Recent Geoid Research at NGS

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Recent Geoid Research at NGS

• Dru A. Smith

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Status of various tasks

• Helmert Anomalies
• Terrain Modeling
• Density Modeling
• Downward Continuation
• Improved Theory
• Ellipsoidal BVP
• Combination of GPS/geoid/leveling

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Overall Status

• Spherical DEM, Outer Zone 95
• Spherical DEM, Inner Zone 25
• Ellipsoidal DEM Ongoing (D. Smith)
• Density Modeling No action
• Downward Continuation No action
• Ellipsoidal Accepted Fei/Sideris. Working on
  similar one-step approach.
• Data Combination Ongoing (D. Roman)

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Spherical DEM, Outer Zone, I

• Uses Global 5 DEM and 1-D FFT
• 160 wall-clock hours on 2 Ultra 2s for attraction
  of 3-D masses computed at surface of Earth
  outside a 4 degree cap
• sub-microGal accuracy, terms to h4
• 30 arcsecond test in progress

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Spherical DEM, Outer Zone, II

• Any cap size can be chosen
• Space domain validations are in progress
• Similar approach will be used for attraction of
  condensed masses (computed at Earths surface or
  at geoid) and for potential changes (I.e. the
  Indirect effect)

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Spherical DEM, Inner Zone

• One paper through review at JG
• Space Domain
• Mix of 1 and 30 DEM
• Maui Supercomputer for final CONUS run

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Ellipsoidal DEM

• Recently begun
• Computations to show how much is improved over
  spherical DEM

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Ellipsoidal Theory

• Fei and Sideris, 2000 (JoG)
• Compute spherical N, then compute corrections
• Appears to reduce N/S tilts wrt GPSBMs by 50
• Investigating other approaches (I.e. 4th
  fundamental BVP?)

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Combination Theory

• Non-isotropic, non-homogenous nature of
  covariance function
• Non-homogenous nature of USA GPS HARN surveys
• Difficult to get full VCV matrices
• Leveling in archives
• Correlation between state-by-state GPS surveys

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Next Steps, in order

• Outer Zone
• Inner Zone
• Combination Theory
• Ellipsoidal DEM
• Downward Continuation
• No new geoid in 2000