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((Short description|Reference frame for geodesy on the continent))
((More citations needed|date=September 2010))
((Use dmy dates|date=January 2020))
[[File:Meade Ranch triangulation station ca. 1940.jpg|thumb|[[Meades Ranch Triangulation Station]], fundamental station for the North American Datum of 1927]]
((Geodesy))

The '''North American Datum''' ('''NAD''') is the [[Geodetic datum#Horizontal datum|horizontal datum]] now used to define the [[Geodesy|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&nbsp;27) and the North American Datum of 1983 (NAD&nbsp;83). Both are [[geodetic reference system]]s based on slightly different assumptions and measurements.

Vertical measurements, based on distances above or below [[Mean High Water]] (MHW), are calculated using the [[North American Vertical Datum of 1988]] (NAVD 88).<ref name=NA88>[https://www.ngs.noaa.gov/datums/vertical/north-american-vertical-datum-1988.shtml North American Vertical Datum of 1988 (NAVD 88), at National Geodetic Survey]</ref>

NAD 83, along with NAVD 88, is set to be replaced with a new [[GPS]]- and gravimetric geoid model-based geometric reference frame and geopotential datum, potentially [[Datum of 2022|in 2025]].<ref name=NEW22>[https://www.ngs.noaa.gov/datums/newdatums/index.shtml New Datums: Replacing NAVD 88 and NAD 83]</ref>

==First North American Datum of 1901==
In 1901 the [[United States Coast and Geodetic Survey]] adopted a national [[Geodetic datum#Horizontal datum|horizontal datum]] called the United States Standard Datum, based on the [[Alexander Ross Clarke|Clarke]] [[Earth ellipsoid#Historical Earth ellipsoids|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.<ref>Shalowitz, USC&GS Field Engineers Bulletin, December 1938</ref><ref>Craig, Cindy (October 2013) [http://archives.profsurv.com/magazine/article.aspx?i=71429 "Where Theory Meets Practice: United States Standard Datum"] ''Professional Surveyor Magazine'' Frederick, Maryland</ref>

==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&nbsp;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 [[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]] that was near the [[geographic center of the contiguous United States]].
<ref>United States Coast and Geodetic Survey, [ftp://ftp.library.noaa.gov/docs.lib/htdocs/rescue/cgs_specpubs/QB275U35no701921.pdf "Triangulation in Kansas"]((Dead link|date=August 2023 |bot=InternetArchiveBot |fix-attempted=yes )), ''Special Publication No. 70'' (1921)</ref><ref>Bulletin of the National Research Council, Issues 77-80, p. 230</ref><ref>Van Zandt, Boundaries of the United States and the several states, p. 265</ref><ref>Walter H. Schoewe, [https://www.jstor.org/stable/pdfplus/3625750.pdf "Kansas and the geodetic datum of North America"], ''Transactions of the Kansas Academy of Science'' 51 (1948) 117–124, www.jstor.org</ref> The datum declares the [[Meades Ranch Triangulation Station]] in [[Osborne County, Kansas]] to be 39°13′26.686″ north latitude, 98°32′30.506″ west longitude.<ref>[http://www.ngs.noaa.gov/cgi-bin/ds_mark.prl?PidBox=kg0640 NGS data]</ref> NAD&nbsp;27 is oriented by declaring the [[azimuth]] from Meades Ranch to Waldo Station (also in Osborne County, about ((convert|4.5|mi|km|1|abbr=on)) northwest of [[Waldo, Kansas|Waldo, Russell County]])<ref>[http://www.ngs.noaa.gov/cgi-bin/ds_mark.prl?PidBox=kg0648 Waldo]</ref> to be 255°28′14.52″ from north. The latitude and longitude of every other point in North America is then 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 [[Alexander Ross Clarke|Clarke]] [[Earth ellipsoid#Historical Earth ellipsoids|Ellipsoid]] of 1866, then its latitude and longitude on that ellipsoid were defined and could be calculated.

{| class="wikitable"
|-
! Ellipsoid
! Semimajor axis<br /><small>(by definition)</small>
! Semiminor axis<br /><small>(by definition)</small>
! Inverse flattening<br /><small>(calculated)</small>
|-
| Clarke 1866
| 6,378,206.4 m
| 6,356,583.8 m
| 294.978698214
|}

These are the defining dimensions for NAD&nbsp;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&nbsp;mm.<ref>Cardarelli, Francois ''Encyclopaedia of scientific units, weights, and measures: their SI equivalences and origins'', Springer-Verlag London Limited 2003, ((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.</ref>

Most [[United States Geological Survey|USGS]] topographic maps were published in NAD&nbsp;27 and many major projects by the United States Army Corps of Engineers and other agencies were defined in NAD&nbsp;27, so the datum remains important, despite more refined datums being available.

