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GRAV-D: Redefining the American Vertical

Learn about GRAV-D, a new method for redefining the vertical datum in the American surveying and mapping industry. Discover why NAVD.88 is no longer sufficient and explore the potential ways to fix it. Find out how GRAV-D can improve accuracy and accessibility in height modernization.

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GRAV-D: Redefining the American Vertical

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  1. Gravity for the Redefinition of the American Vertical: GRAV-D Anchorage, Alaska February 22, 2010 Renee Shields Height Modernization Manager 2010 Alaska Surveying and Mapping Conference

  2. Is NAVD 88 Accessible and “Maintainable”? • Chris Pearson: GPS Height Primer • Dan Roman: National Geodetic Survey’s Geoid Model 2010 Alaska Surveying and Mapping Conference

  3. Outline • Why isn’t NAVD 88 good enough anymore? • Possible ways to fix NAVD 88 • What is GRAV-D? • Why GRAV-D? • What’s the status of GRAV-D? • How will I access the new vertical datum? • Additional Information 2010 Alaska Surveying and Mapping Conference

  4. Re-cap: What is a vertical datum? • A surface representing zero elevation • A system for the determination of heights above a zero elevation surface • A vertical datum always has two components: • Its definition: Parameters and other descriptors • Its realization: Its physical method of accessibility 2010 Alaska Surveying and Mapping Conference

  5. Re-Cap: How has Height Modernization helped? • NAVD88 defined through leveling network • Leveling is the most accurate way to determine heights but is costly/time-consuming • GPS can be used to get accurate ellipsoid heights • With a good geoid model, GPS can be used to get accurate orthometric heights 2010 Alaska Surveying and Mapping Conference

  6. Re-Cap: How accurate is a GPS-derived Orthometric Height? • Relative (local) accuracy in ellipsoid heights between adjacent points will be better than 2 cm, at 95% confidence level • Network accuracy (relative to NSRS) in ellipsoid and orthometric heights will be better than 5 cm, at 95% confidence level • Accuracy of orthometric height is dependent on accuracy of the geoid model 2010 Alaska Surveying and Mapping Conference

  7. How accurate is a GPS-derived Orthometric Height? 89 “GPS on bench marks” in AK 2010 Alaska Surveying and Mapping Conference

  8. Why isn’t NAVD 88 good enough anymore? • NAVD 88 suffers from use of bench marks that: • Are almost never re-checked for movement • Disappear by the thousands every year • Are not funded for replacement • Are not necessarily in convenient places • Don’t exist in most of Alaska • Weren’t adopted in Canada • Were determined by leveling from a single point, allowing cross-country error build up Last Updated 30 Nov 2009 (DAS)

  9. Why isn’t NAVD 88 good enough anymore? • Approximate level of geoid mismatch known to exist in the NAVD 88 zero surface: 2010 Alaska Surveying and Mapping Conference

  10. NGS Gravity Holdings At one time gravity data at NGS was adequate as the base for the Hybrid geoid but … • Most of the historical NGS data is terrestrial • Multiple observers, multiple processors over the past 60 years • Numerous corrections and datums over time • Metadata maintained in paper records • Very limited aerogravity • Deficit in near-shore gravity data 2010 Alaska Surveying and Mapping Conference

  11. Possible ways to fix NAVD 88 • Short term fix: Height Modernization GPS surveys • Have provided a fast way to disseminate NAVD 88 bench mark heights to new marks through the use 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 2010 Alaska Surveying and Mapping Conference

  12. Possible ways to fix NAVD 88 • Long term fix: Re-level some/all of NAVD 88 • 81,500 km of 1st order leveling at least • 625,000 km of mixed 1st and 2nd order leveling • About $3000 / km (average contract cost) • Re-leveling NAVD 88 would cost between $200 Million and $2 Billion • This wouldn’t fix all of the problems associated with the use of bench marks though 2010 Alaska Surveying and Mapping Conference

  13. Possible ways to fix NAVD 88 • Long term fix: Replace NAVD 88 • Find a method of defining a vertical datum that seeks to fix all of the known issues with NAVD 88 • Best option: Define the datum as a given geoid model and realize it through GNSS technology • GRAV-D 2010 Alaska Surveying and Mapping Conference

