Note: Typically, the functional datums are not used to manufacture the part. The manufacturing datums are typically different from the functional datums to save cost, improve process speed, and repeatability. A tolerance analysis may be needed in many cases to convert between the functional datums and the manufacturing datums. Computer software can be purchased for dimensional analysis.
|
|
The datum shift between two particular datums can vary from one place to another within one country or region, and can be anything from zero to hundreds of meters (or several kilometers for some remote islands). The North Pole, South Pole and Equator will be in different positions on different datums, so True North will be slightly different. Different datums use different interpolations for the precise shape and size of the Earth (reference ellipsoids). Because the Earth is an imperfect ellipsoid, localised datums can give a more accurate representation of the area of coverage than WGS 84.
|
|
Datum transformations through the use of empirical multiple regression methods were created to achieve higher accuracy results over small geographic regions than the standard Molodensky transformations. MRE transforms are used to transform local datums over continent-sized or smaller regions to global datums, such as WGS 84. The standard NIMA TM 8350.2, Appendix D, lists MRE transforms from several local datums to WGS 84, with accuracies of about 2 meters. The MREs are a direct transformation of geodetic coordinates with no intermediate ECEF step.
|
|
Ellipsoid-based datums such as WGS 84, GRS80 or NAD83 use a theoretical surface that may differ significantly from the geoid.
|
|
In surveying and geodesy, a datum is a reference system or an approximation of the Earth's surface against which positional measurements are made for computing locations. Horizontal datums are used for describing a point on the Earth's surface, in latitude and longitude or another coordinate system. Vertical datums are used to measure elevations or underwater depths.
|
|
OSGB36, for example, is a better approximation to the geoid covering the British Isles than the global WGS 84 ellipsoid. However, as the benefits of a global system outweigh the greater accuracy, the global WGS 84 datum is becoming increasingly adopted. Horizontal datums are used for describing a point on the Earth's surface, in latitude and longitude or another coordinate system. Vertical datums measure elevations or depths.
|
|
Concrescence is a term coined by Whitehead to show the process of jointly forming an actual entity that was without form, but about to manifest itself into an entity Actual full (satisfaction) based on datums or for information on the universe. The process of forming an actual entity is the case based on the existing datums. Concretion process can be regarded as subjectification process.John B. Cobb and David Ray Griffin.
|
|
The horizontal datum is the model used to measure positions on the Earth. A specific point on the Earth can have substantially different coordinates, depending on the datum used to make the measurement. There are hundreds of local horizontal datums around the world, usually referenced to some convenient local reference point. Contemporary datums, based on increasingly accurate measurements of the shape of the Earth, are intended to cover larger areas.
|
|
The Online Positioning and User Service (OPUS) processes user- input GPS data and outputs position solutions within the NSRS. The agency offers other tools for conversion between datums.
|
|
Equatorial (), polar () and mean Earth radii as defined in the 1984 World Geodetic System revision (not to scale) Currently the most common reference ellipsoid used, and that used in the context of the Global Positioning System, is the one defined by WGS 84. Traditional reference ellipsoids or geodetic datums are defined regionally and therefore non-geocentric, e.g., ED50. Modern geodetic datums are established with the aid of GPS and will therefore be geocentric, e.g.
|
|
NGS will release new datums in 2022. The North American Terrestrial Reference Frame of 2022 (NATRF2022) will supersede NAD83 in defining the geometric relationship between the North American plate and the ITRF. United States territories on the Pacific, Caribbean, and Mariana plates will have their own respective geodetic datums. The North American-Pacific Geopotential Datum of 2022 (NAPGD2022) will separately define the height system of the United States and its territories, replacing NAVD88.
|
|
Datums are used in geodesy, navigation, and surveying by cartographers and satellite navigation systems to translate positions indicated on maps (paper or digital) to their real position on Earth. Each starts with an ellipsoid (stretched sphere), and then defines latitude, longitude and altitude coordinates. One or more locations on the Earth's surface are chosen as anchor "base-points". The difference in co-ordinates between datums is commonly referred to as datum shift.
