The two horizontal datums used almost exclusively in North America are NAD 1927 and NAD 1983.
NAD 1927
NAD 1927 uses the Clarke 1866 spheroid to represent the shape of the earth. The origin of this datum is a point on the earth referred to as Meades Ranch in Kansas. Many NAD 1927 control points were calculated from observations taken in the 1800s. These calculations were done manually and in sections over many years. Therefore, errors varied from station to station.
NAD 1983
Many technological advances in surveying and geodesy—electronic theodolites, Global Positioning System (GPS) satellites, Very Long Baseline Interferometry, and Doppler systems—revealed weaknesses in the existing network of control points. Differences became particularly noticeable when linking existing control with newly established surveys. The establishment of a new datum allowed a single datum to cover consistently North America and surrounding areas.
The North American Datum of 1983 is based on both earth and satellite observations, using the Geodetic Reference System (GRS) 1980 spheroid. The origin for this datum was the earth's center of mass. This affects the surface location of all longitude–latitude values enough to cause locations of previous control points in North America to shift, sometimes as much as 500 feet compared to NAD 1927. A 10-year multinational effort tied together a network of control points for the United States, Canada, Mexico, Greenland, Central America, and the Caribbean.
The GRS 1980 spheroid is almost identical to the World Geodetic System (WGS) 1984 spheroid. The WGS 1984 and NAD 1983 coordinate systems are both earth-centered. When originally published in 1986, NAD 1983 and WGS 1984 could be considered coincident. That is no longer true. WGS 1984 is tied to the International Terrestrial Reference Frame (ITRF). NAD 1983 is tied to the North American tectonic plate to minimize changes to coordinate values over time. This has caused NAD 1983 and WGS 1984 to drift apart. Generally, coordinates in WGS 1984 and NAD 1983 are around one to two meters apart. GPS data is actually reported in the WGS 1984 coordinate system. However, if any type of external control network is being used, such as the Continuously Operating Reference Stations (CORS) service, the GPS coordinates are relative to that coordinate system, not WGS 1984.
HARN or HPGN
There was an ongoing effort at the state level to readjust the NAD 1983 datum to a higher level of accuracy using state-of-the-art surveying techniques that were not widely available when the NAD 1983 datum was being developed. This effort, known as the High Accuracy Reference Network (HARN)—previously the High Precision Geodetic Network (HPGN)—was a cooperative project between the National Geodetic Survey (NGS) and individual states.
Currently, all states except Alaska have been resurveyed, and transformation grid files for 49 states and five territories have been published. Control points that have been adjusted are labeled in the National Geodetic Survey database as NAD83 (19xx) or NAD83 (20xx) where xx represents the year of adjustment. Some points have been adjusted several times, and the year may not be the same as the original HARN readjustment. NGS has never released transformations to convert between an original HARN and later readjustments.
Other NAD 1983 readjustments
NGS maintains a reference network of CORS stations. This set of control points is labeled as NAD 1983 (CORS96), and the points are tied to the ITRF through a transformation. Other geodetic control points are labeled with the adjustment year. Recently, NGS performed a national readjustment. All existing control points except the CORS stations were updated and are now labeled NAD 1983 (NSRS2007). The official name of the readjustment is National Spatial Reference System (NSRS) of 2007. For most of the United States, the differences between HARN coordinates and NSRS2007 are a few centimeters. Because of this, no standardized transformations have been calculated and published to convert between NAD 1983 (NSRS2007) and earlier realizations of NAD 1983. More information is available on the NGS website.
Other United States datums
Alaska, Hawaii, American Samoa, Guam, Puerto Rico and the Virgin Islands, and some Alaskan islands have used other datums besides NAD 1927. New data is referenced to NAD 1983 or one of its readjustments.
Canada
Several readjustments occurred in Canada prior to the adoption of NAD 1983. A national adjustment, called NAD 1927 DEF 1976 (commonly called MAY76), and a regional adjustment for Quebec, NAD 1927 CGQ77, were carried out. The maritime provinces conducted a separate adjustment and defined the Average Terrestrial System of 1977 (ATS 1977). In the 1980s, Canada joined the United States to define NAD 1983. Since then, Canada has readjusted its control network, and the reference system is now known as NAD 1983 (CSRS). CSRS stands for Canadian Spatial Reference System. An excellent paper with more details is Demystifying Reference Systems by Don Junkins and Gordon Garrard.