'Vertical datum' is a common alternative name for 'Geodetic Reference System'. In the simplest terms, a geodetic reference system is used to provide accurate longitude, latitude and elevation to a specific point or location, for example, and in particular, environmental monitoring sites. The Geodetic Reference System utilizes a geoid1, and a reference ellipsoid2 — two mathematical representations of the Earth's surface — along with a base point from which the latitude and longitude of all other points in the system are referenced. For more information, go to the Geospatial Resource Portal at http://www.gisdevelopment.net/index.htm.
The National Geodetic Vertical Datum of 1929 (NGVD29), was established by the National Geodetic Survey (NGS) by connecting the major vertical benchmark3 networks in the country to 26 tidal benchmarks along the Atlantic, Gulf, and Pacific Coasts. For decades the NGVD29 was used to establish monitoring locations, reference points and elevations of natural features such as lake and floodplains, as well as for bridges and levies.
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. Beginning in the early '70s, geodesic scientists4 began developing a new vertical datum that would allow for more accurate and representative establishment of horizontal and vertical locations of all points on Earth's surface. This new datum, North American Vertical Datum of 1988 (NAVD88), was finally established by the NGS in 1991. It provides significant improvement in accuracy and applicability over very large areas. All new NGS benchmarks are established in NAVD88.
"Historically, the most common vertical datum used by Federal Emergency Management Agency (FEMA) has been National Geodetic Vertical Datum of 1929 (NGVD29). However, the NAVD88 datum is more compatible with modern surveying and mapping technologies like Global Positioning Systems and Light Detection and Ranging (LIDAR), more accurate than NGVD29, and it is the only official vertical datum for the continental United States.
"It is important that FEMA accomplish this conversion now for the following reasons:
For more information visit the FEMA website.
1An ellipsoid or reference ellipsoid is a mathematically-defined surface that approximates the "figure of Earth" or another planetary body.
2The 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.
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.
4Geodesic scientist: a specialist in geodesy, a branch of applied mathematics concerned with measuring, or determining the shape of, the earth or a large part of its surface, or with locating exactly points on its surface.