|
|
Every point that is expressed in ellipsoidal coordinates can be expressed as an rectilinear (Cartesian) coordinate. Cartesian coordinates simplify many mathematical calculations. The Cartesian systems of different datums are not equivalent.
|
|
In fact, there are models called datums that apply to different areas of the earth to provide increased accuracy, like NAD83 for U.S. measurements, and the World Geodetic System for worldwide measurements.
|
|
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 (WGS 84, also known as EPSG:4326 ), the default datum used for the Global Positioning System, and the International Terrestrial Reference Frame (ITRF), used for estimating continental drift and crustal deformation. The distance to Earth's center can be used both for very deep positions and for positions in space. Local datums chosen by a national cartographical organization include the North American Datum, the European ED50, and the British OSGB36.
|
|
A geographic coordinate transformation is a translation among different geodetic datums. Both geographic coordinate conversion and transformation will be considered in this article. This article assumes readers are already familiar with the content in the articles geographic coordinate system and geodetic datum.
|
|
The Molodensky-Badekas transform is used to transform local geodetic datums to a global geodetic datum, such as WGS 84. Unlike the Helmert transform, the Molodensky-Badekas transform is not reversible due to the rotational origin being associated with the original datum.
|
|
Other versions of these transformation formulae include parameters to offset the map coordinates so that all x,y values are positive, as well as a scaling parameter relating the radius of the sphere (earth) to the units used on the map. The formulae used for ellipsoidal datums are more involved.
|
|
These two datums are not both in general use in any one place, but for a point in the English Channel halfway between Dover and Calais, the ED50 longitude lines are about 20 m east of the OSGB36 equivalents, and the ED50 latitude lines are about 150 m south of the OSGB36 ones.
|
|
A geodetic datum or geodetic system (also: geodetic reference datum or geodetic reference system) is a coordinate system, and a set of reference points, used for locating places on the Earth (or similar objects). An approximate definition of sea level is the datum WGS 84, an ellipsoid, whereas a more accurate definition is Earth Gravitational Model 2008 (EGM2008), using at least 2,159 spherical harmonics. Other datums are defined for other areas or at other times; ED50 was defined in 1950 over Europe and differs from WGS 84 by a few hundred meters depending on where in Europe you look. Mars has no oceans and so no sea level, but at least two martian datums have been used to locate places there.
|
|
These are referred to as vertical or height datums. For every point on Earth, the local direction of gravity or vertical direction, materialized with the plumb line, is perpendicular to the geoid. On this is based a method, astrogeodetic levelling, for deriving the local figure of the geoid by measuring deflections of the vertical by astronomical means over an area.
|
|
Screenshot from AutoCAD showing usage of T.I.R., perpendicularity, and parallelism GD&T; datum reference symbols. An engineering datum used in geometric dimensioning and tolerancing is a feature on an object used to create a reference system for measurement.ANSI Y14.5M () for engineering datums. In engineering and drafting, a datum is a reference point, surface, or axis on an object against which measurements are made.
|
|
In the polar regions, a different convention is used.DMA Technical Manual 8358.1, Appendix B. Datums, Ellipsoids, Grids, and Grid Reference Systems. South of 80°S, UPS South (Universal Polar Stereographic) is used instead of a UTM projection. The west half-circle forms a grid zone with designation A; the east half-circle forms one with designation B; see figure 3.
|
|
This old astronomical prime meridian has been replaced by a more precise prime meridian. When Greenwich was an active observatory, geographical coordinates were referred to a local oblate spheroid called a datum known as a geoid, whose surface closely matched local mean sea level. Several datums were in use around the world, all using different spheroids, because mean sea level undulates by as much as 100 metres worldwide.
|
|
Title page, first edition ;Part I: Fundamentals, includes an overview of the types and phases of marine navigation and the organizations which support and regulate it. It includes chapters relating to the structure, use and limitations of nautical charts; chart datums and their importance; and other material of a basic nature. ;Part II: Piloting, emphasizes the practical aspects of navigating a vessel in restricted waters. ;Part III: Electronic Navigation, covers the primary means of positioning of the modern navigator.
