Geoid Modeling at noaa dru A. Smith, Ph. D

Geoid Modeling at NOAA

• Dru A. Smith, Ph.D.

• National Geodetic Survey

• National Ocean Service, NOAA

• November 13, 2000

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

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”

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

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

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

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

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% !!

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

NOAA is uniquely qualified to undertake this research

• NOAA is uniquely qualified to undertake this research

• Geoid research is cross-disciplinary

Within NOAA:

• 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

Partner groups:

• 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

Geoid modeling 101

• Gravity measurements every 5-10 km

• Digital elevation models (30-100 m)

• Global gravity field models

• GPS heights on leveled benchmarks

GEOID99 accuracy:

• 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

Today: Surveyors can do sub-cm spirit leveling over lines up to 10 km long

• 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”)

1 cm error budget (absolute and differential)

• 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)

The geoid is the bridge between historical data sets and new cost-saving GPS technology

• 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

Contact Information

• Dr. Dru A. Smith

• 301-713-3202 x 149

• Fax: [phone]

• [e-mail]

• www.ngs.noaa.gov/GEOID

Extra Slides