The GRAV-D ProjectThe GRAV-D Projectandand

The Future of NAD 83 and The Future of NAD 83 and NAVD 88NAVD 88

A briefing for FEMA leadershipA briefing for FEMA leadership

Dru Smith, Chief GeodesistDru Smith, Chief GeodesistNOAA’s National Geodetic Survey NOAA’s National Geodetic Survey

Outline

• FEMA, NGS and datums

• GRAV-D

• Current status

• Future

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

NFIP Elevation Certificates

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!

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?

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 subsidence1925 ground location1955 ground location1977 ground location

-18 cm per year!

(See also LA Times, Sept 13, 2009)

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

Orthometric Height (H)• The distance along the plumb line from the geoid

up to the point of interest

H

Earth’sSurface

The Geoid

NAVD 88 reference level

H (NAVD 88)

Errors in NAVD 88 : ~50 cm ave, 100 cm CONUS tilt, 1-2 meters ave in Alaska NO tracking

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!

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…

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

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

• 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

Q: What is GRAV-D?

A: A Plan (released Dec 2007)

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

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”

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.

1991 FEMA Sea Level Rise Study

The total cost of restudies andremapping 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…”

2009 Sea Level Rise Study

“…FEMA intends to assess the integrity of the flood hazard data by reviewingthe 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…”

GRAV-D status

GRAV-D thus far

NGA fundedNRL plane

USACE funded

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)

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

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/