North American Datum
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Geodesy |
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The North American Datum (NAD) is the
Vertical measurements, based on distances above or below
NAD 83, along with NAVD 88, is set to be replaced with a new
First North American Datum of 1901
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 previously collected for regional datums, which by that time had begun to overlap. In 1913, Canada and Mexico adopted that datum, so it was also renamed the North American Datum.[3][4]
North American Datum of 1927
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 (NAD 27) was based on surveys of the entire continent from a common reference point that was chosen in 1901, because it was as near the center of the contiguous United States as could be calculated: It was based on a
Ellipsoid | Semimajor axis (by definition) |
Semiminor axis (by definition) |
Inverse flattening (calculated) |
---|---|---|---|
Clarke 1866 | 6,378,206.4 m | 6,356,583.8 m | 294.978698214 |
These are the defining dimensions for NAD 27, but Clarke actually 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. The length of a foot or meter at the time could not practically be benchmarked to better than about 0.02 mm.[11]
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.
North American Datum of 1983
Because Earth deviates significantly from a perfect
As
Ellipsoid | Semimajor axis (by definition) |
Semiminor axis (by definition) |
Inverse flattening (calculated) |
---|---|---|---|
GRS 80 | 6,378,137 m | 6,356,752.3141 m | 298.257222101 |
Comparing NAD 27 to NAD 83
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 (Meades Ranch Triangulation Station in Kansas), the direction of a line between this point and a specified second point, and two dimensions that define the spheroid. The North American Datum of 1983 is based on a newer defined spheroid (GRS 80); it is an Earth-centered (or "geocentric") datum having no initial point or initial direction.
Comparing NAD 83 to WGS 84
The definition of NAD 83(1986) is based on the GRS 80 spheroid, as was WGS 84, so many older publications indicate no difference. WGS 84 subsequently changed to a slightly less flattened spheroid. This change in flattening is about 0.1 mm, a difference so small that computational programs often do not distinguish between the two ellipsoids.[13] However, due to differences in how the reference ellipsoids are centered and oriented, coordinates in the two datums differ from each other by amounts on the order of a meter over much of the United States. Each datum has undergone refinements with more accurate and later measurements. One well-known difference is the placement of the center of the Earth, with the two systems differing by about 2.2 metres (7.2 ft).[14]
In addition, 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 naturally 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.
Current implementation of NAD 83
The United States National Spatial Reference System NAD 83(2011/MA11/PA11) epoch 2010.00, is a refinement of the NAD 83 datum using data from a network of very accurate GPS receivers at Continuously Operating Reference Stations (CORS). The NAD 83(2011) describes the main North American plate, while the MA11 and PA11 solutions are for the
New Datum of 2022
To improve the National Spatial Reference System, NAD 83, along with
model is the product of the Gravity for the Redefinition of the American Vertical Datum (GRAV-D) project.These new reference frames are intended to be easier to access and to maintain than NAD 83 and NAVD 88, which rely on physical survey marks that deteriorate over time.[2]
See also
References
- ^ North American Vertical Datum of 1988 (NAVD 88), at National Geodetic Survey
- ^ a b c New Datums: Replacing NAVD 88 and NAD 83
- ^ Shalowitz, USC&GS Field Engineers Bulletin, December 1938
- ^ Craig, Cindy (October 2013) "Where Theory Meets Practice: United States Standard Datum" Professional Surveyor Magazine Frederick, Maryland
- ^ United States Coast and Geodetic Survey, "Triangulation in Kansas"[permanent dead link], Special Publication No. 70 (1921)
- ^ Bulletin of the National Research Council, Issues 77-80, p. 230
- ^ Van Zandt, Boundaries of the United States and the several states, p. 265
- ^ Walter H. Schoewe, "Kansas and the geodetic datum of North America", Transactions of the Kansas Academy of Science 51 (1948) 117–124, www.jstor.org
- ^ NGS data
- ^ Waldo
- ISBN 1-85233-682-X, p. 5, table 2.1, data from Giacomo, P., Du platine a la lumiere, Bull. Bur. Nat. Metrologie, 102 (1995) 5–14.
- ^ NOAA's NAD 27 vs. NAD 83 converter
- ^ "What is the Difference between WGS84 and NAD83?" (PDF).
- ^ "How to Transition to the United States 2022 National Coordinate System Without Getting Left Behind" (PDF).
- ^ "NA2011".
External links
- NOAA-NGS-coordinates CORS-active network Archived 15 May 2013 at the Wayback Machine - explanation of NAD 83(2011/MA11/PA11) epoch 2010.00
- NOAA-NGS-coordinates passive network - explanation of most recent adjustment of passive network
- NADCON – a free utility for Microsoft Windows to convert between NAD 27 and NAD 83
- nadcon.prl – a web-based utility for NADCON
- NAD 83: What Is It and Why You Should Care by Dane E. Ericksen, P.E., Hammett & Edison, Inc., Consulting Engineers. 1994 SBE National Convention and World Media Expo. Archived 26 June 2011 at the Wayback Machine