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Regional Models


  Authors: D.A. Smith, D.G. Milbert     Created: 1996    Resp: D.R. Roman  
      Status: PUBLIC    

G96SSS is a gravimetric geoid model for conterminous United States computed by the National Geodetic Survey (NGS). It is referred to the GRS80 normal ellipsoid in the ITRF94 frame (epoch 1996.0). It has a 2'x2' spatial resolution in latitude and longitude. It is based on over 1.8 million terrestrial and marine gravity values, augmented by gravity data contributions from NIMA. The geoid determination is based on the use of the one-dimensional spherical Fast Fourier Transform (FFT) to evaluate Stokes' integral. This computation is performed in the framework of a remove-restore procedure, where long wavelengths come from the EGM96 geopotential model. The digital elevation model used for the terrain correction is mainly based on the 30"x30" TOPO30 database. By comparing the G96SSS model with GPS/levelling data (ITRF94/NAVD88), a systematic offset at the level of 30-45 cm is present, with an overall rms of 15.6 cm. The GEOID96 hybrid model is computed by fitting 2951 GPS/levelling points (NAD83/NAVD88) with the G96SSS gravimetric model, also taking into account the relationships between NAD83 and ITRF94 reference frames. The fitting is performed by collocation after removing a tilted-plane from the residuals. Therefore the GEOID96 model directly relates NAD83(86) ellipsoid heights and NAVD88 orthometric heights. The differences between GEOID96 and GPS/levelling (NAD83/NAVD88) benchmarks have an rms of 5.5 cm (no tilts, zero average), due primarily to GPS error. The differences between GEOID96 and GEOID93 range from -1.22 to 3.74 m, due primarily to the non-geocentricity of the NAD83 ellipsoid.

D.A. Smith, D.G. Milbert (1999). The GEOID96 high-resolution geoid height model for the United States. Journal of Geodesy, 73(5), pp. 219-236. DOI: 10.1007/s001900050239

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