The figure portrays a perspective view to the north of the GEOID96 geoid
height model in the area 40-48N, 90-98W (890 x 640 km).  The geoid heights
are relative to the GRS80 ellipsoid, and range from -25.0 to -35.9 meters.
The contour interval in the lower figure is 0.5 meter.  The sinuous feature
stretching diagonally across the region is the Mid-Continent Gravity High.  
This is the result of a rift that produced a high density, igneous formation.  
At the steepest part, north of Des Moines, Iowa, one may find a geoid height 
difference in excess of 1 meter over a distance of 10 kilometers.