|
|
Illustration of vertical datums in the United States. NAVD 88 was established in 1991 by the minimum-constraint adjustment of geodetic leveling observations in Canada, the United States, and Mexico. It held fixed the height of the primary tide gauge benchmark (surveying), referenced to the International Great Lakes Datum of 1985 local mean sea level (MSL) height value, at Rimouski, Quebec, Canada. Additional tidal bench mark elevations were not used due to the demonstrated variations in sea surface topography, i.e.
|
|
An old geodetic pillar (triangulation pillar) (1855) at Ostend, Belgium Geodesy () is the Earth science of accurately measuring and understanding Earth's geometric shape, orientation in space and gravitational field. The field also incorporates studies of how these properties change over time and equivalent measurements for other planets (known as planetary geodesy). Geodynamical phenomena include crustal motion, tides and polar motion, which can be studied by designing global and national control networks, applying space and terrestrial techniques and relying on datums and coordinate systems.
|
|
Conversely, if a ship is to safely pass under a low bridge or overhead power cable, the mariner must know the minimum clearance between the masthead and the obstruction, which will occur at high tide. Consequently, bridge clearances etc. are given relative to a datum based on high tide, such as Highest Astronomical Tide or Mean High Water Springs. Sea level does not remain constant throughout geological time, and so tidal datums are less useful when studying very long- term processes.
|
|
At that time, an insurrection was in progress and the Islands were under military law. The field surveys were conducted under a joint agreement between the USCGS and the Insular Government. From 1901 to 1911, the USCGS established several triangulation networks across the country which had different origins. In 1911, these different networks on different datums and with different origins were consolidated into one nationwide network and the Luzon Datum was established with triangulation station Balanacan as its datum origin.
|
|
The National Transformation version 2 (NTv2) is a Canadian version of NADCON for transforming between NAD 1927 and NAD 1983. HARNs are also known as NAD 83/91 and High Precision Grid Networks (HPGN). Subsequently, Australia and New Zealand adopted the NTv2 format to create grid-based methods for transforming among their own local datums. Like the multiple regression equation transform, grid-based methods use a low-order interpolation method for converting map coordinates, but in two dimensions instead of three.
|
|
This gravity field includes mean anomaly values compiled directly from observed gravity data wherever the latter was available in sufficient quantity. The value for areas of sparse or no observational data were developed from geophysically compatible gravity approximations using gravity- geophysical correlation techniques. Approximately 45 percent of the 410 mean free air gravity anomaly values were determined directly from observed gravity data. The astrogeodetic data in its basic form consists of deflection of the vertical components referred to the various national geodetic datums.
|
|
These deflection values were integrated into astrogeodetic geoid charts referred to these national datums. The geoid heights contributed to the Unified WGS Solution by providing additional and more detailed data for land areas. Conventional ground survey data was included in the solution to enforce a consistent adjustment of the coordinates of neighboring observation sites of the BC-4, SECOR, Doppler and Baker–Nunn systems. Also, eight geodimeter long line precise traverses were included for the purpose of controlling the scale of the solution.
|
|
On average, sea level is higher over mountains and ridges than over abyssal plains and trenches. In the United States the United States Army Corps of Engineers performs or commissions most surveys of navigable inland waterways, while the National Oceanic and Atmospheric Administration (NOAA) performs the same role for ocean waterways. Coastal bathymetry data is available from NOAA's National Geophysical Data Center (NGDC),NCEI-Bathymetry & Relief which is now merged into National Centers for Environmental Information. Bathymetric data is usually referenced to tidal vertical datums.
|
|
The Molodensky transformation converts directly between geodetic coordinate systems of different datums without the intermediate step of converting to geocentric coordinates (ECEF). It requires the three shifts between the datum centers and the differences between the reference ellipsoid semi-major axes and flattening parameters. The Molodensky transform is used by the National Geospatial-Intelligence Agency (NGA) in their standard TR8350.2 and the NGA supported GEOTRANS program. The Molodensky method was popular before the advent of modern computers and the method is part of many geodetic programs.
