1. GRAV-D Gravity for the Re-definition of the American Vertical Datum

2. Why this briefing? NGS recently acquired a new airborne gravimeter
Intended to support GRAV-D
GRAV-D is an ambitious new plan for NGS to accomplish part of its mission
Up front costs
Long-term savings
A huge contribution to GEOSS
Part of the NGS, NOS and NOAA missions

3. Q: What is GRAV-D? A: Gravity to determine heights accurately
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

4. Dominant Height Systems in use in the USA
Requires
Gravity
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!
More on this later…
Requires
Gravity

5. Heights Most heights need to refer to some “reference surface”
The reference surface is part of the “vertical datum”

6. Orthometric Height (H)
The distance along the plumb line from the geoid up to the point of interest
Earth’s
Surface H
The Geoid
(Shape and location
determined by Earth’s
gravity field)

7. Ellipsoid Height (h)
The distance along the ellipsoidal normal from some ellipsoid up to the point of interest
Earth’s
Surface h
Another Ellipsoid
(Shape and location
are mathematical abstractions) h
An Ellipsoid
(Shape and location
are mathematical abstractions)

8. H ≈ h-N Geoid Undulation (N)
The distance along the ellipsoidal normal from some ellipsoid up to the geoid
H ≈ h-N h H
The Geoid N
A chosen Ellipsoid

9. Vertical Datums in the USA
Ellipsoid heights: NAD 83
Orthometric heights: NAVD 88

10. 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!

11. Leveling to define and use NAVD 88
Measure geometric changes point to point
Correct for multiple physical effects
Attempts to yield differential geopotential (energy) levels
Convert from geopotential to dynamic height or orthometric height
Very time consuming and tedious

12. Why isn’t NAVD 88 good enough anymore?
The GPS era brought fast, accurate ellipsoid heights – naturally this drove a desire for fast, accurate orthometric heights
Leveling the country can not be done again
Too costly in time and money
Leveling yields cross-country error build-up
Leveling requires leaving behind marks
Bulldozers and crustal motion do their worst
NAVD 88 H=0 level is known not to be the geoid
Biases , Tilts

13. Orthometric Height (H)
The distance along the plumb line from the geoid up to the point of interest
Earth’s
Surface
H (NAVD 88) H
NAVD 88 reference level
The Geoid
Errors in NAVD 88 : ~50 cm ave,
100 cm CONUS tilt,
1-2 meters ave in Alaska
NO tracking

14. Fast, Accurate Orthometric Heights
GPS already gives fast accurate ellipsoid heights
If the geoid were determined to highest accuracy…
Voila… Fast, accurate orthometric heights
Anywhere in the nation
Time-changes to H determined through:
GPS on CORS (h changes)
Absolute gravity spot checks (N changes)

15. Intermission…
To get fast, accurate, inexpensive orthometric heights, all pieces are in place except one:
A well known, and time-tracked geoid model
Thus “GRAV-D”
Gravity for the Re-definition of the American Vertical Datum
Get good gravity once (airborne gravity)
Track gravity on a broad scale over time (absolute gravity)
Replace NAVD 88 with a sustainable, accurate, inexpensive vertical datum
Up front cost is “high”, but pay off is sustainability at low-cost!

16. Q: What is GRAV-D? A: A Plan (released Dec 2007)
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 2017

17. Mission of NGS
To define, maintain and provide access to the National Spatial Reference System to meet our nation’s economic, social, and environmental needs
And
To be a world leader in geospatial activities, including the development and promotion of standards, specifications, and guidelines.

18. Missions and GRAV-D
NGS can neither fulfill their mission, nor contribute to the NOS and NOAA missions without modernizing the vertical datum component of the NSRS
Only GRAV-D offers a sustainable, accurate method for doing this

19. GRAV-D National Scale has 2 parts:
High Resolution Snapshot (Campaign I)
Low Resolution Movie (Campaign II)
Local/Regional Scale has 1 part:
High Resolution Movie (Campaign III)

20. GRAV-D National High Resolution Snapshot
Predominantly through airborne gravity
With Absolute Gravity for ties and checks
Relative Gravity for expanding local regions where airborne shows significant mismatch with existing terrestrial

21. 20-100 km gravity gaps along coast
Terrestrial gravity
New Orleans
Ship gravity

22. GRAV-D: Campaign I Phase I, Part 1 - Testing
Test varieties of flight heights / speeds / spacings for optimal ratio of
$$ : g-accuracy

23. GRAV-D: Campaign I Phase I, Part 2 - Testing
Test area for proof of concept to define vertical datum from GPS + gravimetric geoid
PR/VI cost would run ~$300k

24. GRAV-D: Campaign I Phase II - Operations
In a carefully chosen order…

40. GRAV-D: Campaign II “Monitor Gravity over time”
National Low Resolution “Movie” of gravity
Epochal (annual?) absolute gravity re-measurements at key areas
GRACE / GRACE-follow-on for more global signatures

41. GRAV-D: Campaign II 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.

42. GRAV-D: Campaign III (Local/Regional Surveys)
Regional High Resolution Movie
Certain regions may require repetitive surveys to model the diversity of mass changes occurring locally
Can not be done with NGS personnel as is
Survey Types:
Gravity (absolute + relative)
Co-located GPS+Leveling in space/time
Minimally constrained

43. GRAV-D
Airborne
Critically needed as a one-time high resolution “snapshot” of gravity in the USA
As opposed to the thousands of surveys, with hundreds of instruments and operators over dozens of years
One time survey
Absolute
Cyclical for episodic checks in fixed locales
Co-incident with foundation CORS?
Two field meters plus one fixed SG
Relative
More frequently attached to “Height Mod” surveys

44. GRAV-D Status
2007 Feb 12: NGS Releases Draft of 10 year plan (first public reference to re-definition of vertical datum redefinition)
2007 Apr 10-11: NGS Holds Gravity Seminar at Corbin
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 Airborne Gravimeter Operators
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 Gulf Coast
Coming soon:
2008 March: NGS seeking MOU’s with Federal, State and Academic partners to assist in funding and executing GRAV-D

45. Should anyone lose sight of why NGS cares about knowing and maintaining accurate heights…

46. Questions/Comments? http://www.ngs.noaa.gov/GRAV-D/ Dr. Dru Smith
Chief Geodesist, National Geodetic Survey
x 144

47. Extra Slides

48. 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

49. Next Steps
NGS has released the GRAV-D plan as its official way forward to fulfill the orthometric height portion of its mission
Airborne “snapshot”
10 years
$38.5M (PPBES , MOUs, ???)
Absolute “movie” (time tracking)
$200k (base)
This is the annual cost of sustaining fast, inexpensive, cm-accurate orthometric heights in a dynamic USA from GPS and a geoid model

50. Mission of NOS
To provide products, services, and information that promote safe navigation, support coastal communities, sustain marine ecosystems, and mitigate coastal hazards.

51. Mission of NOAA
To understand and predict changes in the Earth’s environment and conserve and manage coastal and marine resources to meet our nation’s economic, social and environmental needs