Relativistische Zeitdilatation eines künstlichen Satelliten ” When the Soviet Union launched the first artificial satellite in 1957, two American physicists, William Guier and George Weiffenbach, at Johns Hopkins University's Applied Physics Laboratory decided to monitor its radio transmissions. Within hours they realized that, because of the Doppler effect, they could pinpoint where the satellite was along its orbit; the Director of the APL gave them access to their UNIVAC to do the heavy calculations required. Early the next year, Frank McClure, the deputy director of the APL, asked Guier and Weiffenbach to investigate the inverse problem—pinpointing the user's location, given that of the satellite; this led them and APL to develop the TRANSIT system. In 1959, ARPA played a role in TRANSIT. TRANSIT was first tested in 1960, it used a constellation of five satellites and could provide a navigational fix once per hour. In 1967, the U. S. Navy developed the Timation satellite, which proved the feasibility of placing accurate clocks in space, a technology required for GPS.
In the 1970s, the ground-based OMEGA navigation system, based on phase comparison of signal transmission from pairs of stations
Geodesy, is the Earth science of measuring and understanding Earth's geometric shape, orientation in space, gravitational field. The field incorporates studies of how these properties change over time and equivalent measurements for other planets. Geodynamical phenomena include crustal motion and polar motion, which can be studied by designing global and national control networks, applying space and terrestrial techniques, relying on datums and coordinate systems; the word "geodesy" comes from the Ancient Greek word γεωδαισία geodaisia. It is concerned with positioning within the temporally varying gravity field. Geodesy in the German-speaking world is divided into "higher geodesy", concerned with measuring Earth on the global scale, "practical geodesy" or "engineering geodesy", concerned with measuring specific parts or regions of Earth, which includes surveying; such geodetic operations are applied to other astronomical bodies in the solar system. It is the science of measuring and understanding Earth's geometric shape, orientation in space, gravity field.
To a large extent, the shape of Earth is the result of rotation, which causes its equatorial bulge, the competition of geological processes such as the collision of plates and of volcanism, resisted by Earth's gravity field. This applies to the liquid surface and Earth's atmosphere. For this reason, the study of Earth's gravity field is called physical geodesy; the geoid is the figure of Earth abstracted from its topographical features. It is an idealized equilibrium surface of sea water, the mean sea level surface in the absence of currents and air pressure variations, continued under the continental masses; the geoid, unlike the reference ellipsoid, is irregular and too complicated to serve as the computational surface on which to solve geometrical problems like point positioning. The geometrical separation between the geoid and the reference ellipsoid is called the geoidal undulation, it varies globally between ± 110 m. A reference ellipsoid, customarily chosen to be the same size as the geoid, is described by its semi-major axis a and flattening f.
The quantity f = a − b/a, where b is the semi-minor axis, is a purely geometrical one. The mechanical ellipticity of Earth can be determined to high precision by observation of satellite orbit perturbations, its relationship with the geometrical flattening is indirect. The relationship depends on the internal density distribution, or, in simplest terms, the degree of central concentration of mass; the 1980 Geodetic Reference System posited a 1:298.257 flattening. This system was adopted at the XVII General Assembly of the International Union of Geodesy and Geophysics, it is the basis for geodetic positioning by the Global Positioning System and is thus in widespread use outside the geodetic community. The numerous systems that countries have used to create maps and charts are becoming obsolete as countries move to global, geocentric reference systems using the GRS 80 reference ellipsoid; the geoid is "realizable", meaning it can be located on Earth by suitable simple measurements from physical objects like a tide gauge.
The geoid can, therefore, be considered a real surface. The reference ellipsoid, has many possible instantiations and is not realizable, therefore it is an abstract surface; the third primary surface of geodetic interest—the topographic surface of Earth—is a realizable surface. The locations of points in three-dimensional space are most conveniently described by three cartesian or rectangular coordinates, X, Y and Z. Since the advent of satellite positioning, such coordinate systems are geocentric: the Z-axis is aligned with Earth's rotation axis. Prior to the era of satellite geodesy, the coordinate systems associated with a geodetic datum attempted to be geocentric, but their origins differed from the geocenter by hundreds of meters, due to regional deviations in the direction of the plumbline; these regional geodetic data, such as ED 50 or NAD 27 have ellipsoids associated with them that are regional "best fits" to the geoids within their areas of validity, minimizing the deflections of the vertical over these areas.
