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GRAV-D: NGS Gravity for the Re-definition of the American Vertical Datum Project

GRAV-D: NGS Gravity for the Re-definition of the American Vertical Datum Project. V. A. Childers, D. R. Roman, D. A. Smith, and T. M. Diehl* U.S. National Geodetic Survey. Overview. NAVD 88 A new American vertical datum Gravity and the geoid GRAV-D plan Airborne surveys completed

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GRAV-D: NGS Gravity for the Re-definition of the American Vertical Datum Project

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  1. GRAV-D: NGS Gravity for the Re-definition of the American Vertical Datum Project V. A. Childers, D. R. Roman, D. A. Smith, and T. M. Diehl* U.S. National Geodetic Survey

  2. Overview • NAVD 88 • A new American vertical datum • Gravity and the geoid • GRAV-D plan • Airborne surveys completed • Plans for FY10 and beyond

  3. Vertical Datum – History(Orthometric Heights) • 1807 – 1996 • Defined and Accessed – Leveling/Passive Marks • Currently: North American Vertical Datum 1988 • NAVD 88: 600,000+ Marks • NGS detects hundreds moved/destroyed every year • How many go undetected? • Post-Glacial-Rebound, Subsidence, Tectonics, Frost-Heave – lots of motion out there!

  4. Errors in NAVD 88 • NAVD 88 H=0 level is known not to be the geoid • Average 1 m bias and 1 m tilt across CONUS • 1-2 m bias in Alaska Difference: NAVD 88 – datum derived from GRACE Min = 0.17 m Max = 1.88 m Ave = 0.98 m Std. Dev. = 0.37 m

  5. NAVD 88 Must Be Updated • NAVD 88 H=0 level is known not to be the geoid • 50 cm average bias, 1 m tilt across CONUS • 1-2 m bias in AK • Leveling the country again is impractical • Too costly in time and money • Leveling yields cross-country error build-up • Leveling requires leaving behind marks which are impermanent • The GPS era brought fast, accurate ellipsoid heights – naturally this drove a desire for fast, accurate orthometric heights • 1-2 cm accuracy is needed

  6. Relationships between datums and gravity h = ellipsoidal height H = orthometric height N = geoid height h H ≈ h-N H Geoid N A chosen Ellipsoid

  7. Relationships between datums and gravity W=W3=Constant Surface of equal gravitational potential W=W2=Constant W=W1=Constant W=W0=Constant W=W4=Constant The geoid: one that best fits global mean sea level in the least squares sense

  8. NGS’ Plan: GRAV-D • Official NGS policy: • Re-define the US Vertical Datum by creating a new gravimetric geoid • 10 year program • Two Major Program Elements • Airborne Gravity “Snapshot” for Baseline • Long Term Monitoring of Temporal Changes • Projected program cost: $38.5M over 10 years

  9. Static Snapshot: Airborne • Provides intermediate wavelength gravity data • Reconciles terrestrial datasets • Fills in spatial gaps in the littoral region Terrestrial gravity points 20-100 km gravity gaps along coast Ship gravitytracks New Orleans

  10. Current Status • Completed test phase of plan (flight altitude, speed, line spacing needed) in Alabama • Completed Airborne Surveys: • 2008: Alaska- Anchorage Louisiana- New Orleans • 2009: Puerto Rico and the Virgin Islands Louisiana- Lake Charles Texas coast- Austin Alaska- Fairbanks

  11. Anchorage Alaska- Anchorage • Survey flown out of Anchorage, AK over NOAA’s Hydropalooza Area in July, 2008 • 400km x 500km region coveredin ~100 flight hours

  12. Gulf of Mexico • First testing phase from Montgomery, AL in January 2008 • MS/LA began October 2008 • LA done February 2008 • TX started March 2008 • TX done May 2008

  13. Puerto Rico and the Virgin Islands • Flown in January in 100 flight hours • Completes our second test survey for the GRAV-D plan

  14. Alaska- Fairbanks Survey • Flown from Eielson AFB • Naval Research Lab C-12 King Air Aircraft • Survey sponsored by NGA • Flown at 12,500 ft at ~220 kts, 7.5 km line spacing • ~110 flight hours • Incorporated USGS mag sensor

  15. Alaska- Fairbanks Survey Free-Air Anomaly

  16. GRAV-D Priorities • Highest Survey Priority: Alaska • High Priority: Great Lakes Region East Coast US West Coast US • Ultimate Goal:Entire US and holdings by 2017 (~70% by 2015)

  17. GRAV-D and the Future • Airborne Surveys: • Planning to fly much of AK in FY10 • First Congressional funding likely in FY10 • Leverage funding through partnerships with other federal agencies, industrial groups, and universities

  18. Track low degree-order gravity changes with GRACE and satellite laser ranging Maintain networks of absolute and relative gravity measured in areas of most rapid change Convert to geoid changes over time Use with tracked GPS stations (CORS) to get orthometric height changes over time How to Monitor Temporal Change

  19. GRAV-D and the Future • Long-Term Monitoring: • 2009 Workshop to bring program into focus • All interested parties invited to attend • FY2010 will see launch of the monitoring program • Looking for collaboration opportunities

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