  14. Possible ways to fix NAVD 88 • Long term fix: Replace NAVD 88 (continued) • GRAV-D Trade-offs: Datum is only realizable to 2 cm at best at any given point (GNSS error + geoid error) • However, this is an improvement over NAVD 88 realization error • The datum could then be disseminated locally through very precise geodetic leveling 2010 Alaska Surveying and Mapping Conference

  15. Possible ways to fix NAVD 88 • Long term fix: Replace NAVD 88 (continued) • GRAV-D International Issues • Canada has agreed to move to a geoid based vertical datum • Negotiations with USA underway • Mexico has no plans yet to move to a geoid based datum • Central American, Caribbean: No policy to switch, but the datum will be freely available to them 2010 Alaska Surveying and Mapping Conference

  16. What is GRAV-D? • GRAV-D • 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 2010 Alaska Surveying and Mapping Conference

  17. What is GRAV-D? • 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 2018 (if fully funded beginning in 2009) • Part of the NGS 10 year plan (2008-2018) • Target: 2 cm accuracy orthometric heights from GNSS and a geoid model 2010 Alaska Surveying and Mapping Conference

  18. What is GRAV-D? • 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” 2010 Alaska Surveying and Mapping Conference

  19. “CONUS” Alaska Guam / Northern Marianas Hawaii Puerto Rico / Virgin Islands American Samoa What is GRAV-D? GRAV-D Planned Coverage 2010 Alaska Surveying and Mapping Conference

  20. What is GRAV-D? • 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 2010 Alaska Surveying and Mapping Conference

  21. What is GRAV-D? • 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 2010 Alaska Surveying and Mapping Conference

  22. Why GRAV-D? • 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 2010 Alaska Surveying and Mapping Conference

  23. Why GRAV-D? • 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 2010 Alaska Surveying and Mapping Conference

  24. Why GRAV-D? 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 2010 Alaska Surveying and Mapping Conference

  25. What is the status of GRAV-D? • 2007 Feb 12: NGS Releases Draft of 10 year plan (first public reference to re-definition of vertical datum redefinition) • 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 First Airborne Gravimeter Operators 2010 Alaska Surveying and Mapping Conference

  26. What is the status of GRAV-D? • 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 Alabama Gulf Coast • 2008 July: Flights completed over the NOAA Hydropalooza area of South-Central Alaska (near Anchorage) • 2008 Oct: NGS and NOS send formal letters requesting partnership on GRAV-D to 7 other federal agencies interested in either the data collection or datum change • FEMA, USACE, NASA, ONR, FAA, USGS, NGA 2010 Alaska Surveying and Mapping Conference

  27. What is the status of GRAV-D? • 2008 Oct – Nov: Airborne Gravimeter Flights over most of Louisiana completed • 2009 Jan: Puerto Rico / Virgin Islands region flown • 2009 May: In partnership with the USACE, the Western Gulf of Mexico coastline flownfrom Louisiana through Texas to the Mexican border • 2009 June: Socio-Economic Benefits Scoping study for CORS and GRAV-D released • The benefits to the nation from completing GRAV-D are estimated to be $4.8 billion over 15 years, including $2.2 billion in avoidance costs from improved floodplain management. • This is approximately a 1:100 cost-benefit ratio 2010 Alaska Surveying and Mapping Conference

  28. What is the status of GRAV-D? • 2009 July-Aug: In partnership with NGA and the Naval Research Laboratory, the central Alaskan region is flown • 2009 Dec: FY2010 NOAA Appropriations contains a $4 Million/year request for GRAV-D • Previously GRAV-D was an unfunded project, supported predominantly by NGS base funds and Height Modernization • NGS asked for $5 Million, and got $3.5 Million 2010 Alaska Surveying and Mapping Conference

  29. Anchorage Alaska- Anchorage • Survey flown out of Anchorage, AK over NOAA’s Hydropalooza Area in July, 2008 • 400km x 500km region coveredin ~100 flight hours 2010 Alaska Surveying and Mapping Conference

