Geoid Height and Deflections of the Vertical Models - National Geospatial Data Asset (NGDA) Geoid 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 gravimetric geoid. On the other hand, geoid height at bench marks can also be computed using data of the spirit leveling and the Global Positioning System (GPS). A geoid model that fixed to the GPS/leveling data is called 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 (http://geodesy.noaa.gov/GEOID). The latest models are USGG2012 and GEOID12B, both are at 1 arc-second grids over the territory of the United States.