Transcript ΔH = H 1 - Geodesy at Texas A&M

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
• Geodetic implications of subsidence
• GRAV-D overview
• GRAV-D/Campaign II and subsidence
From t0 to t1, subsidence occurs
T=t0
T=t1
Surface
g0
h = h1-h0
Surface
H = H1-H0
=h-N
H0
N=N1-N0
h0
g1
H1
h1
N0=f(g0,H0)
N1=f(g1,H1)
Geoid
N0
Geoid
N1
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
E
B
A
D
C
H≈h–N
HA
hA0
0
NA
hC0
hB0
0
hE0
hD0
HC0
HB0
NB0
NC0
HD
ND0
HE0
0
NE0
Geoid
Ellipsoid
Time: t= t0
h easily measured / monitored by GPS
h
h
h
h
h
Ellipsoid Does Not Change!!
Effect of Subsidence on Ellipsoid Heights
Ellipsoid
h
h
h
h
h
Earth’s mass distribution has changed!
So, Earth’s gravity field has changed….
H ≈ h – N
Including the location and shape of the geoid…
(wanted)
(GPS)
(GRAV-D)
Which means new orthometric heights rely on new locations of:
-The crust and
-The geoid
N
N
N
N
N
Geoid
Effect of Subsidence on Orthometric Heights
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
gE1
gA 0
gA 1
gB0
gB1
gC0
gC1
gE0
gD0
gD1
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
Orthometric Height Monitoring by Leveling
Geoid
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
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
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
Earth’s
Surface
H (NAVD 88)
H
The Geoid
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)