Status and Future of the Geoid at NGS

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

3. Height Modernization – the importance of accurate elevations • Accurate water levels referenced to accurate, standardized land elevations enable safe and efficient maritime shipping and navigation • Integration of surface weather and water with road elevations benefits commerce, tourism, and emergency preparedness (planning) by providing near real-time monitoring and positioning information on weather and road conditions (response) • Terrain modeling derived from GPS/Remote Sensing data results in more accurate field boundaries and slope (contour) management for land utilization • Geographic information (GIS) integrating planting and yield rates results in well-defined fertilizer and pesticide application saving resources and reducing run-off • GPS controlled precise steering of farm equipment available today, with remotely operating machinery anticipated in the near future Height Modernization provides the basis for Intelligent Transportation Systems (highway, rail, air, water) for improved safety and efficiency. Modern society is becoming increasingly dependent on geographic data that is spatially referenced – horizontally and vertically. Precision Agriculture applies GPS technology and management strategies to individual fields to protect the environment, improve productivity, and save time and money. • In FEMA’s National Flood Insurance Program, risk is measured by elevations, so accuracy is critical • Denser control reduces survey costs and improves accuracy of geospatially related data such as LIDAR, photogrammetry, GIS, RTK, construction plans, and evacuation routes • Benefits include more accurate floodplain and insurance rate maps • Monitoring and management of water resources (groundwater vs. surface water) is improved in areas susceptible to subsidence or erosion Relative (local) and absolute (national) height inconsistencies are eliminated through the use of a common vertical datum -- NAVD88. NOAA’s National Geodetic Survey Height Modernization Program Juliana Blackwell, Program Manager Email: htmod@noaa.gov Visit: www.ngs.noaa.gov/heightmod/ University of California, San Diego/Scripps • Wisconsin Department of Transportation • Harris-Galveston Coastal Subsidence District • California Spatial Reference Center • North Carolina Geodetic Survey • South Carolina Geodetic Survey • NOAA’s National Ocean Service Federal Emergency Management Agency • Louisiana Spatial Reference Center • NOAA’s National Weather Service • Spatial Reference Center of Washington • U.S. Army Corps of Engineers • Washington Department of Natural Resources • Alabama Department of Revenue Louisiana State University • NOAA’s Coastal Services Center • U.S. Geological Survey • Alabama Department of Transportation • North Carolina A & T State University • University of Southern Mississippi • NOAA’s Center for Operational Oceanographic Products & Services

4. Draft Guidelines for Establishing GPS-Derived Orthometric Heights (IP)

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.

8. Internal vs. Absolute Model Accuracy NAVD 88 datum “true” geoid

9. tidal benchmarks with a NAVD88 tie tidal benchmarks without a NAVD88 tie

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