| Geodesy | |
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| Fundamentals | |
| Geodesy · Geodynamics Geomatics · Cartography |
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| Concepts | |
| Datum · Distance · Geoid Figure of the Earth Geodetic system Geog. coord. system Hor. pos. representation Map projection Reference ellipsoid Satellite geodesy Spatial reference system |
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| Technologies | |
| GNSS · GPS · ... | |
| Standards | |
| ED50 · ETRS89 · NAD83 NAVD88 · SAD69 · SRID UTM · WGS84 · ... |
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| History | |
| History of geodesy NAVD29 · ... |
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The Sea Level Datum of 1929 was the vertical control datum established for vertical control surveying in the United States of America by the General Adjustment of 1929. The datum was used to measure elevation (altitude) above, and depression (depth) below, mean sea level (MSL).
Mean sea level was measured at 26 tide gauges: 21 in the United States and 5 in Canada. The datum was defined by the observed heights of mean sea level at the 26 tide gauges and by the set of elevations of all bench marks resulting from the adjustment. The adjustment required a total of 66,315 miles (106,724 km) of leveling with 246 closed circuits and 25 circuits at sea level.
Since the Sea Level Datum of 1929 was a hybrid model, it was not a pure model of mean sea level, the geoid, or any other equipotential surface. Therefore, it was renamed the National Geodetic Vertical Datum of 1929 (NGVD 29) in 1973. The NGVD 29 was subsequently replaced by the North American Vertical Datum of 1988 (NAVD 88) based upon the General Adjustment of the North American Datum of 1988.