Crustal motion may exceed 1 meter even after accounting for the shift of the 1964 Prince William Sound Earthquake. One key problem is deciding which orthometric height datum to use. NGVD 29 is not a sea-level datum, and the heights are not true orthometric heights. The datum of NAVD 88 is selected to maintain reasonable conformance with existing height datums, and its Helmert heights are good approximations of true orthometric heights. And, while differential ellipsoidal heights obtained from GPS are precise, they are often expressed in the NAD 83 datum, which is not exactly geocentric.
Studies are underway, and many less stringent requirements can be satisfied by geoid modeling.
The XNTG program can perform various functions, none of which are required to use the INTG program. In addition, from the gravimetric geoid side, there is always ongoing research to improve the theory of geoid determination.