1. 45 th Annual Alaska Surveying & Mapping Conference February 21-25, 2011 Hilton Anchorage Hotel Impact of Airborne Gravity Surveys on Geoid Modeling in Alaska Part I: A New National Vertical Datum Daniel R. Roman and Vicki A. Childers

2. NGS’s Role and Authority on Vertical Datums Coast and Geodetic Survey Act (Public Law 80-373) gives the Department of Commerce the right to (amongst numerous other things): –“…conduct …geodetic control surveys…” http://uscode.house.gov/download/pls/33C17.txt 45th Annual Alaska Surveying & Mapping Conference February 21-25, 2011 2 Impact of Airborne Gravity Surveys on Geoid Modeling in Alaska Thursday, 0800-1200

3. NGS’s Role and Authority on Vertical Datums OMB Circular A-16 (revised)*: Names DOC and NOAA as “lead agency” for Geodetic Control, and says: “All NSDI [National Spatial Data Infrastructure] framework data and users' applications data require geodetic control to accurately register spatial data.” “The National Spatial Reference System is the fundamental geodetic control for the United States.” *[originally issued in 1953, revised in 1967 and again in 1990] 45th Annual Alaska Surveying & Mapping Conference February 21-25, 2011 3 Impact of Airborne Gravity Surveys on Geoid Modeling in Alaska Thursday, 0800-1200

4. NGS’s Role and Authority on Vertical Datums OMB Circular A-16 (revised): Hence, the NSRS responsibility falls to NGS NGS Mission: “To define, maintain, and provide access to the National Spatial Reference System (NSRS) to meet our nation’s economic, social, and environmental needs” –NSRS includes the horizontal and vertical datums, the national shoreline, and a variety of other components. www.whitehouse.gov/omb/Circulars/a016/a016_rev.html 45th Annual Alaska Surveying & Mapping Conference February 21-25, 2011 4 Impact of Airborne Gravity Surveys on Geoid Modeling in Alaska Thursday, 0800-1200

5. NGS’s Role and Authority on Vertical Datums Federal Geodetic Control Subcommittee (FGCS) Federal Register Notice (Vol. 58, No. 120) Affirms “NAVD 88 as the official civilian vertical datum for surveying and mapping activities in the United States performed or financed by the Federal Government. “To the extent practicable, legally allowable, and feasible, require that all Federal agencies using or producing vertical height information undertake an orderly transition to NAVD 88.” http://www.ngs.noaa.gov/PUBS_LIB/FedRegister/FRdoc93-14922.pdf 45th Annual Alaska Surveying & Mapping Conference February 21-25, 2011 5 Impact of Airborne Gravity Surveys on Geoid Modeling in Alaska Thursday, 0800-1200

6. NGS’s Role and Authority on Vertical Datums Summary: –OMB A-16 establishes DOC/NOAA (implying NGS) as lead agency for NSDI geodetic control (the NSRS) –NAVD 88 is the vertical datum of the NSRS –FGCS requires that NAVD 88 heights be used in all civilian federal surveying and mapping These regulations do not apply to DoD nor to state and local surveying, but these groups often do adopt NAVD 88 45th Annual Alaska Surveying & Mapping Conference February 21-25, 2011 6 Impact of Airborne Gravity Surveys on Geoid Modeling in Alaska Thursday, 0800-1200

7. What is a Vertical Datum? Strictly speaking, a vertical datum is a surface representing zero elevation Traditionally, a vertical datum is a system for the determination of heights above a zero elevation surface Vertical datum comprised of: –Its definition: Parameters and other descriptors –Its realization: Its physical method of accessibility "topographic map." Online Art. Britannica Student Encyclopædia. 17 Dec. 2008 http://student.britannica.com/ebi/art-53199 45th Annual Alaska Surveying & Mapping Conference February 21-25, 2011 7 Impact of Airborne Gravity Surveys on Geoid Modeling in Alaska Thursday, 0800-1200

8. History of Vertical Datums in the USA Pre-National Geodetic Vertical Datum of 1929 (NGVD 29) –The first geodetic leveling project in the United States was surveyed by the Coast and Geodetic Survey (C&GS) from 1856 to 1857. –General adjustments of leveling data yielded datums in 1900, 1903, 1907, and 1912. –NGS does not offer products which transform from these older datums into newer ones (though some users still work in them!) 45th Annual Alaska Surveying & Mapping Conference February 21-25, 2011 8 Impact of Airborne Gravity Surveys on Geoid Modeling in Alaska Thursday, 0800-1200

