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Gravity and the Geoid at NGS

This presentation discusses the review of height systems, geoid modeling, the GEOID99 model, and future directions in the field. It covers topics such as gravity measurements, digital elevation models, and the computation of geoid undulations using the Stokes Integral and the G99SSS method. It also highlights the comparison of GEOID99 with GEOID96 and the accuracy of GPS-derived orthometric heights.

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Gravity and the Geoid at NGS

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  1. Gravity and the Geoid at NGS Dru A. Smith National Geodetic Survey Presented at the 2000 Geodetic Advisor Convocation Silver Spring, MD April 11, 2000

  2. Topics of Discussion • Review of height systems • Geoid modeling • GEOID99 • Future Directions

  3. Review of Height Systems • (Helmert) Orthometric Heights • Ellipsoid Heights • Geoid undulations • NAD 83 & NAVD 88 • Mean Sea Level

  4. Geoid Modeling • Data Sets • Methodology

  5. Data Contributions • “Short Wavelength” (local) • Digital Elevation Models • “Medium Wavelength” (regional) • Surface Gravity Measurements • “Long Wavelength” (global) • Geopotential Model (EGM96)

  6. Need: Gravity Measurements • Terrestrial • Ship-borne • Altimetrically-derived

  7. Need: Digital Elevation Models • TOPO30 (30 arcsecond) for CONUS • GTOPO30 (30 arc second) for Canada • NGSDEM99 (1 arc second) for PNW

  8. Step 1: Grid Bouguer Anomalies • Gravity measurements • DEM for Bouguer plate • DEM for Terrain Corrections

  9. Step 2: Compute EGM96 gravity anomalies

  10. Step 3: Residual Free-air anomalies • Begin with Bouguer • Restore Bouguer plate • Remove EGM96

  11. Step 4: The Stokes Integral • Input: residual free-air anomalies • Output: residual co-geoid undulations • 1-D FFT method

  12. Step 5: Compute EGM96 geoid undulations

  13. Step 6: G99SSS • Begin with residual co-geoid undulations • Add EGM96 • Add Indirect Effect • G99SSS is • GEOCENTRIC and • GRAVIMETRIC

  14. Need: GPS on Benchmarks • NAD 83 ellipsoid heights • NAVD 88 Helmert orthometric Heights

  15. Step 7: Conversion Surface Use Least Squares Collocation to combine: ITRF96 / NAD 83 transformation NAVD 88 bias (-52 cm) Tilt (0.15 ppm, 327 azimuth) Local GPS/Leveling/Geoid misfit G99SSS - Conversion Surface = GEOID99

  16. GEOID99 • Comparison to GEOID96 • GPS-derived orthometric heights • Accuracy

  17. 2’x2’ North: 54 DEM: TOPO30 2951 GPS/BMs -31 cm NAVD 88 bias 5.5 cm RMS wrt GPS/BMs 1’x1’ North: 58 DEM: TOPO30 + NGSDEM99 6169 GPS/BMs -52 cm NAVD 88 bias 4.6 cm RMS wrt GPS/BMs GEOID96 vs GEOID99

  18. GPS-derived Orthometric heights • Local accuracy is difficult to gauge • Few GPSBMs near each other • Depends on: • Quality of GPS • “Simplicity” of local geoid • Method of adjustment

  19. Future Directions • Gravimetric • Improved gravimetric theory • GRACE/CHAMP • 1 arcsecond DEM over USA • Hybrid Geoid • 1 cm (1 s) GPS heights • Leveling near CORS

  20. Summary • Gravity coverage is adequate to exceptional • 1 arc second DEM needed for all USA territories • GEOID99 improved on the success of GEOID96 • GPS derived orthometric heights at the <1 to 8 cm level

  21. ON-LINE NGS GEOID RESOURCES • Official NGS Geoid Page: • http://www.ngs.noaa.gov/GEOID • Links to all geoid models • Links to all geoid papers from NGS • Lots of useful geoid information • Slideshows like this one (coming soon)

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