|
|
In 2005, IOGP absorbed the European Petroleum Survey Group or EPSG (1986–2005) into its structure as IOGP Geomatics Committee. EPSG was a scientific organization with ties to the European petroleum industry consisting of specialists working in applied geodesy, surveying, and cartography related to oil exploration. The EPSG Geodetic Parameter Dataset is a widely used database of Earth ellipsoids, geodetic datums, geographic and projected coordinate systems, units of measurement, etc, which was originally created by EPSG and still carries the EPSG initials to this day.
|
|
He was to persuade various organizations doing the surveys including the railroads to begin using similar datums so the data could interconnect. In 1885 he published Boundaries of the United States and of the Several States and Territories, with a Historical Sketch of the Territorial Changes, which was the first attempt to standardize the history of the boundaries in the country. In 1888 Gannett was one of founding members of the National Geographic Society. He served as its first secretary, and later as treasurer, then vice-president, and finally president, in 1909.
|
|
The tool wear and consequent machine loads were predicted from the STEP-NC data and verified using a dynamometer. In the second half of 2010, the testing forum applied STEP-NC to set up compensation with on-machine measurement of part and fixture datums using a FaroArm portable measurement device. In 2012, the testing focused on machine tool accuracy calculations, culminating in a demonstration in June at the KTH production engineering labs in Stockholm. The test case milled a forged blank for a Crown Wheel Gear on an older Mazak VQC 20.
|
|
In geodesy, conversion among different geographic coordinate systems is made necessary by the different geographic coordinate systems in use across the world and over time. Coordinate conversion is composed of a number of different types of conversion: format change of geographic coordinates, conversion of coordinate systems, or transformation to different geodetic datums. Geographic coordinate conversion has applications in cartography, surveying, navigation and geographic information systems. In geodesy, geographic coordinate conversion is defined as translation among different coordinate formats or map projections all referenced to the same geodetic datum.
|
|
The different possible paths for transforming geographic coordinates from datum A to datum B Transformations among datums can be accomplished in a number of ways. There are transformations that directly convert geodetic coordinates from one datum to another. There are more indirect transforms that convert from geodetic coordinates to ECEF coordinates, transform the ECEF coordinates from one datum to the another, then transform ECEF coordinates of the new datum back to geodetic coordinates. There are also grid-based transformations that directly transform from one (datum, map projection) pair to another (datum, map projection) pair.
|
|
For the Coast and Geodetic Survey, Marmer wrote Tides and Currents in New York Harbor, Coastal Currents along the Pacific Coast of the United States, Tidal Datum Planes, and Chart Datums. He also wrote numerous articles on tides, currents, and general oceanography in various journals and magazines in the United States and other countries. He was the author of two books, The Tide and The Sea, which became standard texts on their subjects. His publications were considered so authoritative that the United States Supreme Court used them in cases involving riparian rights.
|
|
The 1st Topographic Platoon is a military unit located on Camp Pendleton, California. It is composed of members of the Geographic Intelligence Specialist Military Occupational Specialty (MOS) 0261. Most members of the 0261 field are highly trained intelligence operators with extensive knowledge of geospatial intelligence, terrain analysis, land navigation techniques, datums and ellipsoids, coordinate systems, and cartography that make them a valuable asset to the Marine Expeditionary Force. All members are required to attend a 7 month course at the National Geospatial-Intelligence College on Fort Belvoir, Virginia.
|
|
EPSG Geodetic Parameter Dataset (also EPSG registry) is a public registry of geodetic datums, spatial reference systems, Earth ellipsoids, coordinate transformations and related units of measurement. Each entity is assigned an EPSG code between 1024-32767, along with a standard machine-readable well- known text (WKT) representation. The dataset is actively maintained by the IOGP Geomatics Committee. Most geographic information systems (GIS) and GIS libraries use EPSG codes as Spatial Reference System Identifiers (SRIDs) and EPSG definition data for identifying projections and performing transformations between these systems, while some also support SRIDs issued by other organizations (such as Esri).