  30. What is the status of GRAV-D? • Gravimeter operating well • Vendor-provided gravity processing software has unresolved artifacts rendering it unusable for final gravity processing • NGS will replace this black-box software with a new in-house suite of software by early calendar year 2010. • No gravity data releases until this process is complete (late Spring 2010) • Next flights scheduled in Alaska for Summer 2010 2010 Alaska Surveying and Mapping Conference

  31. Puerto Rico and the Virgin Islands • Flown in January, 2009 in 100 flight hours • Completes our second test survey for the GRAV-D plan 2010 Alaska Surveying and Mapping Conference

  32. What is the status of GRAV-D? NGA funded NRL plane USACE funded As of Nov 2009: 6.7% of planned areas have been flown 2010 Alaska Surveying and Mapping Conference

  33. New Instruments for GRAV-D • A-10 absolute gravimeter from Micro-g Lacoste • Honeywell H-765 ACE Inertial Navigation System 2010 Alaska Surveying and Mapping Conference

  34. GRAV-D FY10 Surveys • FY10 First Congressional Funding for GRAV-D • Priority 1 is Alaska • Perform 3 surveys that cover the southwestern coast, connect up 2008 and 2009 surveys • Operate from Anchorage and Dillingham • Use the NOAA Turbo Commander Alaska NGA Survey 2009 FY10 FY10 AK08 FY10 2010 Alaska Surveying and Mapping Conference

  35. Long-Term Geoid Monitoring and GRAV-D • Second GRAV-D goal is to monitor long-term change to the geoid • Ultimately, a primary mission goal will be 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 will re-convene at CMOS-CGU meeting in Ottawa in May/June 2010 Alaska Surveying and Mapping Conference

  36. What is the status of GRAV-D? Other news: • Developing a prototype “vertical datum access” tool through OPUS • Will use the best gravimetric geoid available, prior to completion of GRAV-D • Should give users the ability to see how the new datum will be accessed and test its heights relative to NAVD 88 heights 2010 Alaska Surveying and Mapping Conference

  37. How will I access the new vertical datum? Primary access (NGS mission) • Users with geodetic quality GNSS receivers will continue to use OPUS suite of tools • Ellipsoid heights computed, and then a gravimetric geoid removed to provide orthometric heights in the new datum • No passive marks needed • But, could be used to position a passive mark 2010 Alaska Surveying and Mapping Conference

  38. How will I access the new vertical datum? • Secondary access (Use at your own risk) • Passive marks that have been tied to the new vertical datum • NGS will provide a “data sharing” service for these points, but their accuracy (due to either the quality of the survey or the age of the data) will not be a responsibility of NGS 2010 Alaska Surveying and Mapping Conference

  39. How will I access the new vertical datum? • NAVD 88 conversion to new datum • A conversion will be provided between NAVD 88 and the new datum • Only where recent GNSS ellipsoid heights exist to provide modern heights in the new datum 2010 Alaska Surveying and Mapping Conference

  40. Goal of NHMP • Access to accurate, reliable heights nationally • Standards that are consistent across the nation • Data, technology, and tools that yield consistent results regardless of terrain and circumstances • A system/process that will stand the text of time – “Maintain-able”

  41. From NGS’ 10-Year Plan NGS Mission: “Modernize the Geopotential (“Vertical”) Datum” “The gravimetric geoid, long used as the foundation for hybrid geoid models, becomes the most critical model produced by NGS.”

  42. 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 2010 Alaska Surveying and Mapping Conference

  43. Additional Information • Contacts: • Always begin with your state advisor: • http://www.ngs.noaa.gov/ADVISORS/AdvisorsIndex.shtml • NGS Information Center • Phone: (301) 713-3242 • E-mail: ngs.infocenter@noaa.gov 2010 Alaska Surveying and Mapping Conference

  44. Questions Contact information Renee Shields Height Modernization Manager N/NGS1, SSMC3, Room 9357 1315 East-West Highway Silver Spring, MD 20910 301-713-3231, x116 Renee.Shields@noaa.gov

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