The NGS Gravity for the Redefinition of the American Vertical Datum (GRAV-D) project to modernize the national height system also is related to modernization of the NAD (Childers et al., 2009b; see Box 5.2). In effect, GRAV-D would provide the data necessary to define the national height datum as the new NAD ellipsoidal surface plus a geoid correction, modeled using gravimetric measurements. The combined NAD and geoid correction model would then be part of a new NSRS that would be consistent with the ITRF and also would meet the needs of the most demanding applications, including scientific experiments and monitoring for natural hazards. One aspect of this modernization that the NGS cannot deal with (at least not directly) is the legal definition of the NAD83. Such a radical improvement to the NAD might require Congressional legislation to redefine the national reference system; if that is the case, the committee urges that initiatives to propose such legislation be taken in consultation with the NGS.

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