North American Datum
The North American Datum ( NAD) is the common reference system for the land surveying the states of North and Central America.
History requires a distinction between the means of classical triangulation founded North American Datum of 1927 (NAD 27) ( with a market on the earth's surface fundamental point ) and the more modern built up by means of satellite geodesy North American Datum of 1983 ( NAD 83 ) ( with a geocentric origin).
The first national geodetic datum used in the United States was the New England Date of 1879th As a fundamental point Principio was set at Perryville in Maryland ( at approximately 39 ° 35 '30 " N, 76 ° 0' 20" W39.591666666667 - 76.005555555556 ). This location was chosen because he was still at that time in the center of the geographical interest of the United States.
From 1871 to 1899, the National Geodetic Survey conducted a large-scale transcontinental surveying campaign along the 39th parallel, to join the already well -developed survey network of the eastern states with the Pacific West Coast. As part of this expansion, a shift of the origin point of Principle in the middle of the North American continent was necessary. As a new fundamental point was chosen Meades Ranch, between the towns of Lucas and Tipton in Kansas ( at 39 ° 13 ' 27 " N, 98 ° 32 ' 32 " W39.224086111111 - 98.54215 (WGS84) ). Consequently, we named the New England date to date in U.S. standard.
North American Datum of 1927 (NAD 27)
After 1913 Canada and Mexico had also decided to build their surveying networks based on the standard U.S. date, this was renamed the North American Datum. It is based, like its predecessors, on the Clarke ellipsoid of 1866 and his fundamental point is Meades Ranch in Kansas.
1927, the positions of 25,000 fixed points were recalculated by adjustment methods based on past survey data. From the thus-formed frame of reference fixed points, the term NAD 27 is shown.
North American Datum of 1983 ( NAD 83)
With the proliferation of electronic surveying methods, it came in the 1970s increased to contradictions between the reference system of NAD 27 and the results of local surveys. Also, the use of the Clarke ellipsoid presented as the basis of the reference system, they are too vague and out of date. So it was decided a new reference system based on the geodetic reference system 1980 build as Earth model ( GRS 80) with a geocentric origin. Thanks to modern methods of satellite geodesy it took no longer a fundamental point on the earth's surface to determine. Implementation of the new reference system based on several hundred stations with Doppler satellite observations.
Developed in cooperation of the United States with Canada, Mexico and Greenland reference system North American Datum of 1983 ( NAD 83) was first published in 1986. 300 people worked for seven years on this project, which cost a total of 37 million U.S. dollars by then.
High Accuracy Reference Network ( HARN )
In the late 1980s, it came to the development of the Global Positioning System (GPS) and thus again to a substantial progress of methods of land surveying. From 1989 to 1997 the reference system NAD 83 was therefore further refined through a network of GPS observations - the so-called High Accuracy Reference Network ( HARN ).
From 1994, they built a network constant GPS monitoring stations: the Continuously Operating Reference Station ( CORS ), which is a part of the International Terrestrial Reference Frame. These stations continuously measure their current position and transmit the data to a central billing agency. 2011 already included over 1800 GPS stations from 200 different organizations operating on this network.
In addition, isolated CORS stations are operated outside of North America as required ( the U.S. military or the United Nations ); For example, in Benin, Ethiopia or Iraq, where the U.S. Army to build up in cooperation with the National Geodetic Survey since 2004, an Iraqi reference system, the Iraqi Geospatial Reference System ( IGRS ).
Applied to the situation in Europe is the NAD 83 comparable to the European Terrestrial Reference System ( ETRS 89), and the CORS with the EUREF Permanent GPS Network.
Coordinate transformation from NAD 27 to the NAD 83
The conversion of NAD 27 to NAD 83 was not only connected with the election of a new reference ellipsoid, but also with a complete distortion correction and recalculation of the reference frame. In the Continental United States, the deviation between the two systems is 10-100 m, Alaska, and Puerto Rico than 200 meters and in Hawaii over 400 m. The conversion of coordinates from NAD 27 to the NAD 83 is therefore possible to not have a single formula.
The National Geodetic Survey has published for the transformation is a special software ( NADCON ) that each uses a grid- based interpolation based on the two systems known fixed-point data. The accuracy of the transformation by means NADCON is on the land masses of the Continental United States in about 15 cm and in Alaska, Puerto Rico, Hawaii about 50 cm. In areas with low density of the fixed-point error can also be up to 1 m.
For Canada, Earth Sciences Sector of (ESS ) of Natural Resources Canada has developed a similar software called NTv2, which also uses a grid- based transformation method.
Relationship of NAD 83 to the World Geodetic System 1984 ( WGS 84)
The World Geodetic System 1984 ( WGS 84 ) is a global reference system which has been created specifically for use in GPS. The reference ellipsoid used in the WGS 84 is connected to the NAD 83 (GRS 80 ) are almost identical. Therefore, both date definitions are usually equated; on maps of the United States can be found as a date often marked ' NAD 83/WGS 84 ".
Since the NAD However, in contrast to the WGS 84 is primarily intended to be a reference system for North America, its realization is bound (in the form of fixed points ) to the North American Plate. Thus, the measured by connecting to such fixed points within the NAD coordinates have the advantage that they remain ( in relation to North America ) from the effects of global plate shifts largely unaffected.
That is, in NAD 83 coordinates observed can not be directly compared with coordinates from the WGS 84, if it comes to accuracy less than one meter.