Subsidence Monitoring and the GRAV-D project Dru Smith, Dan Roman, Daniel Winester, Mark Eckl NOAA’s National Geodetic Survey Subsidence Workshop -

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Presentation transcript:

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 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

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

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

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

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

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

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 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

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)