1. Geoid Modeling at NOAA Dru A. Smith, Ph.D. National Geodetic Survey National Ocean Service, NOAA November 13, 2000

2. Geoid Modeling at NOAA Dr. Dru A. Smith Topics of Discussion The many definitions of “height” Why does anyone need a geoid model? NOAA resources and partnerships Geoid modeling 101 Current status and future directions

3. November 13, 2000 Geoid Modeling at NOAA Dr. Dru A. Smith The many definitions of “height” Orthometric Height (H) –Height from the geoid to the Earth’s surface –Frequently (informally) called “height above mean sea level” –On most topographic maps e.g. “The Height of Mount Everest is 8848 meters”

4. November 13, 2000 Geoid Modeling at NOAA Dr. Dru A. Smith

5. November 13, 2000 Geoid Modeling at NOAA Dr. Dru A. Smith The many definitions of “height” Ellipsoidal Height (h) –Height from the ellipsoid to the Earth’s surface –Usually derived from GPS measurements –Easy to obtain through GPS –Has few physical applications e.g. water does not necessarily run ‘downhill’ in ellipsoid heights

6. November 13, 2000 Geoid Modeling at NOAA Dr. Dru A. Smith The many definitions of “height” Geoid Height (N) –Height from ellipsoid up to geoid –Also called “geoid undulation” –N = h - H –“N” determined from gravity measurements

7. November 13, 2000 Geoid Modeling at NOAA Dr. Dru A. Smith

8. November 13, 2000 Geoid Modeling at NOAA Dr. Dru A. Smith Why do we need a geoid model? It saves money It saves time (but time = money) Additional scientific and economic uses: –Geophysical: Oil and other subsurface features –Oceanographic: Sea level rise and current modeling

9. November 13, 2000 Geoid Modeling at NOAA Dr. Dru A. Smith Saving money with a geoid model The geographic community needs orthometric heights, H Old way: H = H 0 +  H (spirit leveling) –Time consuming, 4 person team. Expensive! New way: H = h - N (GPS & geoid) –Fast, accurate, 1 person team. Cheap! Part of NOAA’s “Height Modernization” program

10. November 13, 2000 Geoid Modeling at NOAA Dr. Dru A. Smith Saving money with a geoid model Baltimore County GPS project, Phase III –Installed new vertical control benchmarks –290 square miles (1/3 of Baltimore county) –Traditional leveling: 7 1/2 months / $174,000 –GPS and geoid: 3 weeks / $ 49,000 Savings: $125,000 = 72% !!

11. November 13, 2000 Geoid Modeling at NOAA Dr. Dru A. Smith Saving money with a geoid model Baltimore county GPS project, Phase III –1/3 of 1 county out of 3142 counties –Informal extrapolation: The geoid could save 150-1500 million dollars for this application alone –Depends on achieving an accurate geoid not just in Baltimore, but everywhere

12. November 13, 2000 Geoid Modeling at NOAA Dr. Dru A. Smith NOAA resources / partnerships NOAA is uniquely qualified to undertake this research Geoid research is cross-disciplinary

13. November 13, 2000 Geoid Modeling at NOAA Dr. Dru A. Smith NOAA resources Within NOAA: –NOS : NGS -- gravity database, GPS on leveled benchmarks and research on high resolution geoid modeling –NESDIS : NGDC -- Gravity and topography data –NESDIS : ORA : ORAD -- Satellite altimetry and oceanic gravity

14. November 13, 2000 Geoid Modeling at NOAA Dr. Dru A. Smith NOAA partnerships Partner groups: –NIMA -- Significant gravity and DEM data –USGS -- DEM data –NASA -- Global gravity field with NIMA –NTIA -- DEM data –Universities -- Data and research –States -- GPS cooperative surveys –Other countries (Canada, Mexico, Caribbean, Denmark) -- Data and tech transfers

15. November 13, 2000 Geoid Modeling at NOAA Dr. Dru A. Smith Geoid modeling 101 Gravity measurements every 5-10 km Digital elevation models (30-100 m) Global gravity field models GPS heights on leveled benchmarks

16. November 13, 2000 Geoid Modeling at NOAA Dr. Dru A. Smith

17. November 13, 2000 Geoid Modeling at NOAA Dr. Dru A. Smith

18. November 13, 2000 Geoid Modeling at NOAA Dr. Dru A. Smith Current Status GEOID99 accuracy: Absolute +/- 2.5 cm about North American Vertical Datum of 1988 (NAVD 88) Differential +/- 0.2 to 3.5 cm between points 1 to 40+ km apart

19. November 13, 2000 Geoid Modeling at NOAA Dr. Dru A. Smith Positioning for the Future Today: Surveyors can do sub-cm spirit leveling over lines up to 10 km long Tomorrow: Surveyors expect to do 100+ km lines at similar accuracy with GPS and geoid (“GPS leveling”)

20. November 13, 2000 Geoid Modeling at NOAA Dr. Dru A. Smith Achieving Higher Accuracies 1 cm error budget (absolute and differential) –Better theory –More data (Terabytes, USA; Petabytes, Globe) –More computational power Supercomputers? –Occasional, one shot, massive computations –Maui supercomputer: Fall 2000 (UNM/USAFRL)

21. November 13, 2000 Geoid Modeling at NOAA Dr. Dru A. Smith Conclusions The geoid is the bridge between historical data sets and new cost-saving GPS technology Geoid modeling is an integral part of Height Modernization The “1 cm geoid” is achievable in the USA within 10 years

22. November 13, 2000 Geoid Modeling at NOAA Dr. Dru A. Smith Contact Information Dr. Dru A. Smith 301-713-3202 x 149 Fax: 301-713-4172 Dru.Smith@noaa.gov www.ngs.noaa.gov/GEOID

23. November 13, 2000 Geoid Modeling at NOAA Dr. Dru A. Smith Extra Slides

24. November 13, 2000 Geoid Modeling at NOAA Dr. Dru A. Smith