Somewhat old-fashioned name for the separation N between the geoid and the reference ellipsoid of the Earth.
Because geophysical heights H "above sea level" refer to the geoid, but GPS positioning provides in a direct geometric fashion heights h above the GRS80reference ellipsoid, it has become a matter of great importance to determine accurate representations, or maps, of geoid undulations
over the territories of nations, to allow the inexpensive GPS technology to be harnessed for the precise determination of heights.
The behavior of the geoid in the Pacific Northwest is of special interest due to its rugged terrain and difficulty in matching GPS benchmarks with the geoid in the area.
Geoidundulations were evaluated directly on the geoid (rather than using an up/down procedure) for both EGM-X05 and EGM96, with the differences (EGM96 minus EGM-X05) shown in Figure 8 (95K), covering the range of 24-53 degrees North in latitude, and 230-294 degrees East in longitude.
The differences between the two geoids has been calculated, and a plot (minus a few degrees along the edges to reduce the visual impact of edge effects) of the differences is shown in Figure 12.
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