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

Δh = h1-h0

ΔN=N1-N0

N0=f(g0,H0)

ΔH = 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

=Δh-ΔΔNN

hA0

hB0

hC0

hD0

hE0

HA0 HE

0

HD0

HC0

HB0

Ellipsoid

Geoid

A

E

CD

B

Time: t= t0

NA0

NB0

NC0 ND

0

NE0

H ≈ h – N

Ellipsoid Does Not Change!!

Δh easily measured / monitored by GPS

Δh

Δh

Δh

ΔhΔh

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

ΔN

ΔN

ΔNΔN

ΔN

Δh

Δh

Δh

ΔhΔh

ΔH ≈ Δh – ΔN(wanted) (GPS) (GRAV-D)

ΔH 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

0gB

0

gA1

gE1

gD1gC

1gB

1

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 level

H (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)