GRAV-D: Redefining the American Vertical Datum with GNSS Technology and Airborne Gravity

1. by David Conner Geodetic Advisor to the State of Ohio National Geodetic Survey, NOAA Dave.Conner@noaa.gov International Great Lakes Datum& GRAV-DPresented at a meeting of the Great Lakes Regional Height Modernization ConsortiumHarrisburg, PAApril 13-14, 2011

2. NGS and the NSRSWho are those guys and what is CO-OPS? • NOAA's National Geodetic Survey (NGS) defines and manages a National coordinate system. This network, the National Spatial Reference System (NSRS), provides the foundation for transportation and communication; mapping and charting; and a multitude of scientific and engineering applications. • NGS was known as the US Coast and Geodetic Survey (USC&GS) until 1970 when it was renamed to become part of the newly formed National Oceanic and Atmospheric Administration (NOAA). NOAA is within the US Dept of Commerce. NGS is within NOAA’s National Ocean Service (NOS). • The Center for Operational Oceanographic Products and Services (CO-OPS) is another element of NOS. They are responsible for monitoring tides/water levels and currents, and maintain a network of some 54 water level gauges throughout the Great Lakes and their connecting channels. They are responsible for determining heights associated with the International Great Lakes Datum (IGLD), in cooperation with NGS and our International partners in Canada.

3. What is GRAV-D? (1) • Gravity for the Redefinition of the American* Vertical Datum • An NGS project whose target is to redefine the official civilian vertical datum as the geoid, realized through the use of GNSS technology and a gravimetric geoid model over at least the United States and its territories Last Updated 12 October 2010 (DAS)

4. What is GRAV-D? (2) • Official NGS policy as of Nov 14, 2007 • Airborne Gravity Snapshot • Absolute Gravity Tracking • Re-define the Vertical Datum of the USA by 2022 (at current funding levels) • Part of the NGS 10 year plan (2008-2018) • Target: 2 cm accuracy orthometric heights from GNSS and a geoid model Last Updated 12 October 2010 (DAS)

5. What is GRAV-D? (3) • GRAV-D means fast, accurate, consistent orthometric heights everywhere in the USA • GPS already gives fast accurate ellipsoid heights • If the geoid were modeled (and monitored) to highest accuracy…fast, accurate orthometric heights, anywhere, anytime • No need to use leveling to “bring in the datum” Last Updated 12 October 2010 (DAS)

6. What is GRAV-D? (4) • GRAV-D will mean: • Primary access to the vertical datum will be through a GNSS receiver and a gravimetric geoid model • One consistent vertical datum for all of North America • CONUS, Alaska, Hawaii, PR, VI • Available for adoption by • Canada, Mexico, Caribbean, Central America Last Updated 12 October 2010 (DAS)

7. What is GRAV-D? (5) • GRAV-D will mean: • As the H=0 surface, the geoid will be tracked over time to keep the datum up to date • The reliance on bench marks will dwindle to: • Secondary access to the datum • Minimal NGS involvement • Maintenance/checking in the hands of users • Use at your own risk Last Updated 12 October 2010 (DAS)

8. Why GRAV-D? (1) • A relatively small workforce can update the geoid as compared to the large workforce needed to re-level bench marks • A 2 cm target accuracy anywhere that GNSS receivers can be used, kept up to date through monitoring CORS and the geoid, is better than the accuracy and accessibility of NAVD 88 today • It is cheaper than leveling • The geoid can’t be bulldozed out of usefulness • The effect of subsidence upon the realization will be known (and accounted for) by monitoring CORS and monitoring the geoid Last Updated 12 October 2010 (DAS)

9. Why GRAV-D? (2) • Geoid accuracy depends on quality of data and quality of theory • Theory being revised by Y. Wang (IAG study group) • Data being updated using airborne techniques • Airborne gravimetry is the best technique to: • Cover the country quickly and consistently • Connect terrestrial to near-shore oceanic data • Fill in the gap between point-by-point terrestrial measurements and 200 x 200 km footprint satellite (GRACE) measurements Last Updated 12 October 2010 (DAS)

