A description of the earth's gravity field in spherical harmonics through degree and order 30 is obtained by combining satellite-to-sea-surface altimetry data from Geos 3, terrestrial gravity measurements, and satellite perturbation analysis. Reference orbits and a tide model are used with the altimetry data to calculate sea-surface heights, and these heights are assumed to be the geoid. The altimeter-determined geoid heights are averaged for each 1¿¿1¿ ocean-surface area, with an average root-mean-square uncertainty of approximately 2 m. The 25.295 1¿¿1¿ estimates are then treated by autocovariance analysis to obtain 550-km¿550-km block geoid undulations. A set of 37.415 1¿¿1¿ surface-gravity estimates is treated in the same way to obtain block gravity anomalies. The combined sets of blocks provide an estimate of the geopotential in each of 1635 out of 1654 possible surface elements. The two sets of block anomalies then give a set of normal equations for the spherical-harmonics coefficients and are combined with the normal equations obtained from laser data on ten close-earth satellites. Also obtained is the earth's semimajor axis: ae=6,378,138.23¿1.3 m.