![]() ![]() To improve the National Spatial Reference System (NSRS), the National Geodetic Service will replace the North American Datum of 1983 (NAD 83) and the North American Vertical Datum of 1988 (NAVD 88) with a new geometric reference frame and geopotential datum in 2022. Since NGVD 29 used a simple model of gravity based on latitude to calculate the geoid and did not take into account other variations, elevation difference between points in a local area in it and NAVD 88 will show negligible change from one datum to the other, even though the elevation of both does change between datums. The NAVD 88 model is based on then-available measurements, and remains fixed despite later improved geoid models. The definition of NAVD 88 uses the Helmert orthometric height, which calculates the location of the geoid (which approximates MSL) from modeled local gravity. Although many papers on NAVD 88 exist, no single document serves as the official defining document for that datum. In 1993 NAVD 88 was affirmed as the official vertical datum in the National Spatial Reference System (NSRS) for the Conterminous United States and Alaska. North American Vertical Datum of 1988 (NAVD 88) consists of a leveling network on the North American Continent, ranging from Alaska, through Canada, across the United States, affixed to a single origin point on the continent. Additional tidal bench mark elevations were not used due to the demonstrated variations in sea surface topography, i.e., that MSL is not the same equipotential surface at all tidal bench marks. It held fixed the height of the primary tide gauge benchmark (surveying), referenced to the International Great Lakes Datum of 1985 local mean sea level (MSL) height value, at Rimouski, Quebec, Canada. NAVD 88 was established in 1991 by the minimum-constraint adjustment of geodetic leveling observations in Canada, the United States, and Mexico. The benchmark locations are depicted on maps and used as reference points for accurately establishing the coordinates of another or other points.Illustration of vertical datums in the United States. While the latter has excursions of +8,000 m (Mount Everest) and –11,000 m (Marianna Trench), the geoid varies by only about ☑00 m about the reference ellipsoid of revolution.ĢAn ellipsoid or reference ellipsoid is a mathematically-defined surface that approximates the "figure of Earth" or another planetary body.ģA benchmark is a permanent, stationary object on which is set a surveyor's mark. 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. The geometrical separation between it and the reference ellipsoid is called the geoidal undulation. 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. ![]() and continued under the continental masses. It is an idealized equilibrium surface of sea water, the mean sea level surface in the absence of currents, air pressure variations etc. 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.ġThe geoid is essentially the figure of the Earth abstracted from its topographic features. 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. Because it took time for gages to be recalibrated, the switch to NAVD88 was gradual. 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. NAVD88 replaced the National Geodetic Vertical Datum of 1929 (NGVD29), which connected the major vertical benchmark 3 networks in the country to 26 tidal benchmarks along the Atlantic, Gulf, and Pacific Coasts. 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. The currently accepted vertical datum is the North American Vertical Datum of 1988 (NAVD88), which was formally adopted in 1992. 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. ![]()
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