GRAV-D Project A Height Mod. Project September 9, 2009 Fairbanks, Alaska September 9, 2009 Mark C. Eckl (Presenter – Chief, Observation and Analysis Division) Dr. Dru Smith (Chief Geodesist) Dr. Vicki Childers (GRAV-D Project Manager) Dr. Dan Roman (GRAV-D Principal Investigator) Slide 1

Dominant Height Systems in use in the USA Orthometric –Colloquially, but incorrectly, called “height above mean sea level” –On most topographic maps –Is a >99% successful method to tell which way water will flow Ellipsoid –Almost exclusively from GPS –Won’t tell water flow / floodplains Dynamic –Directly proportional to potential energy : always tells which way water will flow –Dynamic heights are not lengths! Requires Gravity Requires Gravity Slide 2

Orthometric Height (H) The distance along the plumb line from the geoid up to the point of interest 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 Slide 3

The only “technical slide”… Hmm..I wonder if my yard will flood next time we have a hurricane… …well, to know if I’m in a floodplain, I guess my “height above sea level” is needed… …wait a minute… height “above sea level”??? There’s no “sea” below me!! …oh yeah…NGS taught me that the correct term is “orthometric height”… …which is the height above “the geoid”…which is a global gravity surface related to sea level… ???????????????? …except it extends underneath land masses, unlike “sea level”! Orthometric Height The Geoid Slide 4

So…how do I figure my orthometric height… I know that “leveling” is one way…sort of… …but that means finding a “benchmark”…and walking with a survey crew for miles and miles…and even then, I’ll only get an NAVD 88 height… …which has decimeter to meter errors at this time… The Geoid NAVD 88 reference level NAVD 88 Height Errors in NAVD 88 : ~50 cm ave, 100 cm CONUS tilt, 1-2 meters ave in Alaska NO tracking Orthometric Height Slide 5

The Geoid Maybe GPS will help? Oh wait, GPS just gives me an “ellipsoid height” which doesn’t tell me anything about my orthometric height. Orthometric Height Ellipsoid Height Some simple ellipsoid model used by GPS receivers If only I knew the geoid/ellipsoid separation really accurately… …I guess that’s why NGS is proposing GRAV-D, so they can model the gravity-based geoid/ellipsoid separation… and I can get my orthometric height quickly and accurately from GPS! Slide 6

Ellipsoids and Geoids GRACE Satellite Gravity GPS Reference Ellipsoid Slide 7

Vertical Datum – History (Orthometric Heights) 1807 – 1996 –Defined and Accessed – Leveling/Passive Marks –NAVD 88: 600,000+ Marks NGS detects hundreds moved/destroyed every year How many go undetected? –Post-Glacial-Rebound, Subsidence, Tectonics, Frost-Heave – lots of motion out there! Slide 8

Could NAVD 88 Be Fixed by Leveling? Measure geometric changes point to point by “leveling” to the gravity field Very time consuming and tedious Too difficult and expensive to re-level –Estimated cost is $2.25 Billion Slide 9

Gravity is used to generate a geoid height model, which is essentially a transformation tool for heights This model should be as accurate as GPS (2 cm; ~1 in) Develops a consistent model for the entire nation that meets all needs NGS cannot fulfill their mission without modernizing the vertical datum component of the NSRS GRAV-D Gravity for the Redefinition of the American Vertical Datum Slide 10

Official NGS policy as of Nov 14, 2007 –$38.5M over 10 years Airborne Gravity Snapshot of entire US and its holdings Absolute Gravity Tracking of regions of rapid change Re-define the Vertical Datum of the USA in 10 years Q: What is GRAV-D? A: A Plan (released Dec 2007) Slide 11

National Scale has 2 parts: –High Resolution Snapshot Most of the costs arises here Most efficient means is an airborne survey Spans land and water Helps validate existing 2 million surface data –Low Resolution Movie Detects and models long term trends Example: Glacial Isostatic Adjustment An ongoing effort that will continue indefinitely Local/Regional Scale has 1 part: –High Resolution Movie –Accounts for such things as Louisiana subsidence –In collaboration with state/local/university partners Scope of Work Slide 12

GRAV-D Survey Priorities Puerto Rico/Virgin Islands as final test Coastal Alaska Southern AK US Eastern Seaboard Gulf of Mexico Slide 13

Shorelines first – to include the Great Lakes region for IGLD 15 Then the remainder of the interior to strengthen ties to coasts IGLD 15 Slide 14

Coastal Alaska and Pacific Islands first The interior Alaska Slide 15

Monitor Gravity (“Low Resolution Movie”) Measure gravity at each point, annually. Model gravity changes over time. Convert to geoid changes over time. Use with tracked GPS stations (CORS) to get orthometric height changes over time. Slide 16

NOAA cares about knowing and maintaining accurate heights… Slide 17

Fast, Accurate Orthometric Heights GPS already gives fast accurate ellipsoid heights Combining GPS with the GRAV-D model results in fast, accurate orthometric heights (H = h - N) Which is what everyone wants Slide 18

