Geoid Modeling at NOAA

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Geoid Modeling at NOAA

• Dru A. Smith, Ph.D.
• National Geodetic Survey
• National Ocean Service, NOAA
• November 13, 2000

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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

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The many definitions of height

• Orthometric Height (H)
• Height from the geoid to the Earths surface
• Frequently (informally) called height above mean
  sea level
• On most topographic maps
• e.g. The Height of Mount Everest is 8848 meters

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The many definitions of height

• Ellipsoidal Height (h)
• Height from the ellipsoid to the Earths 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

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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

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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

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Saving money with a geoid model

• The geographic community needs orthometric
  heights, H
• Old way H H0 DH (spirit leveling)
• Time consuming, 4 person team. Expensive!
• New way H h - N (GPS geoid)
• Fast, accurate, 1 person team. Cheap!
• Part of NOAAs Height Modernization program

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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 !!

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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

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NOAA resources / partnerships

• NOAA is uniquely qualified to undertake this
  research
• Geoid research is cross-disciplinary

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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

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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

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Geoid modeling 101

• Gravity measurements every 5-10 km
• Digital elevation models (30-100 m)
• Global gravity field models
• GPS heights on leveled benchmarks

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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

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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)

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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)

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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

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Contact Information

• Dr. Dru A. Smith
• 301-713-3202 x 149
• Fax [phone]
• Dru.Smith_at_noaa.gov
• www.ngs.noaa.gov/GEOID

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Extra Slides
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