A mean sea surface map has been determined for the global ocean areas between +62¿ and -62¿ latitude using GEOS 3 altimeter data. A grid of laser reference orbits computed using the GEM 10B gravity model has been used to orient the altimeter data in a center of mass coordinate system. The density of the altimeter tracks has enabled the computation of the sea surface heights above the reference ellipsoid on a 1¿¿1¿ grid in most of the oceanic areas. In the northwest Atlantic the dense coverage has enabled computations on a 0.25¿¿0.25¿ grid. Comparisons of the global surface with an independently computed mean sea surface based upon SEASAT altimeter data indicate an rms agreement of a little over a meter. Comparisons of the regional solution in the northwest Atlantic with SEASAT profiles indicate a precision of a few decimeters in this surface. An analysis of the global crossover differences has indicated the possibility of a timing error in the altimeter data. Timing bias values of 9.2¿0.5 ms for 1975 and 18.7¿0.6 ms for 1976 have been recovered from an analysis of the altimeter data.