9. History of Vertical Datums in the USA NGVD 29: National Geodetic Vertical Datum of 1929 – Original name: “Sea Level Datum of 1929” – “Zero height” held fixed at 26 tide gauges –Did not account for Local Mean Sea Level variations from the geoid Thus, not truly a “geoid based” datum 45th Annual Alaska Surveying & Mapping Conference February 21-25, 2011 9 Impact of Airborne Gravity Surveys on Geoid Modeling in Alaska Thursday, 0800-1200

10. History of Vertical Datums in the USA NAVD 88: North American Vertical Datum of 1988 Definition: The surface of equal gravity potential to which orthometric heights shall refer in North America*, and which is 6.271 meters (along the plumb line) below the geodetic mark at “Father Point/Rimouski” (NGSIDB PID TY5255). Realization: Over 500,000 geodetic marks across North America with published Helmert orthometric heights, most of which were originally computed from a minimally constrained adjustment of leveling and gravity data, holding the geopotential value at “Father Point/Rimouski” fixed. Father Point Lighthouse, Quebec *Not adopted in Canada 45th Annual Alaska Surveying & Mapping Conference February 21-25, 2011 10 Impact of Airborne Gravity Surveys on Geoid Modeling in Alaska Thursday, 0800-1200

11. NAVD 88 (continued) – Defined only on the conterminous North American continent – Does not (and cannot) exist for any place you can’t level to from Father Point, such as: Guam, American Samoa, Hawaii, Puerto Rico, American Virgin Islands, Commonwealth of the Northern Marianas, Aleutian Islands History of Vertical Datums in the USA 45th Annual Alaska Surveying & Mapping Conference February 21-25, 2011 11 Impact of Airborne Gravity Surveys on Geoid Modeling in Alaska Thursday, 0800-1200

12. History of Vertical Datums in the USA NAVD 88 (continued) –Height chosen was to minimize 1929/1988 differences in USGS maps –“Zero height surface” of NAVD 88 wasn’t chosen for its closeness to the geoid (but it was close…few decimeters) 45th Annual Alaska Surveying & Mapping Conference February 21-25, 2011 12 Impact of Airborne Gravity Surveys on Geoid Modeling in Alaska Thursday, 0800-1200

13. History of Vertical Datums in the USA NAVD 88 (continued) –Use of one fixed height removed local sea level variation problem of NGVD 29 –Use of one fixed height did open the possibility of unconstrained cross-continent error build up –But the H=0 surface of NAVD 88 was supposed to be parallel to the geoid…(close again) 45th Annual Alaska Surveying & Mapping Conference February 21-25, 2011 13 Impact of Airborne Gravity Surveys on Geoid Modeling in Alaska Thursday, 0800-1200

14. NAVD 88 relies upon bench marks that: –Disappear by the thousands every year –Are not funded for replacement –Are almost never re-checked for movement –Are not necessarily in convenient places –Don’t exist in most of Alaska –Weren’t adopted in Canada –Cross-country error build up Problems with NAVD 88 45th Annual Alaska Surveying & Mapping Conference February 21-25, 2011 14 Impact of Airborne Gravity Surveys on Geoid Modeling in Alaska Thursday, 0800-1200

15. NAVD 88 suffers from a zero height surface that: –Has been proven to be ~50 cm biased from the latest, best geoid models (GRACE satellite) –Has been proven to be ~ 1 meter tilted across CONUS (again, based on the independently computed geoid from the GRACE satellite) Problems with NAVD 88 45th Annual Alaska Surveying & Mapping Conference February 21-25, 2011 15 Impact of Airborne Gravity Surveys on Geoid Modeling in Alaska Thursday, 0800-1200

16. H Earth’s Surface The Geoid NAVD 88 reference level H (NAVD 88) Errors in NAVD 88 : ~50 cm average, 100 cm CONUS tilt, 1-2 meters average in Alaska NO tracking of temporal changes Problems with NAVD 88 45th Annual Alaska Surveying & Mapping Conference February 21-25, 2011 16 Impact of Airborne Gravity Surveys on Geoid Modeling in Alaska Thursday, 0800-1200

17. Approximate level of geoid mismatch known to exist in the NAVD 88 zero surface Problems with NAVD 88 45th Annual Alaska Surveying & Mapping Conference February 21-25, 2011 17 Impact of Airborne Gravity Surveys on Geoid Modeling in Alaska Thursday, 0800-1200

18. Accuracy vs Precision High Accuracy, Low Precision High Precision, Low Accuracy http://en.wikipedia.org/wiki/Accuracy_and_precision NAVD 88 is precise but not accurate. GEOID09 is precise but not accurate. 45th Annual Alaska Surveying & Mapping Conference February 21-25, 2011 18 Impact of Airborne Gravity Surveys on Geoid Modeling in Alaska Thursday, 0800-1200

