A “geodetic reference system” is used to precisely describe the location of a specific point on the Earth and is composed of latitude, longitude, and elevation. Its basis is composed of a geoid1 and a reference ellipsoid2 — two mathematical representations of the Earth's surface — along with base points to which the latitude, longitude and elevation of all other points in the system are referenced. These base points are known as “datums”. The latitude-longitude base point is known as a horizontal datum, and the elevation base point is known as a vertical datum. For more information, see the discussion of datums on the National Geodetic Survey's website: https://geodesy.noaa.gov/datums/
Vertical datums are used to establish the elevation of monitoring locations, reference points and natural features such as lake levels and floodplains, as well as for bridges and levies.
The currently accepted vertical datum is the North American Vertical Datum of 1988 (NAVD88), which was formally adopted in 1992. It consists of a leveling network that applies to the entire North American continent and which is affixed to a single origin point in Quebec, Canada.
NAVD88 replaced the National Geodetic Vertical Datum of 1929 (NGVD29), which connected the major vertical benchmark3 networks in the country to 26 tidal benchmarks along the Atlantic, Gulf, and Pacific Coasts. As technology improved, it was determined that the NGVD29 did not accurately represent sea and lake levels, nor did it allow for accurate delineation of flood zones and it was replaced. Because it took time for gages to be recalibrated, the switch to NAVD88 was gradual. Elevations were still recorded relative to NGVD29 for a long time, and even now dual elevation values are sometimes reported, in both NGVD29 and NAVD88.
Soon, another change will occur. In 2022, the National Geodetic Survey will replace the current horizontal datum, the North American Datum of 1983 (NAD83), and NAVD88 with a new geometric reference frame and geopotential datum.
1The geoid is essentially the figure of the Earth abstracted from its topographic features. It is an idealized equilibrium surface of sea water, the mean sea level surface in the absence of currents, air pressure variations etc. and continued under the continental masses. The geoid, unlike the reference ellipsoid, is irregular and too complicated to serve as the computational surface on which to solve geometrical problems like point positioning. The geometrical separation between it and the reference ellipsoid is called the geoidal undulation. It varies globally between ±110 m. Put another way, the geoid surface is irregular, unlike the reference ellipsoid often used to approximate the shape of the physical Earth, but considerably smoother than Earth's physical surface. While the latter has excursions of +8,000 m (Mount Everest) and –11,000 m (Marianna Trench), the geoid varies by only about ±100 m about the reference ellipsoid of revolution.
2An ellipsoid or reference ellipsoid is a mathematically-defined surface that approximates the "figure of Earth" or another planetary body.
3A benchmark is a permanent, stationary object on which is set a surveyor's mark. The benchmark locations are depicted on maps and used as reference points for accurately establishing the coordinates of another or other points.