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 1.NAVD 88 2.A new American vertical datum 3.Gravity and the geoid 4.GRAV-D plan 5.Airborne surveys completed 6.Plans for FY10 and beyond

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!

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 Errors in NAVD 88 Difference: NAVD 88 – datum derived from GRACE Min = 0.17 m Max = 1.88 m Ave = 0.98 m Std. Dev. = 0.37 m

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

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

Relationships between datums and gravity The geoid: one that best fits global mean sea level in the least squares sense W=W 1 =Constant W=W 2 =Constant W=W 3 =Constant W=W 4 =Constant W=W 0 =Constant Surface of equal gravitational potential

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

Static Snapshot: Airborne Provides intermediate wavelength gravity data Reconciles terrestrial datasets Fills in spatial gaps in the littoral region Ship gravity tracks Terrestrial gravity points New Orleans km gravity gaps along coast

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

Alaska- Anchorage Anchorage Survey flown out of Anchorage, AK over NOAA’s Hydropalooza Area in July, km x 500km region covered in ~100 flight hours

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

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

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

Alaska- Fairbanks Survey Free-Air Anomaly

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)

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

How to Monitor Temporal Change 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

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