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.

GEOID12B is used to convert the ellipsoidal height acquired by the Global Positioning System (GPS) in NAD 83 into an NAVD 88 orthometric height-the datum used in many maps and charts and other applications. USGG12 is computed using gravity data and can be used for circulation studies and other applications.

1301

USGG2012 is a gravimetric model that refers to the Geodetic Reference System 1980 (GRS80) while the hybrid model GEOID12B is fixed to the GPS/leveling data at bench marks of the North America Vertical Datum 1988 (NAVD88) - the official US height system. Therefore, GEOID12B converts the ellipsoidal height of GPS into the official NAVD88 orthometric height. More specific, GEOID12B converts the ellipsoidal height in the North American Datum of 1983 (NAD 83 [2011.0]) into orthometric height in the NAVD 88 for the conterminous United States and Alaska. In regions where there is no NAVD88, GEOID12B converts the ellipsoidal height in NAD 83 (PA11) into the local vertical datums in Hawaii and American Samoa. The ellipsoidal height in NAD 83 (MA11) can also be converted to heights of the local vertical datums in Guam and the Commonwealth of the Northern Mariana Islands by using GEOID12B.

USDOV2012 and DEFLEC12B are Deflection of the Vertical models that describe the angular difference between the norm of the zero-height-surfaces and the norm of the reference ellipsoid. USDOV2012 was computed from USGG2012, whereas DEFLEC12B was determined from GEOID12B. Both are given 1 arc-second grids.

NGS offers technical information of the geoid models and practical information concerning its use on the NGS website at http://geodesy.noaa.gov/GEOID. NGS also offers a workshop concerning GPS-derived heights and the use of the geoid models to convert from GPS-derived heights to orthometric heights (see http://geodesy.noaa.gov/PC_PROD/WorkShops/).

NAVD 88 coverage in Alaska is incomplete and may not be suitable for all regions. Notice that Hawaii, Guam, CNMI, American Samoa, Puerto Rico (PR), and the U.S. Virgin Islands (VI) all have their own separate vertical datums distinct from NAVD 88. GEOID12B converts between the appropriate local version of NAD 83 and the respective vertical datums for each of these regions. The GEOID12B product comprises eight files for the conterminous United States: four files for Alaska, and one each for Hawaii, PR/VI, American Samoa, and Guam/CNMI. Single grids also are available for CONUS and Alaska. Each file contains geoid heights on a one-minute by one-minute grid in both big-endian and little-endian binary (PC compatible) formats. The INTG program is supplied at the download site to interpolate to a specific position within these grids. Another program, XNTG, is supplied to extract subsets and convert the grids between ASCII and binary. Both are in FORTRAN, are available compiled and as source code, and can byte-swap the grids if needed on a Unix platform. There is also an on-line geoid computational facility within the NGS Geodetic Tool Kit at http://www.ngs.noaa.gov/TOOLS. Similar files, programs, and on-line computational facilities exist for USGG2012, USDOV2012 and DEFLEC12B.

Possibly 2015 or later depending on changes in control data in the NGS database.

none

none

none