NGVD29

NGVD29, or the National Geodetic Vertical Datum of 1929, is a historical vertical datum that was used in the United States to define elevations. It was established in 1929 by the U.S. Coast and Geodetic Survey (now part of the National Geodetic Survey) as the national standard for measuring height above a common reference surface. NGVD29 is based on mean sea level observed at a network of tide gauges along the Atlantic and Gulf coasts, with a nationwide leveling adjustment to produce a single continent-wide surface. Elevations are expressed in feet (US survey feet) above this reference surface and were widely used by engineers, surveyors, and mapping agencies throughout much of the 20th century.

In 1988, the United States adopted a new national vertical datum, NAVD88, which is tied to a fixed geopotential surface rather than directly to mean sea level. As a result, NGVD29 has been largely superseded for new work, but it remains present in historical datasets, older maps, and some legacy projects. Because NGVD29 originated from sea level observations and was produced through a leveling network rather than a gravity-based surface, the difference between NGVD29 and modern datums varies by location and can amount to several feet in some areas.

Conversions between NGVD29 and more current datums are available, notably via transformation tools such as VERTCON, which enable the conversion of elevations between NGVD29 and NAVD88. When integrating legacy NGVD29 data with contemporary datasets, applying appropriate transformations is essential to ensure consistency.