Outline

1. Subsidence Monitoring and the GRAV-D projectDru Smith, Dan Roman, Daniel Winester, Mark EcklNOAA’s National Geodetic SurveySubsidence Workshop - Geodetic Monitoring TechniquesTexas Spatial Reference CenterTexas A&M University-Corpus ChristiCorpus Christi, Texas

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

3. From t0 to t1, subsidence occurs T=t0 T=t1 Surface Δh = h1-h0 g0 Surface ΔH = H1-H0 =Δh-ΔN g1 H0 H1 ΔN=N1-N0 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. E B D A C H ≈ h – N hE0 hD0 hA0 hC0 HA0 HE0 HC0 hB0 HB0 HD0 NC0 ND0 NB0 Geoid NA0 NE0 Ellipsoid Time: t= t0

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

6. 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 gD0 gC0 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 Geoid Thus, NGS has adopted GRAV-D as its future policy toward defining and monitoring the vertical datum 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)