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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. Outline. - PowerPoint PPT PresentationTRANSCRIPT
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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
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Outline
• Geodetic implications of subsidence
• GRAV-D overview
• GRAV-D/Campaign II and subsidence
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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
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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
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Ellipsoid Does Not Change!!
Δh easily measured / monitored by GPS
Δh
Δh
Δh
ΔhΔh
Effect of Subsidence on Ellipsoid Heights
Ellipsoid
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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)
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ΔH from Leveling?
• Why not just re-level to monitor orthometric height changes?
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Leveling and Gravity
“Leveling without gravity measurements, although applied in practice, is meaningless from a rigorous point of view”
– Physical Geodesy• Heiskanen and Moritz
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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
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• 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)
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• 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
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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
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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
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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)