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The GRAV-D Project and The Future of NAD 83 and NAVD 88 A briefing for FEMA leadership

The GRAV-D Project and The Future of NAD 83 and NAVD 88 A briefing for FEMA leadership Dru Smith, Chief Geodesist NOAA’s National Geodetic Survey. Outline. FEMA, NGS and datums GRAV-D Current status Future. FEMA and NGS. Long history of collaboration and cooperation

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The GRAV-D Project and The Future of NAD 83 and NAVD 88 A briefing for FEMA leadership

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  1. The GRAV-D Project and The Future of NAD 83 and NAVD 88A briefing for FEMA leadership Dru Smith, Chief Geodesist NOAA’s National Geodetic Survey

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

  3. 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” • We’re from the government and we’re here to help

  4. NFIP Elevation Certificates

  5. 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!

  6. 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?

  7. 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 hasn’t 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)

  8. 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

  9. 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

  10. 2007 National Research Council Report “…FEMA’s 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!

  11. 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…

  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. 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

  14. 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

  15. 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

  16. 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”

  17. 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.

  18. 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 term…implementation of measures that would reduce the impact of relative rise in sea level along the Louisiana coast…”

  19. 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…”

  20. GRAV-D status

  21. GRAV-D thus far NGA funded NRL plane USACE funded

  22. 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 isn’t 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)

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

  24. Questions/Comments? • Dr. Dru Smith • Chief Geodesist, National Geodetic Survey • Dru.Smith@noaa.gov • 301-713-3222 x 144 • http://www.ngs.noaa.gov/GRAV-D/

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