This figure portrays the distribution of 2951 GPS benchmarks used in the
computation of the GEOID96 geoid height model.  The GPS ellipsoidal heights
are in the NAD 83 datum, and the benchmark orthometric heights are in the 
NAVD 88 datum.  These data, plus known relationships between the NAD 83
and the ITRF94 geocentric reference frame enable GEOID96 to support the
direct conversion between the NAD 83 GPS ellipsoidal heights and NAVD 88
orthometric heights.