10. Why GRAV-D? (3) 20-100 km gravity gaps along coast Airborne gravity is the only technique that can adequately connect existing terrestrial data to existing ship and altimetry data in the oceans and fill coverage gaps. Airborne data will not replace existing data, but will be used as a baseline for correcting that data to be consistent across the country. Terrestrial gravity Ship gravity New Orleans Last Updated 12 October 2010 (DAS)

11. Why GRAV-D? (4) • Decades of disparate gravity surveys are inconsistent with one another • Airborne gravity will provide a baseline for removing these inconsistencies % Last Updated 12 October 2010 (DAS)

12. FY12 Plan for CONUSFY11 = Alaska and California

13. FY12-13 Great Lakes Plan FY12 = White, FY13 = Orange

14. IGLD … what is it? The IGLD is a dynamic height system. Dynamic heights, by definition, define a water level surface. Their use is not limited to the Great Lakes but they are the base height system used for IGLD 85 in the Great Lakes. Dynamic heights are the reference of choice for all large hydraulic systems everywhere. NAVD 88 heights are orthometric heights. Orthometric heights do not define a water level surface. IGLD heights are corrected dynamic heights. Hydraulic correctors (HC) are applied to dynamic heights to remove various observational and other inconsistencies in the NSRS. HC’s are the difference in the mean water level for the reference period 1982 -88 (7 year) between a subordinate gauge and the primary gauge on a lake.

15. IGLD … “I” is for International! Management of these waters is governed by International Treaty administered by the International Joint Commission with representatives from the US and Canada. US scientific agencies have Canadian counterparts. Published Great Lakes water level data are coordinated by government agencies in both countries to ensure there is agreement. Coordinated basic hydraulic and hydrologic data are required to solve numerous international problems related to joint use of Great Lakes waters

16. IGLD … Why is it important? The Great Lakes are a vast hydraulic system with water levels and flows influenced by engineered channels and control structures. The entire region is tilting due to post-glacial rebound (aka glacial isostatic adjustment … GIA) The absolute rate of change at a given location must be considered along with the relative rate of change across the region. The overall Great Lakes basin is tilting at a rate of about 60 cm (two) feet per century.

18. National Spatial Reference System (NSRS) NOS has two fundamental backbone systems … - GPS CORS (3-D) by NGS - Tide and Water Level Gauges (height) by CO-OPS Cleveland Water Level Gauge and GPS CORS

19. Vertical rate of change due to Post-Glacial Rebound International Great Lakes Datum of 1985 (IGLD 85), IGLD 2015 is coming

22. Seasonal fluctuation ~ +/- 1 ft, Long term fluctuation ~ +/- 6 ftStorm events can cause short term fluctuations of up to ~ 8 ft at Buffalo

23. IGLD … Why is it useful? Because it is a dynamic height system and can be used to relate various data to a common water level surface It provides a mechanism for the US and Canada to reference heights to a common datum throughout the Great Lakes region Each country has adopted other vertical datums for general use. The Canadian vertical datum is CGVD 28. The US vertical datum is NAVD 88.

24. IGLD … When/where is it used? It is used for basic hydraulic and hydrologic data related to engineering and scientific works on or near Great Lakes waters in both the US and Canada These works include things related to navigation and management of water … navigation charts, dredging, marine boundary determination, bridges, locks, dams, sewage and drainages systems, and much more.

25. IGLD … Why is there more than one? The IGLD is revised about every 30 years to remove the effect of GIA from published heights. IGLD 85 is the current datum. It was preceded by IGLD 55, USLS 35, and various outer datums. Efforts are underway now to compute IGLD 2015

26. IGLD … How are IGLD heights made available? Dynamic heightsare published by NGS on all bench mark datasheets IGLD 85 heights are not displayed on NGS’ datasheets Hydraulic correctors are determined by CO-OPS NOS provided software can be used to convert NAVD88 heights to IGLD 85 heights. This software applies a hydraulic corrector when appropriate. http://www.ngs.noaa.gov/TOOLS/IGLD85/igld85.shtml IGLD water levels are found on the CO-OPS water level web page at http://www.tidesandcurrents.noaa.gov/

