Geoid Height and Deflections of the Vertical Models

In Gauss' words, the geoid is "the mathematical figure of the Earth". This figure is an equipotential surface coincident with the idealized mean sea surface. The geoid can be computed from the geodetic boundary value problems that use gravity data as its boundary value. A geoid model computed using gravity data is called a gravimetric geoid. On the other hand, geoid height at bench marks can also be computed using data from spirit leveling and the Global Positioning System (GPS). A geoid model that is fixed to the GPS/leveling data is called a hybrid geoid. Both geoid models can serve as the zero-height-surface of a country's height system by selection. To satisfy this need, National Geodetic Survey has published a series of geoid models (https://geodesy.noaa.gov/GEOID).

The vast majority of navigation and positioning applications utilize a hybrid geoid model with the latest model being GEOID18 for CONUS and Puerto Rico/U.S. Virgin Islands and GEOID12B for all other states and territories of the United States. The corresponding gravimetric geoid for these regions is xGEOID19B and USGG2012, respectively. All models are provided at 1 arc-minute resolution.