Why NOAA Needs Accurate “Heights” We need accurate “orthometric” heights, or those that are based upon the gravity field –Gravity controls which way water flows GPS receivers give accurate heights but based upon a different reference frame –We must develop a model to transform from GPS heights to orthometric heights We need to establish “what is zero” with respect to both land and nautical charts –No other way to predict flooding extent Slide 19

GRAV-D is the most ambitious project within the National Height Mod Program National Height Modernization needs to repair and maintain NAVD 88 while transitioning (via GRAV-D) to a new vertical datum in 10 years All National Height Mod funds (internal or grants) should support access to accurate heights, in general, but: –With emphasis on NAVD 88 today –With emphasis on the new vertical datum in 5 years Slide 20

Climate Oceans and Marine Life Coasts High-Impact Weather Water Transportation Infrastructure Annual Guidance Memorandum Slide 21

GRAV-D and the AGM priorities Climate –GRAV-D data allow monitoring of sea-level rise including its secular changes. Because the geoid is (by definition) tied to the global ocean surface, it is both a provider and user of sea-level rise data. Slide 22

GRAV-D and the AGM priorities Ecosystem Applications –Coastal ecosystems can be highly sensitive to the most minor flooding events. –GRAV-D data will allow determination of elevations in estuaries to an accuracy of 2 cm (almost an inch) –GRAV-D data will allow more accurate determination of the small gradients in topography which affect the flow of water Slide 23

GRAV-D and the AGM priorities Coasts –Almost no gravity measurements exist near shore Thus heights are poorly modeled at the coast, and coastal heights are some of the least accurate –Building resilience to coastal flooding hazards depends on knowing heights accurately –Subsidence occurs at the coast GRAV-D monitors the geoid, so subsidence is accounted for in GPS-determined heights! –GRAV-D data allow for coastal heights to be determined accurately and for changes like subsidence to be tracked over time Slide 24

GRAV-D and the AGM priorities High Impact Weather –Many high impact weather events coincide with a flooding event GRAV-D data allow for accurate evacuation routes to be mapped GRAV-D data allow for accurate determination of floodplains Slide 25

GRAV-D and the AGM priorities Water –Discrepancies in water table depths of just a few cm (~1 inch) in Hawaii mean the difference between developing land or not – GRAV-D aids in this decision –GRAV-D data will allow efficient fertilizer & pesticide use, reducing impact of pollution from chemical runoff –GRAV-D data will aid in determining areas prone to coastal flooding –Extremely low gradients in water pipelines reduce pumping costs by letting gravity move water. This can only be economically done over large areas if GRAV-D is fully in place –Preservation and monitoring of the Great Lakes is of keen interest – GRAV-D will directly impact IGLD 15 Slide 26

GRAV-D and the AGM priorities Transportation –GRAV-D’s impact on IGLD 15 also affects shipping and dredging in the Great Lakes –GRAV-D allows for height differences between airports to be determined to 2 cm (~ 1 inch) over vast areas, including Alaska Accounting for this means greater air safety –Roads can be constructed which are less prone to flooding, by accurately knowing their heights. If the roads are evacuation routes, then building them above flood levels will ensure safety during storm events Slide 27

GRAV-D and the AGM priorities Infrastructure –NOAA is mandated (OMB Circular No. A-16) with providing the geodetic control portion of the National Spatial Data Infrastructure –GRAV-D will assist in monitoring levee subsidence… –GRAV-D will help counties will save thousands of dollars in establishing geospatial frameworks… –GRAV-D will reduce errors in FEMA floodplain insurance maps… –By providing a vertical datum that is accurate and accessible to all USA territories, GRAV-D provides one of the most crucial pieces of infrastructure at very low cost. Slide 28

Conclusions GRAV-D will reduce the errors that exist today that limit monitoring capabilities GRAV-D will provide a consistent vertical model for all U.S. regions GRAV-D will allow for monitoring of height at the 2 cm level nationwide NOAA will be the lead, but can’t do it alone Slide 29

GRAV-D: Gravity for the Redefinition of the American Vertical Datum Will redefine the vertical component of the National Spatial Reference System (NSRS). Nationwide airborne gravity collection is essential for the definition of an improved national vertical reference system Will have Profound implications for GPS elevation accuracy: national elevations with GPS to ~ 2cm compared to as much as 2 meters today Enabling at-risk communities to: plan evacuation routes model storm surge monitor sea-level rise monitor levee subsidence improve flood plain maps GRAV-D Overview Slide 30

Airborne Meter First build (Oct 2007) Initial road tests (Nov 2007) Unit is TAGS (Turn-key Airborne Gravity System) Sensor, Software and Training sold as a package from Micro-G/LaCoste Has been flight tested and proven as the most accurate airborne meter available Slide 31

The first, middle and last point of GRAV-D: Gravity and Heights are inseparably connected Or (to borrow from a common bumper sticker): –No gravity, no height –Know gravity, know height Q: What is GRAV-D? A: Gravity to determine heights accurately Slide 32

Questions/Comments? Dr. Dru Smith Chief Geodesist, National Geodetic Survey x 144 Dr. Vicki Childers GRAV-D Program Manager, National Geodetic Survey x 161 Dr. Daniel Roman GRAV-D Principal Investigator, National Geodetic Survey x Slide 33