28. www.ngs.noaa.gov

32. IGLD 85 heights… some important details Hydraulic correctors are applied to dynamic heights at locations on or very near the lake surface, which is assumed to be flat. Hydraulic correctors are not applied on rivers and connecting channels, which are sloping water surfaces. On sloping water surfaces the IGLD 85 height and NAVD 88 dynamic height are one and the same

33. Hydraulic Correctors - DIY • Maps of each lake show the hydraulic corrector at each water level gauge. • Correctors are interpolated between each gauge based on location of interest • Accuracy of an interpolated HC is one or two cm as stated in the IGLD 85 toolkit • HC are applied only on the lake, not on sloping surfaces of rivers or connecting channels

39. IGLD 2015 … how and when? • Preliminary data collection work is ongoing in the US and Canada • Schedule of the main effort is uncertain and contingent upon funding. Cost of the US effort is estimated at approx $15 M over 5 years (~ $3M per year), including the cost of GRAV-D in the region

40. IGLD 2015 Update • A funding request for 2012 was denied • Another attempt is in the 2013 budget request • Canada has similar budget uncertainties • No funding identified for FY 10, 11, or 12 • Great Lakes Commission Resolution http://www.glc.org/about/resolutions/10/pdf/Final_IGLD_Resolution_20100223.pdf

41. From the Great Lakes Commission http://www.glc.org/ Resolution: Maintaining Accuracy of the International Great Lakes Datum (IGLD) Adopted on February 23, 2010 Whereas, movement of the earth’s crust across the Great Lakes - St. Lawrence River system occurs on a very gradual, continuous and non-uniform basis (in some areas in the magnitude of centimeters per decade), referred to as “glacial isostatic adjustment” or rebounding of the land surface from the weight of glaciers that retreated over 10,000 years ago; and Whereas, water levels in the Great Lakes - St. Lawrence system fluctuate on a short-term, seasonal, and long-term basis, requiring careful and accurate measurements within an accurately established vertical datum; and Whereas, the safety and economic viability of the Great Lakes commercial navigation industry, the extensive recreational boating community and ports/harbors across the region all rely heavily on accurate water level forecasts and maintenance of congressionally authorized dredging programs to provide depths in the navigation channels including waterways connecting the Great Lakes; and

42. page 2 of 3 Whereas, all coastal zone management, including erosion prediction, flood prediction and response, and coastal structure design, construction and maintenance, rely upon an accurate vertical elevation datum; and Whereas, all coastal habitat restoration, rehabilitation, creation, enhancement, improvement and protection activities currently underway and expected to be implemented under the Great Lakes Restoration Initiative require an accurate vertical datum; and Whereas, prediction of the magnitude, timing and duration of climate variability and global climate change as it affects the Great Lakes – St. Lawrence River system, and the development of adaptive management approaches thereto, require accurate water level measurements and forecasts; and Whereas, the vertical elevation datum for the Great Lakes, known as the International Great Lakes Datum (IGLD), first established in 1955 and last updated in 1985, needs to be updated every 25 to 35 years to reflect continuous and differential changes in land surface elevations across the region.

43. page 3 of 3 Therefore, Be It Resolved, that the Great Lakes Commission urges the National Oceanic and Atmospheric Administration (NOAA) to prioritize the IGLD update to complete this work by 2015, and to coordinate their activities with Canadian federal and provincial partners, to ensure that this foundational vertical datum is maintained to the highest accuracy possible using the best available technologies; and Be It Finally Resolved, that the Great Lakes Commission urges the U.S. Congress to provide necessary financial resources to complete the IGLD by 2015, and further requests that the Canadian federal government provide a commensurate share to ensure timely completion of this important endeavor. Adopted at the 2010 Seminnual Meeting of the Great Lakes Commission, Washington, D.C., Feb. 22-23, 2010.

44. Additional Information 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 Last Updated 12 October 2010 (DAS) 44

45. Further Information • NOAA, NOS, National Geodetic Survey www.ngs.noaa.gov • NOAA, NOS, Center for Operational Oceanographic Products and Services http://www.co-ops.nos.noaa.gov/ For data request information incl IGLD55 and USLS35: User Services: (301) 713-2815