It is only because GPS satellites orbit about the geocenter, that this point becomes the origin of a coordinate system defined by satellite geodetic means, as the satellite positions in space are themselves computed in such a system. Geocentric coordinate systems used in geodesy can be divided into two classes: Inertial reference systems, where the coordinate axes retain their orientation relative to the fixed stars, or equivalently, to the rotation axes of ideal gyroscopes; the X-axis lies within the Greenwich observatory's meridian plane. The coordinate transformation between these two systems is described to good approximation by sidereal time, which takes into account variations in Earth's axial rotation. A more accurate description takes polar motion into account, a phenomenon monitored by geodesists. In surveying and mapping, important fields of application of geodesy, two general types of coordinate systems are used in the plane
Figure of the Earth
Figure of the Earth is a term of art in geodesy that refers to the size and shape used to model Earth. The size and shape it refers to depend on context, including the precision needed for the model; the sphere is an approximation of the figure of the Earth, satisfactory for many purposes. Several models with greater accuracy have been developed so that coordinate systems can serve the precise needs of navigation, cadastre, land use, various other concerns. Earth's topographic surface is apparent with its variety of land forms and water areas; this topographic surface is the concern of topographers and geophysicists. While it is the surface on which Earth measurements are made, mathematically modeling it while taking the irregularities into account would be complicated; the Pythagorean concept of a spherical Earth offers a simple surface, easy to deal with mathematically. Many astronomical and navigational computations use a sphere to model the Earth as a close approximation. However, a more accurate figure is needed for measuring distances and areas on the scale beyond the purely local.
Better approximations can be had by modeling the entire surface as an oblate spheroid, using spherical harmonics to approximate the geoid, or modeling a region with a best-fit reference ellipsoids. For surveys of small areas, a planar model of Earth's surface suffices because the local topography overwhelms the curvature. Plane-table surveys are made for small areas without considering the size and shape of the entire Earth. A survey of a city, for example, might be conducted this way. By the late 1600s, serious effort was devoted to modeling the Earth as an ellipsoid, beginning with Jean Picard's measurement of a degree of arc along the Paris meridian. Improved maps and better measurement of distances and areas of national territories motivated these early attempts. Surveying instrumentation and techniques improved over the ensuing centuries. Models for the figure of the earth improved in step. In the mid- to late 20th century, research across the geosciences contributed to drastic improvements in the accuracy of the figure of the Earth.
The primary utility of this improved accuracy was to provide geographical and gravitational data for the inertial guidance systems of ballistic missiles. This funding drove the expansion of geoscientific disciplines, fostering the creation and growth of various geoscience departments at many universities; these developments benefited many civilian pursuits as well, such as weather and communication satellite control and GPS location-finding, which would be impossible without accurate models for the figure of the Earth. The models for the figure of the Earth vary in the way they are used, in their complexity, in the accuracy with which they represent the size and shape of the Earth; the simplest model for the shape of the entire Earth is a sphere. The Earth's radius is the distance from Earth's center to about 6,371 kilometers. While "radius" is a characteristic of perfect spheres, the Earth deviates from spherical by only a third of a percent, sufficiently close to treat it as a sphere in many contexts and justifying the term "the radius of the Earth".
The concept of a spherical Earth dates back to around the 6th century BC, but remained a matter of philosophical speculation until the 3rd century BC. The first scientific estimation of the radius of the Earth was given by Eratosthenes about 240 BC, with estimates of the accuracy of Eratosthenes’s measurement ranging from 2% to 15%; the Earth is only spherical, so no single value serves as its natural radius. Distances from points on the surface to the center range from 6,353 km to 6,384 km. Several different ways of modeling the Earth as a sphere each yield a mean radius of 6,371 kilometers. Regardless of the model, any radius falls between the polar minimum of about 6,357 km and the equatorial maximum of about 6,378 km; the difference 21 kilometers correspond to the polar radius being 0.3% shorter than the equator radius. Since the Earth is flattened at the poles and bulges at the Equator, geodesy represents the figure of the Earth as an oblate spheroid; the oblate spheroid, or oblate ellipsoid, is an ellipsoid of revolution obtained by rotating an ellipse about its shorter axis.
