NGVD 29, or the National Geodetic Vertical Datum of 1929, is a vertical control datum used in the United States. It serves as a reference point for measuring elevations and is based on a network of survey benchmarks across the country. The datum was established to provide a consistent standard for elevation measurements. NGVD 29 was replaced by NAVD 88, the North American Vertical Datum of 1988, which offers improved accuracy and is based on a more extensive network of data. However, NGVD 29 is still used in some applications and remains important for historical data comparisons.