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The geoid undulations were computed from the OSU91A potential coefficient model. The undulations were calculated on a 0.25 x 0.25 degree grid. The geoid undulations given on the grid refer to an ellipsoid whose flattening is 1.0/298.257--- and whose equatorial radius is approximately 6378.13655 km.
Rapp, R.H., 1992. Computation and accuracy of global geoid undulation models, presented at the Sixth International Geodetic Symposium on Satellite Positioning, Columbus, Ohio.
