Geoid Modeling at NOAA
Dru A. Smith, Ph.D.National Geodetic SurveyNational Ocean Service, NOAA
November 13, 2000
November 13, 2000Geoid 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
November 13, 2000Geoid 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”
November 13, 2000Geoid Modeling at NOAA
Dr. Dru A. Smith
November 13, 2000Geoid 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
November 13, 2000Geoid 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
November 13, 2000Geoid Modeling at NOAA
Dr. Dru A. Smith
November 13, 2000Geoid 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
November 13, 2000Geoid Modeling at NOAA
Dr. Dru A. Smith
Saving money with a geoid model
• The geographic community needs orthometric heights, H
• Old way: H = H0 + 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
November 13, 2000Geoid 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% !!
November 13, 2000Geoid 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
November 13, 2000Geoid Modeling at NOAA
Dr. Dru A. Smith
NOAA resources / partnerships
• NOAA is uniquely qualified to undertake this research
• Geoid research is cross-disciplinary
November 13, 2000Geoid 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
November 13, 2000Geoid 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
November 13, 2000Geoid 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
November 13, 2000Geoid Modeling at NOAA
Dr. Dru A. Smith
November 13, 2000Geoid Modeling at NOAA
Dr. Dru A. Smith
November 13, 2000Geoid 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
November 13, 2000Geoid 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”)
November 13, 2000Geoid 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)
November 13, 2000Geoid 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
November 13, 2000Geoid Modeling at NOAA
Dr. Dru A. Smith
Contact Information
• Dr. Dru A. Smith
• 301-713-3202 x 149
• Fax: 301-713-4172
• www.ngs.noaa.gov/GEOID
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