Subsidence Monitoring Subsidence Monitoring and the GRAV-D projectand the GRAV-D project

Dru Smith, Dan Roman, Daniel Winester, Mark EcklDru Smith, Dan Roman, Daniel Winester, Mark EcklNOAA’s National Geodetic SurveyNOAA’s National Geodetic Survey

Subsidence Workshop - Geodetic Monitoring TechniquesSubsidence Workshop - Geodetic Monitoring Techniques

Texas Spatial Reference CenterTexas Spatial Reference CenterTexas A&M University-Corpus ChristiTexas A&M University-Corpus Christi

Corpus Christi, TexasCorpus Christi, Texas

Outline

• Geodetic implications of subsidence

• GRAV-D overview

• GRAV-D/Campaign II and subsidence

N0

H0

h0

g0

N1

H1

h1

g1

Surface

Ellipsoid

Geoid

Surface

Geoid

Ellipsoid

T=t0 T=t1

h = h1-h0

N=N1-N0

N0=f(g0,H0)

H = H1-H0

N1=f(g1,H1)

g1≠g0 : This is the essence of “Campaign II” of GRAV-D

g0 hasn’t been measured in decades: This is the essence of “Campaign I” of GRAV-D

From t0 to t1, subsidence occurs

=h-NN

hA0

hB0

hC0

hD0

hE0

HA0 HE

0HD

0

HC0HB

0

Ellipsoid

Geoid

AE

C DB

Time: t= t0

NA0

NB0

NC0 ND

0NE

0

H ≈ h – N

Ellipsoid Does Not Change!!

h easily measured / monitored by GPS

h

hh

hh

Effect of Subsidence on Ellipsoid Heights

Ellipsoid

Earth’s mass distribution has changed!

So, Earth’s gravity field has changed….

Including the location and shape of the geoid…

Which means new orthometric heights rely on new locations of:-The crust and-The geoid

Geoid

Effect of Subsidence on Orthometric Heights

NN

NNN

h

hh

hh

H ≈ h – N(wanted) (GPS) (GRAV-D)

H from Leveling?

• Why not just re-level to monitor orthometric height changes?

Leveling and Gravity

“Leveling without gravity measurements, although applied in practice, is meaningless from a rigorous point of view”

– Physical Geodesy• Heiskanen and Moritz

Geoid

Orthometric Height Monitoring by Leveling

gA0

gE0

gD0gC

0gB0

gA1

gE1

gD1gC

1gB1

Issue #1: Find a provably “stable” point

Issue #2: Presumption that gravity hasn’t changed

Issue #3: Failure to measure gravity while leveling, in general

Issue #4: Reliance on finite point-cloud of vulnerable benchmarks

Thus, NGS has adopted GRAV-D as its future policytoward defining and monitoring the vertical datum

• 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

Q: What is GRAV-D?

A: A Plan (released Dec 2007)

• 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

Q: What is GRAV-D?

A: Gravity to determine heights accurately

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

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 levelH (NAVD 88)

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

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)

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.

Mission of NOS

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

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

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

• National Scale has 2 parts:– High Resolution Snapshot– Low Resolution Movie

• Local/Regional Scale has 1 part:– High Resolution Movie

GRAV-D

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

Ship gravity

Terrestrial gravity

New Orleans

20-100 km gravity gaps along coast

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-FO for more global signatures

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 heightchanges over time.

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

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

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 TBD: NGS seeking MOU’s with Federal, State and Academic partners to assist in funding and executing GRAV-D

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

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/

Extra Slides

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