19. Short term fix: Height Modernization GPS surveys –Quick way to propagate NAVD 88 bench mark heights to new marks through the use of GPS and a constrained least squares adjustment –NOAA TM NOS NGS 58 and 59 guidelines – Keeps NAVD 88 useful and accessible, but does not address the majority of problems of NAVD 88 itself Lack of roads prevents Height Modernization surveys from propagating heights to western AK Can NAVD 88 be fixed? 45th Annual Alaska Surveying & Mapping Conference February 21-25, 2011 19 Impact of Airborne Gravity Surveys on Geoid Modeling in Alaska Thursday, 0800-1200

20. Long term fix: Re-level some/all of NAVD 88 –81,500 km of 1 st order leveling at least –625,000 km of mixed 1 st and 2 nd order leveling Re-leveling NAVD 88 estimated to cost between $200 Million and $2 Billion Still have problems related to passive control Can NAVD 88 be fixed? 45th Annual Alaska Surveying & Mapping Conference February 21-25, 2011 20 Impact of Airborne Gravity Surveys on Geoid Modeling in Alaska Thursday, 0800-1200

21. Long term fix: Replace NAVD 88 GRAV-D: Gravity for the Redefinition of the American Vertical Datum GRAV-D will: define the datum as a given geoid model realize it through GNSS technology spanning at least the United States and its territories Can NAVD 88 be fixed? 45th Annual Alaska Surveying & Mapping Conference February 21-25, 2011 21 Impact of Airborne Gravity Surveys on Geoid Modeling in Alaska Thursday, 0800-1200

22. Official NGS policy as of Nov 14, 2007 as part of 10 yr plan 2008-18 – Estimated $38.5M over 10 years Three thrusts of project: – Airborne survey “snapshot” – Long-term monitoring of geoid change – Partnership surveys Re-define the Vertical Datum of the USA by 2022 (given funding levels) What is GRAV-D? 45th Annual Alaska Surveying & Mapping Conference February 21-25, 2011 22 Impact of Airborne Gravity Surveys on Geoid Modeling in Alaska Thursday, 0800-1200

23. 45th Annual Alaska Surveying & Mapping Conference February 21-25, 2011 Impact of Airborne Gravity Surveys on Geoid Modeling in Alaska Thursday, 0800-1200 23 GRAV-D Target: 2 cm accuracy orthometric heights from GNSS and a geoid model GRAV-D would mean fast, accurate, consistent orthometric heights everywhere in the USA

24. GRAV-D One consistent vertical datum for the entire US: Continental US, Alaska, Hawaii and Pacific Islands, and Puerto Rico and the US Virgin Islands Plan to coordinate our datum with Canada, Mexico, Caribbean, Central America Long-term objective: North American datum from equator to Arctic 45th Annual Alaska Surveying & Mapping Conference February 21-25, 2011 Impact of Airborne Gravity Surveys on Geoid Modeling in Alaska Thursday, 0800-1200 24

25. A good-enough geoid? Target 2 cm accuracy in orthometric heights assumes: – 1 cm accuracy in GNSS positioning – 1 cm accuracy in Geoid An accurate geoid requires: – A sufficiently good gravity field – Good geoid theory Under revision by Y. Wang (IAG study group) 45th Annual Alaska Surveying & Mapping Conference February 21-25, 2011 Impact of Airborne Gravity Surveys on Geoid Modeling in Alaska Thursday, 0800-1200 25

26. Gravity Field Before GRAV-D Blend of various gravity data: – GRACE satellite gravity (wavelengths ≥ 350 km) – NGS gravity holdings: land surface and marine measurements 45th Annual Alaska Surveying & Mapping Conference February 21-25, 2011 Impact of Airborne Gravity Surveys on Geoid Modeling in Alaska Thursday, 0800-1200 26

27. 45th Annual Alaska Surveying & Mapping Conference February 21-25, 2011 Impact of Airborne Gravity Surveys on Geoid Modeling in Alaska Thursday, 0800-1200 27 Ship gravity Terrestrial gravity New Orleans 20-100 km gravity gaps along coast Field is not sampled uniformly Data range in age and quality, some w/o metadata Some surveys have systematic errors Data gaps in littoral areas Problems with Gravity Holdings

28. 45th Annual Alaska Surveying & Mapping Conference February 21-25, 2011 Impact of Airborne Gravity Surveys on Geoid Modeling in Alaska Thursday, 0800-1200 28 Building a Gravity Field Long Wavelengths: (≥ 350 km) GRACE Satellite Intermediate Wavelengths (500 km to 20 km) Airborne Measurement Surface Measurement Short Wavelengths (< 200 km) + +

