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

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

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 3

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 4 Re-cap: What is a vertical datum?

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 Alaska Surveying and Mapping Conference

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 Alaska Surveying and Mapping Conference

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

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)8 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 9 Why isn’t NAVD 88 good enough anymore?

NGS Gravity Holdings At one time gravity data at NGS was adequate as the base for the Hybrid geoid but … Alaska Surveying and Mapping Conference 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

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 11 Possible ways to fix NAVD 88

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 – 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 12 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 13 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 14 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 15 Possible ways to fix NAVD 88

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 16 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 ( ) Target: 2 cm accuracy orthometric heights from GNSS and a geoid model 2010 Alaska Surveying and Mapping Conference 17 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 18 What is GRAV-D?

Hawaii Alaska “CONUS” Puerto Rico / Virgin Islands Guam / Northern Marianas American Samoa GRAV-D Planned Coverage 2010 Alaska Surveying and Mapping Conference 19 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 20 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 21 What is 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 22 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 23 Why GRAV-D?

2010 Alaska Surveying and Mapping Conference 24 Ship gravity Terrestrial gravity New Orleans 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. Why 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 25 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 26 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 flown from 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 27 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 28 What is the status of GRAV-D?

Alaska- Anchorage Anchorage Survey flown out of Anchorage, AK over NOAA’s Hydropalooza Area in July, km x 500km region covered in ~100 flight hours Alaska Surveying and Mapping Conference

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 No gravity data releases until this process is complete (late Spring 2010) Next flights scheduled in Alaska for Summer Alaska Surveying and Mapping Conference 30 What is the status of GRAV-D?

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

32 NGA funded NRL plane USACE funded As of Nov 2009: 6.7% of planned areas have been flown What is the status of GRAV-D?

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 33

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 2010 Alaska Surveying and Mapping Conference 34 Alaska NGA Survey 2009 AK08 FY10

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 Alaska Surveying and Mapping Conference

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 36 What is the status of GRAV-D?

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 37 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 38 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 39 How will I access the new vertical datum?

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” Goal of NHMP

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.”

2010 Alaska Surveying and Mapping Conference 42 The NGS 10 year plan ( ) The GRAV-D Project Socio-Economic Benefits of CORS and GRAV-D Additional Information

Contacts: Always begin with your state advisor: – NGS Information Center – Phone: (301) – Alaska Surveying and Mapping Conference 43 Additional Information

Questions Renee Shields Height Modernization Manager N/NGS1, SSMC3, Room East-West Highway Silver Spring, MD , x116 Contact information