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Regional Models


  Authors: D.A. Smith, D.R. Roman     Created: 1999     Resp: D.R. Roman  
      Status: PUBLIC    

G99SSS 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 ITRF96 frame (epoch 1997.0). It has a 1'x1' spatial resolution in latitude and longitude. It is based on 2.6 million gravity measurements, that is approximately 0.8 million more gravity data points than in G96SSS; note that most of these additional points are due to expanding the computational area of G99SSS with respect to G96SSS. 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 terrain correction is based on 30-arc-second and 3-arc-second digital elevation models. By comparing the G99SSS model with GPS/levelling data (ITRF97/NAVD88), a systematic offset is present with an average of 51.7 cm and a standard deviation of 21.7 cm. The GEOID99 hybrid model is computed by fitting 6169 GPS/levelling points (NAD83/NAVD88) with the G99SSS gravimetric model, also taking into account the relationships between NAD83 and ITRF96 reference frames. The fitting is performed by least squares collocation after removing a trend to the residuals. Therefore the GEOID99 model directly relates NAD83(86) ellipsoid heights and NAVD88 orthometric heights. The differences between GEOID99 and GPS/levelling (NAD83/NAVD88) benchmarks have an rms of 4.6 cm, while the differences between GEOID99 and GEOID96 have an rms of 20.9 cm.

D.A. Smith, D.R. Roman (2001). GEOID99 and G99SSS: 1-arc-minute geoid models for the United States. Journal of Geodesy, 75(9), pp. 469-490.