29. Airborne gravity “snapshot”: survey the entire region –Cover the country quickly and consistently with 20 km resolution –Fill in littoral data gaps –Connect terrestrial to near-shore oceanic data and satellite altimetric gravity fields Enhance the surface measurements with predicted gravity from topography 45th Annual Alaska Surveying & Mapping Conference February 21-25, 2011 Impact of Airborne Gravity Surveys on Geoid Modeling in Alaska Thursday, 0800-1200 29 Gravity Field Recovery for Geoid

30. Airborne Gravity Objectives Survey the entire US and its holdings by 2022 Line spacing of 10 km Extend flight lines out beyond shelf break Complete the survey according to the following metric: 45th Annual Alaska Surveying & Mapping Conference February 21-25, 2011 Impact of Airborne Gravity Surveys on Geoid Modeling in Alaska Thursday, 0800-1200 30 FY09 (baseline) Targets FY10FY11FY12FY13FY14FY15FY16FY17FY18FY19FY20FY21FY22 6.14%7.5%13%20%28%36%44%52%60%68%76%84%92% 100% and Implement

31. GRAV-D Coverage 45th Annual Alaska Surveying & Mapping Conference February 21-25, 2011 Impact of Airborne Gravity Surveys on Geoid Modeling in Alaska Thursday, 0800-1200 31 Puerto Rico US Virgin Is.

32. GRAV-D Coverage 45th Annual Alaska Surveying & Mapping Conference February 21-25, 2011 Impact of Airborne Gravity Surveys on Geoid Modeling in Alaska Thursday, 0800-1200 32 Am. Samoa Hawaii Guam

33. 45th Annual Alaska Surveying & Mapping Conference February 21-25, 2011 Impact of Airborne Gravity Surveys on Geoid Modeling in Alaska Thursday, 0800-1200 33 Long-Term Geoid Monitoring and GRAV-D A primary mission goal is to model the dynamic geoid for North America IAG-sponsored workshop was held in Boulder to address how to monitor geoid change Included representatives from the absolute/relative gravity communities, GRACE, and CORS Formed a steering committee of interested parties to help craft plan to monitor geoid change Group did re-convene at CMOS-CGU meeting in Ottawa in June 2010 Concluded that this aspect would require a combination of absolute gravity monitoring and geoid change modeling

34. 45th Annual Alaska Surveying & Mapping Conference February 21-25, 2011 Impact of Airborne Gravity Surveys on Geoid Modeling in Alaska Thursday, 0800-1200 34 Steering Committee Name Joe Henton Hans-Peter Plag Michael Sideris David Avalos-Naranjo Karla Edwards Dan Roman Dan Winester Vicki Childers David Schmerge Dennis Milbert Affiliation Natural Resources Canada NV Bur. of Mines&Geology Univ. of Calgary INEGI, Mexico The Ohio State University NGS NGS (retired)

35. 45th Annual Alaska Surveying & Mapping Conference February 21-25, 2011 Impact of Airborne Gravity Surveys on Geoid Modeling in Alaska Thursday, 0800-1200 35 Regional Partnerships & Collaborations MOU letters to Other Federal Agencies – USACE already funded Gulf of Mexico survey – NGA funded Alaska 2009 survey – USGS High level briefing in planning stages Active science participation with Crustal Imaging and Characterization Science Center: Magnetometer States – North Carolina, South Carolina Industry Partners: Micro-g LaCoste, Fugro University Researchers

36. Break time! Next up: GRAV-D Airborne Project Status

37. The NGS 10 year plan (2008-2018) http://www.ngs.noaa.gov/INFO/NGS10yearplan.pdf The GRAV-D Project http://www.ngs.noaa.gov/GRAV-D Socio-Economic Benefits of CORS and GRAV-D http://www.ngs.noaa.gov/PUBS_LIB/Socio-EconomicBenefitsofCORSandGRAV-D.pdf Additional Information (1 of 2) 45th Annual Alaska Surveying & Mapping Conference February 21-25, 2011 37 Impact of Airborne Gravity Surveys on Geoid Modeling in Alaska Thursday, 0800-1200

38. Contacts: Always begin with your state advisor: –http://www.ngs.noaa.gov/ADVISORS/AdvisorsIn dex.shtmlhttp://www.ngs.noaa.gov/ADVISORS/AdvisorsIn dex.shtml NGS Information Center –Phone: (301) 713-3242 –E-mail: ngs.infocenter@noaa.gov Additional Information (2 of 2) 45th Annual Alaska Surveying & Mapping Conference February 21-25, 2011 38 Impact of Airborne Gravity Surveys on Geoid Modeling in Alaska Thursday, 0800-1200