Ellipsoidal height differences have been determined for 13 station pairs in the central Ohio region using measurements made with the Global Positioning System. This information was used to compute geoid undulation differences based on known orthometric heights. These differences were compared to gravimetrically computed undulations (using a Stokes integration procedure and least squares collocation having an internal r.m.s. agreement of ¿1 cm in undulation differences). The two sets of undulation differences have an r.m.s. discrepancy of ¿5 cm while the average station separation is of the order of 14 km. This good agreement suggests that gravimetric data can be used to compute accurate geoid undulation differences that can be used to convert ellipsoidal height differences obtained from GPS to orthometric height differences.