What is the difference between navd 88 and ngvd 29

Have you ever noticed the abbreviation NAVD 88 or NGVD 29 written after an elevation marker on a survey or construction drawing and wondered what it means?

Well, there are a few simple explanations to answer why we need and use these complex systems. Due to the advancement of technology and surveying methods, the increased amount of available data and the level of accuracy of that data led to the new datum.

The original datum used 26 tide stations throughout the United States and Canada and recorded changes in tide levels to establish a starting line of 0 ft. The cooperative network upgrading program began in Tennessee in The last field observations were completed in Indiana in September after horizontally upgrading some 16, survey stations to A-order or B-order status.

Up until that time we only were interested in latitude and longitude, even after we began using GPS. The HARN ushered us into an age where we were concerned with latitude, longitude, and focused us on precise ellipsoid heights and the advent of fixed height poles.

Geodetic Glossary , pp. Mean sea level was held fixed at the sites of 26 tide gauges, 21 in the United States and 5 in Canada. The datum is defined by the observed heights of mean sea level at the 26 tide gauges and by the set of elevations of all bench marks resulting from the adjustment.

A total of , kilometers of leveling was involved, constituting closed circuits and 25 circuits at sea level. The datum was not mean sea level, the geoid, or any other equipotential surface. Therefore, it was renamed in , the National Geodetic Vertical Datum on Additional tidal bench mark elevations were not used due to the demonstrated variations in sea surface topography, i.

About , km of leveling had been added to the NGVD since A datum is a standard position or level i. Originally called the Sea Level Datum of , it was created for vertical control surveying in the United States. This datum was used to measure elevation above altitude and depression below depth mean sea level. However, as technology improved and the demand for greater accuracy increased, inherent inaccuracies were uncovered in NGVD NAVD 88 was established in In , Congress adopted NAVD 88 as the official civilian vertical datum for the United States and instructed all federal agencies to upgrade to the new datum.

Since it was created, more than , benchmarks have been set with NAVD 88 elevations and there are more than one million kilometers of leveling data for this datum.