|
|
The standard ellipsoid for western states in 1849—which is generally congruent with that year's version of the Astronomical Almanac—is implicit in California's constitutional boundary definition; incessant invention of new datums by new and potentially interested parties do not re-render the old boundary definition. Holding assumptions of the earth back-in-time, modern satellite assisted survey techniques can determine location and transform them onto old ellipsoids to within a centimeter. Celestial navigation techniques by contrast, are accurate up to two-fifths of a mile; uncertainty in the latter was known, but precision then was unobtainable. The legacy of this dispute continues.
|
|
Meades Ranch Triangulation Station, fundamental station for the North American Datum of 1927 The North American Datum (NAD) is the horizontal 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, cartography, and land-use planning, two North American Datums are in use for making lateral or "horizontal" measurements: the North American Datum of 1927 (NAD 27) and the North American Datum of 1983 (NAD 83). Both are geodetic reference systems based on slightly different assumptions and measurements.
|
|
483)) Nautical charts display the water's "charted depth" at specific locations with "soundings" and the use of bathymetric contour lines to depict the submerged surface's shape. These depths are relative to a "chart datum", which is typically the water level at the lowest possible astronomical tide (although other datums are commonly used, especially historically, and tides may be lower or higher for meteorological reasons) and are therefore the minimum possible water depth during the tidal cycle. "Drying heights" may also be shown on the chart, which are the heights of the exposed seabed at the lowest astronomical tide.
|
|
Detailed information about NAVD 88 is available here. The definition of NAVD 88 uses the Helmert orthometric height, which calculates the location of the geoid (which approximates MSL) from modeled local gravity. The NAVD 88 model is based on then-available measurements, and remains fixed despite later improved geoid models. Since NGVD 29 used a simple model of gravity based on latitude to calculate the geoid and did not take into account other variations, elevation difference between points in a local area in it and NAVD 88 will show negligible change from one datum to the other, even though the elevation of both does change between datums.
|
|
Depth in a well is not necessarily measured vertically or along a straight line. Because wells are not always drilled vertically, there may be two "depths" for every given point in a wellbore: the measured depth (MD) measured along the path of the borehole, and the true vertical depth (TVD), the absolute vertical distance between the datum and the point in the wellbore. In perfectly vertical wells, the TVD equals the MD; otherwise, the TVD is less than the MD measured from the same datum. Common datums used are ground level (GL), drilling rig floor (DF), rotary table (RT), kelly bushing (KB or RKB) and mean sea level (MSL).
|
|
In some situations sea level does not apply at all — for instance for mapping Mars' surface — forcing the use of a different "zero elevation", such as mean radius. A geodetic vertical datum takes some specific zero point, and computes elevations based on the geodetic model being used, without further reference to sea levels. Usually, the starting reference point is a tide gauge, so at that point the geodetic and tidal datums might match, but due to sea level variations, the two scales may not match elsewhere. An example of a gravity-based geodetic datum is NAVD88, used in North America, which is referenced to a point in Quebec, Canada.
|
|
At the very beginning of her career in mathematics and geodesy, Dr. Fischer had quickly taught herself the basics of geodetic tables, datums, transformations, gravity studies, astronomy, long lines, flare triangulation, and guided missile ballistics. Her updates to geodetic science helped determine the parallax of the moon. Irene Fischer was also intrigued by research into post glacial uplift, and her geoid studies went hand in hand with investigations of the lingering effects of the last ice age. Fischer disagreed with the established figure for the oblateness of the earth (the fraction by which the polar axis is foreshortened by the equatorial radius), which had remained unchallenged since 1924.
|
|
NCEI -Coastal relief models For deep-water bathymetry, this is typically Mean Sea Level (MSL), but most data used for nautical charting is referenced to Mean Lower Low Water (MLLW) in American surveys, and Lowest Astronomical Tide (LAT) in other countries. Many other datums are used in practice, depending on the locality and tidal regime. Occupations or careers related to bathymetry include the study of oceans and rocks and minerals on the ocean floor, and the study of underwater earthquakes or volcanoes. The taking and analysis of bathymetric measurements is one of the core areas of modern hydrography, and a fundamental component in ensuring the safe transport of goods worldwide.
