1	Status and Future of the Geoid at NGS Dan Roman 2005 Convocation of the National Geodetic Survey 27 OCT 2005
2	Outline Height Modernization Requirements Review of Current Models Why Current Models are Insufficient What is Needed in Future Models Required Research and Data Collection Summary
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4	Draft Guidelines for Establishing GPS-Derived Orthometric Heights (IP)
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6	What is a Geoid Though? The equipotential surface of the Earth's gravity field which best fits, in the least-squares sense, mean sea level. In practice, the average position of mean sea level and a corresponding average over the time-varying geopotential must be accepted. The equipotential surface of the Earth's gravity field which would coincide with the ocean surface if the latter were undisturbed and affected only by the Earth's gravity field. This is the original definition as given by Listing (1873). The definition is deficient because it assumes that the oceanic surface specified is an equipotential surface. The equipotential surface coinciding with mean sea level in the oceans. Since mean sea level is not an equipotential surface, the definition is inconsistent. The equipotential surface, through a given point, chosen near mean sea level, that would exist if only the rotation of the Earth and the Earth's gravitational field affected the potential. This definition was given by Jensen (1950). It is a function of the position of the chosen point.
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8	Internal vs. Absolute Model Accuracy
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13	Future Efforts Aerogravity must be downward continued or otherwise combined with existing surface gravity data Clean up gravity DB by combining airborne gravity and surface absolute gravity as tie points Develop new methods of geoid computation using cleansed gravity DB, suitable EGM and DEM’s Lidar data used to evaluate Mean Dynamic Topography (MDT) The models need to be compared for mutual validation: Geoid + MDT + Tide = lidar-obs instantaneous sea surface The next study area will flow from the Florida panhandle through Louisiana into Texas along the Gulf coast MDT issues aren’t as severe there, but terrestrial subsidence issues are very significant in Louisiana Proof of concept for a larger production in FY08 and beyond Ultimately, these data must be fused with forthcoming Earth Gravity Models and terrain models to yield seamless gravity and geoid models across North America
14	Conclusions Potential meter-level errors exist within the NAVD 88 Similar magnitude errors could exist within EGM96 or other EGM’s Future EGM’s will directly impact the gravimetric geoid solutions at the long to intermediate wavelengths Seamless gravity data is needed from deep ocean onto shore and off the opposite national coast(s) to resolve almost all wavelengths These data can be used to find the best fitting EGM for the U.S. Resulting gravimetric models can be studied in the context of MDT models to determine the best fitting littoral geoid The implication then would be that these models would also be suitable in the interior Such geoid models will better resolve orthometric heights and provide an improved relationship between oceanographic and terrestrial applications