NGVD29, or the National Geodetic Vertical Datum of 1929, is a reference system used in the United States to measure elevations. It was established to provide a consistent baseline for determining the height of land and water surfaces across the country. The datum is based on a network of survey points and uses mean sea level as a reference. This system has been widely used in various applications, including engineering, construction, and floodplain management. Although it has been largely replaced by the more modern NAVD88 (North American Vertical Datum of 1988), NGVD29 remains important for historical data and comparisons.