|
|
A tide table for Monterey Bay Aquarium Tide tables, sometimes called tide charts, are used for tidal prediction and show the daily times and levels of high and low tides, usually for a particular location. Tide heights at intermediate times (between high and low water) can be approximated by using the rule of twelfths or more accurately calculated by using a published tidal curve for the location. Tide levels are typically given relative to a low- water vertical datum, e.g. the mean lower low water (MLLW) datum in the US.Tidal Datums And Their Applications, NOAA Special Publication NOS CO-OPS 1, Silver Spring MD, June 2000.
|
|
The burrowing invertebrates that make up large portions of sandy beach ecosystems are known to travel relatively great distances in cross-shore directions as beaches change on the order of days, semilunar cycles, seasons, or years. The distribution of some species has been found to correlate strongly with geomorphic datums such as the high tide strand and the water table outcrop. Since the foreshore is alternately covered by the sea and exposed to the air, organisms living in this environment must have adaptions for both wet and dry conditions. Hazards include being smashed or carried away by rough waves, exposure to dangerously high temperatures, and desiccation.
|
|
In 1897, well after the observatory closed in 1892, the Coast and Geodetic Survey reported that its meridian was west of Greenwich,3618 Serial Set 254 which was quoted for the next 50 years in the list of observatories in the Almanac (as . When referred to later datums, this meridian has been variously specified as or (both presumably NAD 27). If NAD27, the latter would be which seems to be within a few meters of the actual longitude, at worst. This meridian was repealed by Congress on August 22, 1912 to allow the Greenwich meridian to become the legal prime meridian of the United States.
|
|
The difference between the coordinates on different datums varies from place to place. The longitude and latitude positions on OSGB 36 are the same as for WGS 84 at a point in the Atlantic Ocean well to the west of Great Britain. In Cornwall, the WGS 84 longitude lines are about 70 metres east of their OSGB 36 equivalents, this value rising gradually to about 120 m east on the east coast of East Anglia. The WGS 84 latitude lines are about 70 m south of the OSGB 36 lines in South Cornwall, the difference diminishing to zero in the Scottish Borders, and then increasing to about 50 m north on the north coast of Scotland.
|
|
EUREF is driven by a voluntary agreement of scientific organisations and national geodetic/geographic agencies of the member countries, who are willing to replace their legacy national geodetic datums with a unified European Reference Frame and join their national networks of reference stations into a common European network. The mandate of EUREF is to foster and coordinate the activities of the member countries, which are financed and managed on the national level. EUREF realizes and maintains ETRS89, the European geodetic datum, which is to become a single datum used for geodetic applications in all the member countries. A key instrument in maintaining ETRS89 is the EUREF Permanent Network (EPN), a network of reference stations which covers the European continent.
|
|
The corresponding parallel data formats, defined by SMPTE 274, SMPTE 296, and several other standards, are 20-bit standards; thus SMPTE 292M uses a 20-bit word size. Each 20-bit word consists of two 10-bit datums, coming from two logical (and parallel) data channels, one ("Y") which encodes luminance video samples, the other ("C") which encodes chrominance information. The C channel is further time-multiplexed into two half-bandwidth channels, known as Cr (the "red color difference" channel), and Cb (the "blue color difference" channel). The nominal datarate of the Y channel is 75 Mwords/sec (1.5 Gbit/s divided by 20), and the nominal datarate of each of the two chroma channels is 37.5 Mwords/sec.
|
|
It will not remove the effects of local gravity strength, and so the height of MSL, relative to a geodetic datum, will vary around the world, and even around one country. Countries tend to choose the mean sea level at one specific point to be used as the standard “sea level” for all mapping and surveying in that country. (For example, in Great Britain, the national vertical datum, Ordnance Datum Newlyn, is based on what was mean sea level at Newlyn in Cornwall between 1915 and 1921). However, zero elevation as defined by one country is not the same as zero elevation defined by another (because MSL is not the same everywhere), which is why locally defined vertical datums differ from one another.
|
|
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.
