Outline

1. Subsidence Monitoring and the GRAV-D projectDru Smith, Dan Roman, Daniel Winester, Mark EcklNOAA’s National Geodetic SurveySubsidence Workshop - Geodetic Monitoring TechniquesTexas Spatial Reference CenterTexas A&M University-Corpus ChristiCorpus Christi, Texas

2. Outline • Geodetic implications of subsidence • GRAV-D overview • GRAV-D/Campaign II and subsidence

3. From t0 to t1, subsidence occurs T=t0 T=t1 Surface Dh = h1-h0 g0 Surface DH = H1-H0 =Dh-DN g1 H0 H1 DN=N1-N0 h0 h1 N0=f(g0,H0) N1=f(g1,H1) Geoid Geoid N1 N0 Ellipsoid Ellipsoid g0 hasn’t been measured in decades: This is the essence of “Campaign I” of GRAV-D g1≠g0 : This is the essence of “Campaign II” of GRAV-D

4. E B D A C H ≈ h – N hE0 hD0 hA0 hC0 HA0 HE0 HC0 hB0 HB0 HD0 NC0 ND0 NB0 Geoid NA0 NE0 Ellipsoid Time: t= t0

5. Dh easily measured / monitored by GPS Dh Dh Dh Dh Dh Ellipsoid Does Not Change!! Ellipsoid Effect of Subsidence on Ellipsoid Heights

6. Dh Dh Dh Dh Dh Earth’s mass distribution has changed! So, Earth’s gravity field has changed…. DH ≈ Dh – DN (wanted) (GPS) (GRAV-D) Including the location and shape of the geoid… • Which means new orthometric heights rely on new locations of: • The crust and • The geoid DN DN DN DN DN Geoid Effect of Subsidence on Orthometric Heights

7. DH from Leveling? • Why not just re-level to monitor orthometric height changes?

8. Leveling and Gravity “Leveling without gravity measurements, although applied in practice, is meaningless from a rigorous point of view” • Physical Geodesy • Heiskanen and Moritz

9. gE0 gE1 gB0 gD0 gC0 gA0 gD1 gB1 gC1 gA1 Issue #1: Find a provably “stable” point Issue #2: Presumption that gravity hasn’t changed Issue #3: Failure to measure gravity while leveling, in general Issue #4: Reliance on finite point-cloud of vulnerable benchmarks Geoid Thus, NGS has adopted GRAV-D as its future policy toward defining and monitoring the vertical datum Orthometric Height Monitoring by Leveling

10. Q: What is GRAV-D?A: A Plan (released Dec 2007) • Official NGS policy as of Nov 14, 2007 • $38.5M over 10 years • Airborne Gravity Snapshot • Absolute Gravity Tracking • Re-define the Vertical Datum of the USA by 2017

11. Q: What is GRAV-D?A: Gravity to determine heights accurately • The first, middle and last point of GRAV-D: Gravity and Heights are inseparably connected • Or (to borrow from a common bumper sticker): • No gravity, no height • Know gravity, know height

12. Why isn’t NAVD 88 good enough anymore? • The GPS era brought fast, accurate ellipsoid heights – naturally this drove a desire for fast, accurate orthometric heights • Leveling the country can not be done again • Too costly in time and money • Leveling yields cross-country error build-up • Leveling requires leaving behind marks • Bulldozers and crustal motion do their worst • NAVD 88 H=0 level is known not to be the geoid • Biases , Tilts

13. Orthometric Height (H) • The distance along the plumb line from the geoid up to the point of interest Earth’s Surface H (NAVD 88) H NAVD 88 reference level The Geoid Errors in NAVD 88 : ~50 cm ave, 100 cm CONUS tilt, 1-2 meters ave in Alaska Almost NO tracking

14. Fast, Accurate Orthometric Heights • GPS already gives fast accurate ellipsoid heights • If the geoid were determined to highest accuracy… • Voila… Fast, accurate orthometric heights • Anywhere in the nation • Time-changes to H determined through: • GPS on CORS (h changes) • Absolute gravity spot checks (N changes)

15. Mission of NGS • To define, maintain and provide access to the National Spatial Reference System to meet our nation’s economic, social, and environmental needs And • To be a world leader in geospatial activities, including the development and promotion of standards, specifications, and guidelines.

