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Recent Advances in Geoid Research: Status Report on Key Tasks and Future Directions

This report from NGS highlights recent progress and ongoing tasks in geoid research, including Helmert anomalies, terrain and density modeling, and ellipsoidal boundary value problems (BVP). Key findings include the outer Spherical DEM at 95% completion and inner Spherical DEM at 25%, with ongoing work on ellipsoidal DEMs. Current challenges include density modeling and downward continuation. Upcoming steps focus on refining theories of data combination while addressing the complexities of GPS HARN covariance function and leveling data.

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Recent Advances in Geoid Research: Status Report on Key Tasks and Future Directions

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  1. Recent Geoid Research at NGS Dru A. Smith

  2. Status of various tasks • Helmert Anomalies • Terrain Modeling • Density Modeling • Downward Continuation • Improved Theory • Ellipsoidal BVP • Combination of GPS/geoid/leveling

  3. 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)

  4. 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

  5. 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 Earth’s surface or at geoid) and for potential changes (I.e. the Indirect effect)

  6. Spherical DEM, Inner Zone • One paper through review at JG • Space Domain • Mix of 1” and 30” DEM • Maui Supercomputer for final CONUS run

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

  8. 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?)

  9. 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

  10. Next Steps, in order: • Outer Zone • Inner Zone • Combination Theory • Ellipsoidal DEM • Downward Continuation • No new geoid in 2000

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