Figure 3. Physical surface (light green undulating line) of the earth adjusted to spheroidal surface (yellow broken line) by removing mass from continent above mean sea level and filling same mass in ocean below mean sea level. Geoid surface (light blue solid line), on the other hand, depends on the internal mass distribution i.e., geoid moves below spheroid where mass is deficient and it moves above spheroid where mass is excess. Where geoid surface and spheroidal surface coincides is accounted for mass balanced. By definition geoid describes the irregular shape of the earth and is the true zero surface for measuring elevations. Because geoid surface cannot be directly observed, heights above or below the geoid surface can’t be directly measured and are inferred by making gravity measurements and modeling the surface mathematically. MSL surfaces are assumed to be essentially the same, at some spots the geoid can actually differ from MSL by several meters.