gravity for the redefinition of the american vertical: grav-d anchorage, alaska february 22, 2010 1...

Gravity for the Gravity for the Redefinition of the Redefinition of the American Vertical: American Vertical:

GRAV-DGRAV-D

Anchorage, AlaskaFebruary 22, 2010

12010 Alaska Surveying and Mapping Conference

Renee ShieldsHeight Modernization Manager

Is NAVD 88 Accessible and “Maintainable”?

• Chris Pearson: GPS Height Primer

• Dan Roman: National Geodetic Survey’s Geoid Model


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


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


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


How accurate is a GPS-derived Orthometric Height?


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 goodenough anymore?

• Approximate level of geoid mismatch known to exist in the NAVD 88 zero surface:


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 …


• 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


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



• 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



• 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



• 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



• 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 territories2010 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 (2008-2018)

• Target: 2 cm accuracy orthometric heights from GNSS and a geoid model


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”


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


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


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


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


Why GRAV-D?


Ship gravity

Terrestrial gravity

New Orleans

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 thecountry.

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


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



• 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



• 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



Alaska- Anchorage

Anchorage

• Survey flown out of Anchorage, AK over NOAA’s Hydropalooza Area in July, 2008

• 400km x 500km region coveredin ~100 flight hours


• 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



Puerto Rico and the Virgin Islands• Flown in January, 2009 in

100 flight hours

• Completes our second test survey for the GRAV-D plan



NGA fundedNRL plane

USACE fundedAs of

Nov 2009:6.7% of planned areashave been flown


New Instruments for GRAV-D

• A-10 absolute gravimeter from Micro-g Lacoste

• Honeywell H-765 ACE Inertial Navigation System


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 Survey2009

AK08

FY10

FY10

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


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



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


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



• 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



• 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.”


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

• Contacts:

• Always begin with your state advisor:– http://www.ngs.noaa.gov/ADVISORS/AdvisorsIndex.shtml

• NGS Information Center– Phone: (301) 713-3242– E-mail: [email protected]


Additional Information

http://www.ngs.noaa.gov/ADVISORS/AdvisorsIndex.shtml

Questions

Renee Shields

Height Modernization ManagerN/NGS1, SSMC3, Room 93571315 East-West HighwaySilver Spring, MD 20910301-713-3231, [email protected]

Contact information

gravity for the redefinition of the american vertical: grav-d anchorage, alaska february 22, 2010 1...

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