==North American Datum of 1983==
Because Earth deviates significantly from a perfect [[Reference ellipsoid|ellipsoid]], the ellipsoid that best approximates its shape varies region by region across the world. Clarke 1866, and North American Datum of 1927 with it, were surveyed to best suit North America as a whole. Likewise, historically, 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 naturally deal with Earth as a monolithic body. Therefore, the [[GRS&nbsp;80]] ellipsoid was developed for best approximating the Earth as a whole, and it became the foundation for the North American Datum of 1983. Though GRS&nbsp;80 and its close relative, [[World Geodetic System|WGS&nbsp;84]], are generally not the best fit for any given region, a need for the closest fit largely evaporates when a global survey is combined with computers, databases, and software able to compensate for local conditions.

{| class="wikitable"
! Ellipsoid
! Semimajor axis<br /><small>(by definition)</small>
! Semiminor axis<br /><small>(by definition)</small>
! Inverse flattening<br /><small>(calculated)</small>
|-
| GRS&nbsp;80
| 6,378,137 m
| 6,356,752.3141 m
| 298.257222101
|}

===Comparing NAD&nbsp;27 to NAD&nbsp;83===
[[File:Datum Shift Between NAD27 and NAD83.png|thumb|Datum Shift Between NAD&nbsp;27 and NAD&nbsp;83]]
A point having a given latitude and longitude in NAD&nbsp;27 may be displaced on the order of many tens of meters from another point having the identical latitude and longitude in NAD&nbsp;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&nbsp;80); it is an Earth-centered (or "[[Geocentric model|geocentric]]") datum having no initial point or initial direction.

[[NOAA]] provides a converter between the two systems.<ref>[http://www.ngs.noaa.gov/cgi-bin/nadcon.prl NOAA's NAD&nbsp;27 vs. NAD&nbsp;83 converter]</ref> The practical impact is that if you use a modern GPS device set to work in NAD&nbsp;83 or WGS&nbsp;84 to navigate to NAD&nbsp;27 coordinates (as from a [[Quadrangle (geography)|topo map)]] near [[Seattle]], you would be off by about 95 meters (not far enough west), and you'd be about 47 meters off near [[Miami]] (not far enough north-northeast), whereas you would be much closer for points near [[Chicago]].

===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 [[World Geodetic System#WGS84|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.<ref>((cite web |url= https://www.uvm.edu/giv/resources/WGS84_NAD83.pdf |title= What is the Difference between WGS84 and NAD83?))</ref> 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 ((convert|2.2|m|ft)).<ref>((cite web |title=How to Transition to the United States 2022 National Coordinate System Without Getting Left Behind |url=https://www.unoosa.org/documents/pdf/psa/activities/2019/UN_Fiji_2019/S5-24.pdf))</ref>

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&nbsp;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&nbsp;83(2011/MA11/PA11) epoch 2010.00, is a refinement of the NAD&nbsp;83 datum using data from a network of very accurate GPS receivers at Continuously Operating Reference Stations (CORS). The NAD&nbsp;83(2011) describes the main North American plate, while the MA11 and PA11 solutions are for the [[Mariana plate]] and the [[Pacific plate]] respectively.<ref>((Cite web|url=https://www.ngs.noaa.gov/web/surveys/NA2011/|title = NA2011))</ref>

==New Datum of 2022==
((main|Datum of 2022|National Spatial Reference System))
To improve the National Spatial Reference System NAD 83, along with [[North American Vertical Datum of 1988]] (NAVD 88), is set to be replaced with a new [[GNSS]]- and gravimetric geoid model-based geometric reference frame and geopotential datum [[Datum of 2022|in 2022]].<ref name=NEW22/>

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 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.<ref name=NEW22/>

==See also==
* [[North American Vertical Datum of 1988]]
* [[World Geodetic System]]

==References==
((Reflist))

==External links==
*[http://geodesy.noaa.gov/CORS/coords.shtml NOAA-NGS-coordinates CORS-active network ] ((Webarchive|url=https://web.archive.org/web/20130515060158/http://geodesy.noaa.gov/CORS/coords.shtml |date=15 May 2013 )) - explanation of NAD&nbsp;83(2011/MA11/PA11) epoch 2010.00
*[http://geodesy.noaa.gov/web/surveys/NA2011/ NOAA-NGS-coordinates passive network ] - explanation of most recent adjustment of passive network
*[http://www.ngs.noaa.gov/TOOLS/Nadcon/Nadcon.html NADCON] – a free utility for Microsoft Windows to convert between NAD&nbsp;27 and NAD&nbsp;83
*[http://www.ngs.noaa.gov/cgi-bin/nadcon.prl nadcon.prl] – a web-based utility for NADCON
*[http://h-e.com/sites/h-e.com/files/tech_docs/de_sbe94.pdf NAD&nbsp;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.] ((Webarchive|url=https://web.archive.org/web/20110626151629/http://h-e.com/sites/h-e.com/files/tech_docs/de_sbe94.pdf |date=26 June 2011 ))

[[Category:Geodetic datums]]
[[Category:Surveying of the United States]]
[[Category:Surveying of Canada]]

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