16. Mission of NOS • To provide products, services, and information that promote safe navigation, support coastal communities, sustain marine ecosystems, and mitigate coastal hazards.

17. Mission of NOAA • To understand and predict changes in the Earth’s environment and conserve and manage coastal and marine resources to meet our nation’s economic, social and environmental needs

18. Missions and GRAV-D • NGS can neither fulfill their mission, nor contribute to the NOS and NOAA missions without modernizing the vertical datum component of the NSRS • Only GRAV-D offers a sustainable, accurate method for doing this

19. GRAV-D • National Scale has 2 parts: • High Resolution Snapshot • Low Resolution Movie • Local/Regional Scale has 1 part: • High Resolution Movie

20. GRAV-D • National High Resolution Snapshot • Predominantly through airborne gravity • With Absolute Gravity for ties and checks • Relative Gravity for expanding local regions where airborne shows significant mismatch with existing terrestrial

21. 20-100 km gravity gaps along coast Terrestrial gravity New Orleans Ship gravity

22. GRAV-D: Campaign II“Monitor Gravity over time” • National Low Resolution “Movie” of gravity • Epochal (annual?) absolute gravity re-measurements at key areas • GRACE/GRACE-FO for more global signatures

23. GRAV-D: Campaign IIMonitor Gravity (“Low Resolution Movie”) Measure gravity at each point, annually. Model gravity changes over time. Convert to geoid changes over time. Use with tracked GPS stations (CORS) to get orthometric height changes over time.

24. GRAV-D: Campaign III(Local/Regional Surveys) • Regional High Resolution Movie • Certain regions may require repetitive surveys to model the diversity of mass changes occurring locally • Can not be done with NGS personnel as is • Survey Types: • Gravity (absolute + relative) • Co-located GPS+Leveling in space/time • Minimally constrained

25. GRAV-D • Airborne • Critically needed as a one-time high resolution “snapshot” of gravity in the USA • As opposed to the thousands of surveys, with hundreds of instruments and operators over dozens of years • One time survey • Absolute • Cyclical for episodic checks in fixed locales • Co-incident with foundation CORS? • Two field meters plus one fixed SG • Relative • More frequently attached to “Height Mod” surveys

26. GRAV-D Status • 2007 Feb 12: NGS Releases Draft of 10 year plan (first public reference to re-definition of vertical datum redefinition) • 2007 Apr 10-11: NGS Holds Gravity Seminar at Corbin • 2007 Oct 25: NGS Acquires Airborne Gravimeter • 2007 Nov 14: NGS Releases Final Version of the GRAV-D plan • 2007 Nov 15-17: NGS Trains Airborne Gravimeter Operators • 2008 Jan 9: NGS Finalizes 10 year plan (codifying re-definition of vertical datum through GRAV-D) • 2008 Jan 14-Feb 15: Initial Flights of Airborne Gravimeter in Gulf Coast Coming soon: • 2008 TBD: NGS seeking MOU’s with Federal, State and Academic partners to assist in funding and executing GRAV-D

27. Next Steps • NGS has released the GRAV-D plan as its official way forward to fulfill the orthometric height portion of its mission • Airborne “snapshot” • 10 years • $38.5M (PPBES , MOUs, ???) • Absolute “movie” (time tracking) • $200k (base) • This is the annual cost of sustaining fast, inexpensive, cm-accurate orthometric heights in a dynamic USA from GPS and a geoid model

28. Should anyone lose sight of why NGS cares about knowing and maintaining accurate heights…

29. Questions/Comments? • Dr. Dru Smith • Chief Geodesist, National Geodetic Survey • Dru.Smith@noaa.gov • 301-713-3222 x 144 • http://www.ngs.noaa.gov/GRAV-D/

30. Extra Slides

31. Airborne Meter Initial road tests (Nov 2007) First build (Oct 2007) • Unit is TAGS (Turn-key Airborne Gravity System) • Sensor, Software and Training sold as a package from Micro-G/LaCoste • Has been flight tested and proven as the most accurate airborne meter available