1. Subsidence Monitoring and the GRAV-D project Dru Smith, Dan Roman, Daniel Winester, Mark Eckl NOAA’s National Geodetic Survey Subsidence Workshop - Geodetic Monitoring Techniques Texas Spatial Reference Center Texas A&M University-Corpus Christi Corpus Christi, Texas

2. Outline Geodetic implications of subsidence GRAV-D overview
GRAV-D/Campaign II and subsidence

3. From t0 to t1, subsidence occurs
Surface
Δh = h1-h0 g0
Surface
ΔH = H1-H0
=Δh-ΔN g1 H0
ΔN=N1-N0 H1 h0 h1
N0=f(g0,H0)
N1=f(g1,H1)
Geoid
Geoid N1 N0
Ellipsoid
Ellipsoid
g0 hasn’t been measured in decades:
This is the essence of
“Campaign I” of GRAV-D
g1≠g0 : This is the essence
of “Campaign II” of GRAV-D

4. H ≈ h – N E B A D C hE0 hD0 hA0 hC0 HA0 HC0 HE0 hB0 HB0 HD0 NC0 NB0
ND0
Geoid
NA0
NE0
Ellipsoid
Time: t= t0

5. Δh easily measured / monitored by GPS
Ellipsoid Does Not Change!!
Ellipsoid
Effect of Subsidence on Ellipsoid Heights

6. ΔH ≈ Δh – ΔN (wanted) (GPS) (GRAV-D) Δh Δh Δh Δh Δh
Earth’s mass distribution has changed!
So, Earth’s gravity field has changed….
ΔH ≈ Δh – ΔN
(wanted) (GPS) (GRAV-D)
Including the location and shape of the geoid…
Which means new orthometric heights rely on new locations of:
The crust and
The geoid ΔN ΔN ΔN ΔN ΔN
Geoid
Effect of Subsidence on Orthometric Heights

7. ΔH from Leveling?
Why not just re-level to monitor orthometric height changes?

8. Leveling and Gravity
“Leveling without gravity measurements, although applied in practice, is meaningless from a rigorous point of view”
Physical Geodesy
Heiskanen and Moritz

9. gE0
gE1
gB0
gC0
gD0
gA0
gD1
gB1
gC1
gA1
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 policy
toward defining and monitoring the vertical datum
Geoid
Orthometric Height Monitoring by Leveling

10. 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 2017

11. Q: What is GRAV-D? A: Gravity to determine heights accurately
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

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. 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
Almost NO tracking

14. 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)