North American date
The North American Datum ( NAD ) is the common reference system for the national surveying of the states of North and Central America .
For historical reasons, a distinction between the means of classical triangulation founded North American Datum of 1927 (NAD 27) (with one on the earth's surface market fundamentals point ) and the more modern means of satellite geodesy established North American Datum of 1983 (NAD 83) (a geocentric origin).
historical development
The first national geodetic datum used in the United States was the New England date of 1879. Principio was established as the fundamental point at Perryville , Maryland (approximately at 39 ° 35 ′ 30 ″ N , 76 ° 0 ′ 20 ″ W ). This location was chosen because it was still in the center of US geographic interest at the time.
From 1871 to 1899 the National Geodetic Survey carried out a large-scale transcontinental survey campaign along the 39th parallel to connect the already extensive survey network of the eastern states with the Pacific west coast. In the course of this expansion, it was also necessary to relocate the point of origin from Principio to the center of the North American continent. Meades Ranch , between the towns of Lucas and Tipton in Kansas (at 39 ° 13 ′ 26.7 ″ N , 98 ° 32 ′ 31.7 ″ W (WGS84)) was chosen as the new fundamental point . Consequently, the New England date was renamed the US Standard Date .
North American Date of 1927 (NAD 27)
After Canada and Mexico decided in 1913 to also set up their surveying networks on the basis of the US Standard Date , this was renamed North American Date . Like its predecessors, it is based on the Clarke Ellipsoid of 1866 and its fundamental point is Meades Ranch in Kansas.
In 1927, the positions of 25,000 were fixed points by adjustment methods recalculated on the basis of past survey data. The designation NAD 27 emerges from the reference frame of fixed points formed in this way.
North American Date of 1983 (NAD 83)
With the increasing spread of electronic surveying methods, there were increasing inconsistencies in the 1970s between the reference system NAD 27 and the results of local surveys. The use of the Clarke ellipsoid as the basis of the reference system also turned out to be too imprecise and no longer up to date. So it was decided to build a new reference system based on the geodetic reference system 1980 (GRS 80) as an earth model with a geocentric origin. Thanks to modern satellite geodesy methods, it was no longer necessary to define a fundamental point on the earth's surface. The implementation of the new reference system was based on several hundred stations with Doppler satellite observations.
The North American Datum of 1983 (NAD 83), developed in collaboration between the United States, Canada , Mexico and Greenland , was first published in 1986. 300 people worked on this project for seven years, which until then had cost a total of 37 million US dollars.
High Accuracy Reference Network (HARN)
In the late 1980s, the Global Positioning System (GPS) was developed, and with it a further significant advance in land surveying methods. From 1989 to 1997, the NAD 83 reference system was therefore further refined by a network of GPS observations - the so-called High Accuracy Reference Network (HARN) .
From 1994 a network of permanent GPS observation stations was established: the Continuously Operating Reference Station (CORS) , which is part of the International Terrestrial Reference Frame . These stations constantly measure their current position and transmit the data to a central evaluation point. In 2011 there were already over 1800 GPS stations from 200 different operating organizations in this network.
In addition, individual CORS stations are also operated outside of North America, depending on requirements (from the US military or the United Nations); for example in Benin, Ethiopia or Iraq, where the US Army has been building an Iraqi reference system, the Iraqi Geospatial Reference System (IGRS) , in cooperation with the National Geodetic Survey since 2004 .
Applied to the situation in Europe, the NAD 83 is comparable with the European Terrestrial Reference System (ETRS 89) , as well as the CORS with the EUREF Permanent GPS Network .
Coordinate transformation from NAD 27 to NAD 83
The conversion of the NAD 27 to the NAD 83 was not only associated with the selection of a new reference ellipsoid, but also with an equalization and recalculation of the complete reference frame. In the Continental United States , the difference between the two systems is 10–100 m, in Alaska and Puerto Rico over 200 m and in Hawaii over 400 m. The conversion of coordinates from NAD 27 to NAD 83 is therefore not possible using a uniform formula.
The National Geodetic Survey has published special software (NADCON) for the transformation, which works with a grid-based interpolation on the basis of fixed point data known in both systems. The accuracy of the transformation using NADCON is about 15 cm on the land masses of the Continental United States and about 50 cm in Alaska, Puerto Rico, Hawaii. In areas with a low density of fixed points, the error can be up to 1 m.
For Canada, the Earth Sciences Sector (ESS) of Natural Resources Canada has developed a similar software called NTv2, which also works with a grid-based transformation process.
Relationship of NAD 83 to the World Geodetic System 1984 (WGS 84)
The World Geodetic System 1984 (WGS 84) is a global reference system that was specially created for use in GPS. The reference ellipsoid used in the WGS 84 is almost identical to that of the NAD 83 (GRS 80). Therefore, both date definitions are usually equated; The date "NAD 83 / WGS 84" is often found on maps of the USA.
Since the NAD, in contrast to the WGS 84, is primarily intended to be a reference system for North America, its implementation (in the form of fixed points) is tied to the North American plate . The coordinates measured by connection to such fixed points within the NAD therefore have the advantage that they (in relation to North America) remain largely unaffected by the effects of global plate displacements.
This means that the coordinates observed in the NAD 83 cannot be compared directly with the coordinates from the WGS 84, provided that the accuracy is less than one meter.
See also
Web links
- US National Geodetic Survey data sheet with a detailed description of the Meades Ranch reference point
- Continuously Operating Reference Stations (CORS)
Individual evidence
- ^ US Coast and Geodetic Survey: Geodesy. The eastern oblique arc of the United States and osculating spheroid (1902)
- ^ National Geodetic Survey: Continuously Operating Reference Station (CORS) .
- ↑ Development of the Iraqi Geospatial Reference System ( Memento of the original dated February 7, 2009 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. . In: The American Surveyor 11/2005.
- ↑ National Geodetic Survey: NADCON - North American Datum Conversion Utility
- ↑ Canadian Geodetic Service: NTv2 National Transformation software ( Memento of the original from June 26, 2010 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