|
|
The Datum of 2022 is a placeholder name for a new vertical datum set to be produced by the National Geodetic Survey in 2022 to improve the National Spatial Reference System (NSRS) by replacing the North American Datum of 1983 (NAD 83) and the North American Vertical Datum of 1988 (NAVD 88) with a new geometric reference frame and geopotential datum.New Datums: Replacing NAVD 88 and NAD 83 The new reference frames will rely primarily on Global Navigation Satellite Systems (GNSS), such as the Global Positioning System (GPS), as well as on a gravimetric geoid model resulting from NGS's Gravity for the Redefinition of the American Vertical Datum (GRAV-D) Project. These new reference frames are intended be easier to access and to maintain than NAD 83 and NAVD 88, which rely on physical survey marks that deteriorate over time.
|
|
Google Maps displays satellite imagery using the WGS-84 coordinate system, and street maps using the GCJ-02 datum The China GPS shift (or offset) problem is a class of issues stemming from the difference between the GCJ-02 and WGS-84 datums. Global Positioning System coordinates are expressed using the WGS-84 standard and when plotted on street maps of China that follow the GCJ-02 coordinates, they appear off by a large (often over 500 meters) and variable amount. Authorized providers of location-based services and digital maps (such as AutoNavi or NavInfo) must purchase a "shift correction" algorithm that enables plotting GPS locations correctly on the map. Satellite imagery and user-contributed street map data sets, such as those from OpenStreetMap also display correctly because they have been collected using GPS devices (albeit technically illegally - see Legislation).
|
|
Illustration of vertical datums in the United States. The National Geodetic Vertical Datum of 1929 is the official name since 1973National Geodetic Vertical Datum of 1929 (NGVD 29), at National Geodetic Survey of the vertical datum established for vertical control surveying in the United States of America by the General Adjustment of 1929. Originally known as Sea Level Datum of 1929, NGVD 29 was determined and published by the National Geodetic Survey and used to measure the elevation of a point above and depression below mean sea level (MSL). NGVD29 was superseded by the North American Vertical Datum of 1988 (NAVD 88),North American Vertical Datum of 1988 (NAVD 88), at National Geodetic Survey based upon reference to a single benchmark (referenced to the new International Great Lakes Datum of 1985 local mean sea level height value), although many cities and U.S. Army Corps of Engineers projects with established data continued to use the older datum.
|
|
These projects included ground surveys, definition of geodetic datums, air photography, assistance with definition of Exclusive Economic Zones, mapping, provision of equipment and technology transfer and training of officers and technicians. Projects commenced in 1970 in Indonesia and expanded over 25 years to include Solomon Islands, Fiji, Tonga, Kiribati, Nauru, Tuvalu, Vanuatu and Western Samoa. Technical Advisers were posted to national survey and mapping organisations in Fiji, Indonesia, Malaysia, Papua New Guinea, Solomon Islands and Vanuatu. All field survey operations outside of Australia, and indeed in Australia, would not have been possible without essential support of most other Army Corps (Engineers; Signals; Aviation – Cessna, Porter, Nomad, Sioux, Kiowa; Chaplains, Medical, Dental, Transport, Ordnance, Electrical and Mechanical Engineers, Pay, Catering, Service), the Royal Australian Navy (Hydrographic Service, Landing Craft, Patrol Boats) and at times the Royal New Zealand Navy (Hydrographic Service), the Royal Australian Air Force (Canberra, Hercules, Caribou, Iroquois) and civil charter fixed wing and helicopters for aerial survey work and transport.
|
|
A datum reference or just datum is some important part of an object—such as a point, line, plane, hole, set of holes, or pair of surfaces—that serves as a reference in defining the geometry of the object and (often) in measuring aspects of the actual geometry to assess how closely they match with the nominal value, which may be an ideal, standard, average, or desired value. For example, on a car's wheel, the lug nut holes define a bolt circle that is a datum from which the location of the rim can be defined and measured. This matters because the hub and rim need to be concentric to within close limits (or else the wheel will not roll smoothly). The concept of datumsThe plural of this sense of the word datum is datums by convention, in contrast with the other senses of the word in which data often serves as a plural form.) is used in many fields, including carpentry, metalworking, needlework, geometric dimensioning and tolerancing (GD&T;), aviation, surveying, and others.
|
|