Which one is a common vertical datum?

Both NAVD88 and NGVD29 are indeed common vertical datums used in the United States, which makes the combination of both answers the correct choice.

NAVD88, or the North American Vertical Datum of 1988, was established to provide a more accurate and consistent vertical reference point over North America. It is based on a network of precise geodetic benchmarks across the continent and is widely utilized in modern surveying, mapping, and engineering applications.

NGVD29, or the National Geodetic Vertical Datum of 1929, predates NAVD88 and was based on an older standard that also employed a network of benchmarks, but with less accuracy and coverage. Although it has largely been replaced by NAVD88 for new projects, many older documents and datasets still refer to NGVD29.

The existence of both datums highlights the evolution and refinement of vertical measurement systems over time in geospatial data. Understanding these different vertical datums is critical for professionals in fields such as geology, hydrology, and civil engineering, as they play a significant role in ensuring that measurements and analyses are consistent and accurate across different datasets and projects.