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

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GRAV-DGravity for the Re-definition of the
American Vertical Datum

• Abridged Version for the ACSM/LSAW Conference
• Spokane, WA 04-09 MAR 2008
• Daniel R. Roman, Ph.D. Research
  Geodesist

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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

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Geoid Undulation or Height (N)

• The distance along the ellipsoidal normal from
  some ellipsoid up to the geoid

h
H h-N
H
The Geoid
NAVD 88
N
GEOID03/GEOID06
NAD 83
A chosen Ellipsoid
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Why isnt 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 H0 level is known not to be the geoid
• Biases , Tilts

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Orthometric Height (H)

• The distance along the plumb line from the geoid
  up to the point of interest

Earths 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
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GGM02S (GRACE)/GPS vs. NAVD 88
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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)

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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!

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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

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GRAV-D

• National Scale has 2 parts
• High Resolution Snapshot
• Low Resolution Movie
• Local/Regional Scale has 1 part
• High Resolution Movie

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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

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20-100 km gravity gaps along coast
Terrestrial gravity
New Orleans
Ship gravity
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GRAV-D Campaign IPhase I, Part 1 - Testing

• Test varieties of flight heights / speeds /
  spacings for optimal ratio of
• g-accuracy
• Data have been collected and are being analyzed

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GRAV-D Campaign IPhase I, Part 2 - Testing

• Test area for proof of concept to define vertical
  datum from GPS gravimetric geoid
• PR/VI cost would run 300k
• Likely deferred until Fall to allow analysis

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GRAV-D Campaign IPhase II - Operations

• In a carefully chosen order

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Some of the first Alaska work may start this
Summer depending on funding. This will likely be
piggybacked onto a planned mapping project in and
around the Anchorage/Kenai area.
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GRAV-D Campaign IIMonitor Gravity over time

• National Low Resolution Movie of gravity
• Epochal (annual?) absolute gravity
  re-measurements at key areas
• GOCE for more global signatures

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GRAV-D Campaign IIMonitor 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.
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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 GPSLeveling in space/time
• Minimally constrained

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Residual Gravity and Implied Residual Geoid for
Gravity-Lidar 2006 Study
Absolute and relative gravity ground survey in
progress right now.
Approximate region of GRAV-D training and
analysis flights
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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

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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 18-Feb 11 Initial Flights of Airborne
  Gravimeter in Gulf Coast
• Coming soon
• 2008 TBD NGS seeking MOUs with Federal, State
  and Academic partners to assist in funding and
  executing GRAV-D
• 2008 Summer Proof of Concept flights in support
  of IOCM in Kenai, Alaska
• 2008 Fall Proof of Concept flights in PR/VI
  region for a gravimetric geoid datum

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Airborne Meter
Initial road tests (Nov 2007)
First build (Oct 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

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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

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Should anyone lose sight of why NGS cares about
knowing and maintaining accurate heights
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Questions/Comments?

• Dr. Daniel R. Roman
• Research Geodesist, National Geodetic Survey
• Dan.Roman_at_noaa.gov
• 301-713-3202 x 161
• http//www.ngs.noaa.gov/GRAV-D/