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Geoids are approximations of mean sea‐level orthometric height with respect to a reference ellipsoid. Geoids are defined by additional higher‐order shapes, commonly modeled by spherical harmonics of height deviations from an ellipsoid, as illustrated in Figure 3.8. There are many geoid models based on different data, but the more recent, most accurate models depend heavily on GPS data. Geoid heights deviate from reference ellipsoids by tens of meters, typically.

The WGS84 geoid heights vary about 100 m from the reference ellipsoid. As a rule, oceans tend to have lower geoid heights and continents tend to have higher geoid heights. Coarse 20‐m contour intervals are plotted versus longitude and latitude in Figure 3.9, with geoid regions above the ellipsoid shaded gray.