grav-d: ngs gravity for the re-definition of the american vertical datum project
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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 - PowerPoint PPT PresentationTRANSCRIPT
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 882. A new American vertical datum3. Gravity and the geoid4. GRAV-D plan5. Airborne surveys completed6. 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 GRACEMin = 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
NGeoid
A chosen Ellipsoid
H ≈ h-Nh = ellipsoidal heightH = orthometric heightN = geoid height
Relationships between datums and gravity
The geoid: one that best fits global mean sea level in the least squares sense
W=W1=Constant
W=W2=Constant
W=W3=Constant
W=W4=Constant
W=W0=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 gravitytracks
Terrestrial gravity points
New Orleans
20-100 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, 2008
• 400km x 500km region coveredin ~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