The National Geodetic Vertical Datum of 1929 (NGVD 29) is a reference system used in the United States to measure elevations. Established in 1929, it provides 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, which were used to create a standard elevation reference. NGVD 29 was replaced by the North American Vertical Datum of 1988 (NAVD 88), which offers improved accuracy and is based on more modern surveying techniques. However, NGVD 29 is still used in some applications and historical data, making it important for understanding elevation changes over time.