The GRAV-D Project

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The GRAV-D Project and The Future of NAD 83 and
NAVD 88A briefing for FEMA leadership Dru
Smith, Chief Geodesist NOAAs National Geodetic
Survey
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Outline

• FEMA, NGS and datums
• GRAV-D
• Current status
• Future

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FEMA and NGS

• Long history of collaboration and cooperation
• NGS provides to FEMA the datums for their maps
• NAD 83 Latitude / Longitude
• NAVD 88 Heights
• The datums are going to change soon
• Were from the government and were here to help

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NFIP Elevation Certificates
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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!

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FEMA Land Surface ElevationAccuracy Standards
Two-foot contour interval equivalent for flat
terrain (vertical accuracy 1.2 feet at the 95
percent confidence level). Thus, for flat
coastal areas, the allowable standard deviation
of elevations is 19 cm But this is within
the datum. What if the datum realization points
(benchmarks) themselves are wrong? What if they
move without being checked?
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Subsidence
Benchmarks set in the ground move with the
ground Finding an old benchmark might be worse
than not finding it, if the known (published)
elevation hasnt been updated. San Joaquin
Valley subsidence 1925 ground location 1955
ground location 1977 ground location -18 cm per
year! (See also LA Times, Sept 13, 2009)
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Leveling to define, maintain or use NAVD 88

• Leveling
• 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

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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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2007 National Research Council Report
FEMAs requirements for floodplain mapping
would be met in all areas by elevation data with
1-ft to 2-ft equivalent contour accuracy, and
that a national lidar program called Elevation
for the Nation should be carried out to create a
new national DEM But lidar only yields
ellipsoid heights, not orthometric heights!
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2009 National Research Council Report
uniform national standards for FEMA flood maps
cannot be met until an improved orthometric
height datum and geoid model exist
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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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Replacing NAVD 88 using GRAV-D

• NGS recently released the GRAV-D plan as its
  official policy
• GRAV-D is an ambitious new plan for NGS to
  re-define the vertical datum (NAVD 88)
• Tremendous Cost/Benefit ratio
• NGS has begun executing GRAV-D
• Working with other federal partners

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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 2018
• Part of the NGS 10 year plan (2008-2018)
• Target 2 cm accuracy orthometric heights from
  GPS and a geoid model

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NGS 10 year plan

• Official NGS policy as of Jan 2008
• By 2018, without reliance on passive geodetic
  marks
• Replace NAVD 88 with a GPS/geoid datum
• Replace NAD 83 with a geocentric GPS based datum

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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)
• No need to use leveling to bring in the datum

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Socio-Economic Benefits
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.
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1991 FEMA Sea Level Rise Study
The total cost of restudies and remapping of
floodplains was estimated at 30 million (in
1991) Because of the uncertainties in the
projections (of Sea Level Rise) FEMA concluded
that there are no immediate program changes
needed. However continue to monitor
progress in the scientific community in the
near termimplementation of measures that would
reduce the impact of relative rise in sea level
along the Louisiana coast
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2009 Sea Level Rise Study
FEMA intends to assess the integrity of the
flood hazard data by reviewing the flood map
inventory every five years. Where the review
indicates the flood data integrity has degraded
the flood maps (due to outdated data and known
changes in hydrology and floodplain elevation
since the last maps were issued), updates will be
provided or new studies will be performed
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GRAV-D status
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GRAV-D thus far
NGA funded NRL plane
USACE funded
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(No Transcript)
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GRAV-D future

• GRAV-D requires 6M / year to be complete by 2018
• FY2010 likely partial funding (3M)
• GRAV-D will continue, even if extra funding isnt
  provided (50 years out of NGS standard budget)
• Seeking collaboration with 7 other federal
  agencies with a stake in GRAV-D
• Need to work on transitioning to a benchmark-free
  customer base (including FEMA)

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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. Dru Smith
• Chief Geodesist, National Geodetic Survey
• Dru.Smith_at_noaa.gov
• 301-713-3222 x 144
• http//www.ngs.noaa.gov/GRAV-D/