It is the regular geometric shape. A spheroid describing the figure of the Earth or other celestial body is called a reference ellipsoid; the reference ellipsoid for Earth is called an Earth ellipsoid. An ellipsoid of revolution is uniquely defined by two quantities. Several conventions for expressing the two quantities are used in geodesy, but they are all equivalent to and convertible with each other: Equatorial radius a, polar radius b. Eccentricity and flattening are different ways of expressing; when flattening appears as one of the defining quantities in geodesy it is expressed by its reciprocal. For example, in the WGS 84 spheroid used by today's GPS systems, the reciprocal of the flattening 1 / f is set to be 298.257223563. The difference between a sphere and a reference ellipsoid for Earth is small, only about one part in 300. Flattening was computed from grade measurements. Nowadays, geodetic networks and satellite geodesy are used. In practice, many reference ellip
North American Datum
The North American Datum is the datum now used to define the geodetic network in North America. A datum is a formal description of the shape of the Earth along with an "anchor" point for the coordinate system. In surveying and land-use planning, two North American Datums are in use: the North American Datum of 1927 and the North American Datum of 1983. Both are geodetic reference systems based on different assumptions and measurements. In 1901 the United States Coast and Geodetic Survey adopted a national horizontal datum called the United States Standard Datum, based on the Clarke Ellipsoid of 1866, it was fitted to data collected for regional datums, which by that time had begun to overlap. In 1913, Canada and Mexico adopted that datum, so it was renamed the North American Datum; as more data were gathered, discrepancies appeared, so the datum was recomputed in 1927, using the same spheroid and origin as its predecessor. The North American Datum of 1927 was based on surveys of the entire continent from a common reference point, chosen in 1901, because it was as near the center of the contiguous United States as could be calculated: It was based on a triangulation station at the junction of the transcontinental triangulation arc of 1899 on the 39th parallel north and the triangulation arc along the 98th meridian west, near the geographic center of the contiguous United States.
The datum declares the Meades Ranch Triangulation Station to be 39°13′26.686″ north latitude, 98°32′30.506″ west longitude. NAD 27 is oriented by declaring the azimuth from Meades Ranch to Waldo to be 255°28′14.52″ from north. The latitude and longitude of every other point in North America is based on its distance and direction from Meades Ranch: If a point was X meters in azimuth Y degrees from Meades Ranch, measured on the Clarke Ellipsoid of 1866 its latitude and longitude on that ellipsoid were defined and could be calculated. †By definition. ††Calculated. These are the defining dimensions for NAD 27, but Clarke defined his 1866 spheroid as a = 20,926,062 British feet, b = 20,855,121 British feet; the conversion to meters uses Clarke's 1865 inch-meter ratio of 39.370432. Most USGS topographic maps were published in NAD 27 and many major projects by the United States Army Corps of Engineers and other agencies were defined in NAD 27, so the datum remains important, despite more refined datums being available.
Because Earth deviates from a perfect ellipsoid, the ellipsoid that best approximates its shape varies region by region across the world. Clarke 1866, North American Datum of 1927 with it, were surveyed to best suit North America as a whole. Most regions of the world used ellipsoids measured locally to best suit the vagaries of Earth's shape in their respective locales. While ensuring the most accuracy locally, this practice makes integrating and disseminating information across regions troublesome; as satellite geodesy and remote sensing technology reached high precision and were made available for civilian applications, it became feasible to acquire information referred to a single global ellipsoid. This is because satellites deal with Earth as a monolithic body. Therefore, the GRS 80 ellipsoid was developed for best approximating the Earth as a whole, it became the foundation for the North American Datum of 1983. Though GRS 80 and its close relative, WGS 84, are not the best fit for any given region, a need for the closest fit evaporates when a global survey is combined with computers and software able to compensate for local conditions.
†By definition. ††Calculated. A point having a given latitude and longitude in NAD 27 may be displaced on the order of many tens of meters from another point having the identical latitude and longitude in NAD 83, so it is important to specify the datum along with the coordinates; the North American Datum of 1927 is defined by the latitude and longitude of an initial point, the direction of a line between this point and a specified second point, two dimensions that define the spheroid. The North American Datum of 1983 is based on a newer defined spheroid. NOAA provides a converter between the two systems; the practical impact is that if you use current GPS device set to work in NAD 83 or WGS 84 to navigate to NAD 27 coordinates in Seattle, you would be off by about 95 meters, you'd be about 47 meters off in Miami, whereas you would be much closer for points near Chicago. The initial definition of NAD 83 was intended to match GRS 80 and WGS 84, so many older publications indicate no difference.
Subsequent measurements found a difference on the order of a meter over much of the United States. Each datum has undergone refinements with more accurate and measurements. NAD 83 is defined to remain constant over time for points on the North American Plate, whereas WGS 84 is defined with respect to the average of stations all over the world, thus the two systems diverge over time. For much of the United States the relative rate is on the order of 1 to 2 cm per year. Hawaii and the coastal portions of central and southern California west of the San Andreas Fault are not on the North American plate, so their divergence rate differs; the United States National Spatial Reference System NAD 83 epoch 2010.00, is a refinement of the NAD 83 datum using data from a network of accurate GPS receivers at Co