Using radar altimeter data from the Seasat satellite, a geophysical model has been developed that successfully accounts for the anomalous geoid measurements observed over the Tonga Arc-Tonga Trench region. An isostatic and non-isostatic model for the compensation of the ocean bathymetry was analyzed. Airy isostatic compensation was found to over-compensate the Tonga Arc region and under-compensate the Tonga Trench. Gravitational field theory is frequently used to investigate the nature of the Earth’s interior. Even though it is well recognized that inversion of gravity anomalies to obtain density distribution leads to nonunique solutions, information is obtainable from the analysis. In studying the Earth’s interior it is evident that some sort of “compensation” of the surface features is taking place, as evidenced by the distribution of the gravity anomalies. This compensation is governed by the principle of isostasy. The Tonga island and ridge system forms a north to northeast trending linear system.