[coordinate systems, government]
A datum defined by the observed heights of mean sea level at 26 North American tide gauges, as well as the elevations of benchmarks resulting from over 60,000 miles (96,560 kilometers) of leveling across the continent, totaling over 500,000 vertical control points. Established in 1929 by the U.S. Coast and Geodetic Survey as the surface against which elevation data in the United States is referenced. Often abbreviated to NGVD29.