engineering analyses, based on NGVD 29, are used for new studies, based otherwise on NAVD 88. Although conversion programs are available, the old surveys and methods used to establish NGVD 29 elevations are not a robust substitute for new measurements made with modern surveying technology and tied to well-founded, well-maintained NAVD 88 control monuments. Furthermore, the NGVD 29 elevations for benchmarks in areas of active subsidence frequently were not adjusted to account for movement of the terrain.
Finding. FEMA is justified in requiring that all survey data be referenced to the NAVD 88 datum.
Establishing an orthometric height datum that can provide centimeter-level height accuracy requires the use of either geodetic survey leveling observations or GPS measurements and a high-accuracy geoid model. The current version of NAVD 88 does not apply to islands, which cannot be reached with leveling measurements from the continental United States. Therefore, uniform national standards for FEMA flood maps cannot be met until an improved orthometric height datum and geoid model exist. The NGS is engaged in this task through geodetic leveling in U.S. territorial islands and implementation of the Gravity for the Redefinition of the American Vertical Datum (GRAV-D) project, which is estimated to be completed in 2017 (NOAA, 2007). If local island vertical datums are established, efforts should be made to ensure that the observations conform to national geodetic standards and that the data are archived and easily available for later adjustments.
The NGS Height Modernization Program includes the development of a high-accuracy geoid model and tools to assist with datum transformations. Height modernization has been implemented in only a few states (Figure 3.3). Yet it is essential for ongoing maintenance and expansion of NAVD 88 to support FEMA’s standards and requirements for flood studies and floodplain mapping. The control monumenta-