Downward continuation of gravity field

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Gravity is observed on the earth's surface but for the geoid computation it is needed at the sea level (on the geoid): it has to be "continued downward" onto the sea level through topographical masses. In our approach to geoid determination, the best approximation of topographical masses is subtracted from actual topography and we consider the result (Earth without topography) to represent the situation in the "NT space". In fact, we replace topography by a condensation layer on the geoid and call this new space the "Helmert space". Helmert's space is thus the NT space augmented by the condensation layer. In both the NT and Helmert's space, the downward continu-ation is carried out through a part of space that is as void of mass as we can possibly make it.