North American Datum 1983 (NAD83)

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NAD83 definitions

The North American Datum 1983 (NAD83) is a geocentric datum that was established in 1986 for the United States, Canada, Mexico, Central America and the Caribbean Islands. Hawaii and Greenland were also connected to this datum. It is based on a horizontal adjustment of conventional survey data and the inclusion of Transit Satellite Doppler data and Very Long Baseline Interferometry (VLBI) data. The global Doppler and VLBI observations were used to orient the NAD83 reference frame to the BIH Terrestrial System of 1984. The orientation of the ECEF coordinate axes of the NAD83 reference frame is identical to that of the original  WGS84 reference frame.

Defining parameters

NAD83 uses the Geodetic Reference System 1980 (GRS80) ellipsoid as its reference ellipsoid with the geometric center of the ellipsoid coincident with the center of mass of the Earth and the origin of the coordinate system.

NAD83 realizations

The EPSG database lists all NAD83 datums without spaces between 'NAD' and '83', and 'NAD83' and e.g. '(HARN)', whereas the NGS uses 'NAD 83' and e.g. 'NAD 83 (HARN)' most of the time.

Transformations between NAD83 realizations and ITRF are given in ITRF_Transformation_Parameters_.xlsx (NAD83 sheet).

Rotations are for the position vector rotation convention. Units are meters, mas (milliarcsecons) and ppb (parts-per-billion).
1 mas = 0.001 " = 2.77778 e^-7 degrees = 4.84814 e^-9 radians. 0.001 " corresponds to about 0.030 m at the earth's surface.

Note. Not all transformation parameters in the NAD83 sheet have been tested. Always check the original documentation.

United States and Canada

In summary, surveyors and others have witnessed various realizations of NAD83 in the United States. A similar evolution occurred in Canada, but the two countries at least agree on their first and last realizations. The U.S. realizations are consistent in their choice of origin and orientation; they differ, however, in their choice of scale. While the scale difference between NAD83 (1986) and NAD83 (HARN) equals -0.0871 ppm, the scale difference between NAD83 (HARN) and any NAD83 (CORSxx) is smaller than 0.005 ppm in magnitude. It should be noted that the NAD83 (HARN) latitude and/or longitude of a given control point may differ by up to 1 meter from its corresponding NAD83 (1986) coordinate. Fortunately, the horizontal discrepancy between the NAD83 (CORS93) and NAD83 (HARN) positions for a control point is almost always less than 10 cm, and the horizontal discrepancy between any two NAD83 (CORSxx) positions for a control point is almost always less than 2 cm. In addition, as NAD83 has evolved from mostly a horizontal reference system to a full 3D reference system, the number of control points with measured ellipsoidal heights has grown dramatically. 

NAD83, WGS84 and ITRF

Unfortunately, the U.S. National Geospatial-Intelligence Agency (NGA) still considers the G-series realizations of WGS84 to be identical with the original realization. Thus, the zero transformation with respect to NAD83 has never been revised in spite of the bias being clearly measurable. This has created problems when using WGS84-based correction services and trying to convert results to NAD83. Most receiver manufactures include only the original NIMA coordinate shifts (translations) in their receiver firmware, which are zero for NAD83. Consequently, many receivers are producing so-called NAD83 coordinates that are actually still in WGS84 and biased by 1.5 to 2 metres (horizontally; 0 to 0.3 metres vertically) with respect to the true NAD83 reference frame.

New WGS84 realizations (since G730, 1994) are coincident with ITRF at about 10-centimeter level. For these realizations there are no official transformation parameters. This means that one can consider that ITRF coordinates are also expressed in WGS84 at 10 cm level. However, the most recent G1674 realization adopted ITRF2008 coordinates for more than half of the reference stations and velocities of nearby sites for the others. Thus, ITRF2008 and WGS84 (G1674) are likely to agree at the centimeter level, yielding conventional 0-transformation parameters. For more information, check ftp://itrf.ensg.ign.fr/pub/itrf/WGS84.TXT.

Replacing NAD83 and NAVD88

NAD83 and NAVD88 (North American Vertical Datum of 1988) will be replaced in 2022, and there are many related projects to make sure the transition goes smoothly. Read the NGS Ten-Year Plan to learn more and continue to visit NGS's web-page on New Datums for more information.

References