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Leveling After 2022

geodesy.noaa.gov. Leveling After 2022. Dan Gillins, Ph.D., P.L.S. April 25, 2017 Geospatial Summit 2017. Differential Leveling. Rod 1. Setup of Leveling, Δ n = B – F. Rod 2. Foresight. Backsight. F. B. Δ n. S B. S F. S. Differential Leveling. Advantages:

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Leveling After 2022

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  1. geodesy.noaa.gov Leveling After 2022 Dan Gillins, Ph.D., P.L.S. April 25, 2017 Geospatial Summit 2017

  2. Differential Leveling Rod 1 Setup of Leveling, Δn = B – F Rod 2 Foresight Backsight F B Δn SB SF S

  3. Differential Leveling Advantages: • Leveling is used to determine differences in heights between marks • Highly accurate (i.e., mm to sub-mm) height differences can be obtained in less than one km Challenges: • A tedious process • Line-of-sight technology, restricted to 50-100 m sight lengths; prone to blunders • Requires starting from a mark with a “known” height (i.e., vertical control point or bench mark)

  4. Heights on Bench Marks • NGS publishes orthometric heights on bench marks referenced to NAVD 88 • NAVD 88: • Realized by over 700,000 km of leveling lines • Over 500,000 monumented bench marks • Minimally Constrained adjustment, holding fixed the Father Point/Rimouski tide station in Canada

  5. NAVD 88 Courtesy Brian Shaw

  6. Challenges with NAVD 88 • Bench marks may not be in convenient locations or near a project area • Must assume the bench marks have not moved, and their published heights are without error • Bench marks are often destroyed and rarely replaced • Sparse number of bench marks outside of CONUS • Heights for the bench marks were derived from a single point, allowing error to build up across the country

  7. Bias and Tilt of NAVD 88

  8. NAPGD2022 • The North American-Pacific Geopotential Datum of 2022 (NAPGD2022) will replace NAVD 88 • Will not be realized by leveling between passive marks • Accessed by GNSS observations (referenced to NATRF2022) and a high-accuracy gravimetric geoid model (GEOID2022)

  9. GNSS-Derived Orthometric Heights H ≈ h - N HNAPGD2022 = hNATRF2022 – NGEOID2022 h H Ellipsoid N Geoid Ocean

  10. Advantages of NAPGD2022 • Easily accessed by making GNSS observations • Establish vertical control in convenient locations • Level from this control to other stations in your survey • Possible to derive orthometric heights on marks at the time of your survey • Enables monitoring orthometric heights on marks

  11. Estimated Accuracy of GNSS-Derived Height Differences h2 h1 N2 Ellipsoid N1 Geoid Ocean

  12. Accuracy of Leveling

  13. Principles of Least Squares Two Components of a least squares adjustment: • Mathematical Model • Stochastic Model • How to weight your observations and constraints • Weights inversely proportional to variances

  14. Case Study: GSVS11 Austin • 325 km line from Austin to Rockport, TX • Static GPS collected on 218 stations (48-h sessions) • First-order Class 2 geodetic leveling • Surface gravimetry Rockport

  15. Preliminary Adjustments of GSVS11 Austin • Free adjustment of leveling observations: • ΔH = 158.454 m • σH,end = ±0.014 m • Simply differencing GNSS-derived orthometric heights at each end of line: • ΔH = 158.398 m • σH,ends ≈ ±0.04 m Rockport

  16. GNSS+Leveling Adjustments ΔH H2 ΔH H1 Geoid

  17. GNSS+Leveling Adjustments ΔH H2 ΔH H1 Geoid

  18. GNSS+Leveling Adjustments ΔHadj H2,adj ΔHadj Hf He Hd Hc H1,adj Hb Ha Geoid

  19. Test Adjustment Results

  20. Test Adjustment Results

  21. Additional Tests

  22. Conclusions • GNSS and a high-accuracy geoid model connects network to NAPGD2022 • (network accuracy) • Leveling improves accuracy of height differences between marks • (local accuracy) • Addition of leveling with GNSS increases overall redundancy in a survey network

  23. Ongoing Research and Development • Develop models to combine and adjust GNSS-derived heights and/or observations with leveling • More tests with other GNSS+leveling projects • Add a new section to the NOS NGS Manual 3 on “Geodetic Leveling” • Provide guidance for the necessary spacing of vertical control from GNSS • Update FGCS specifications for leveling • Develop software applications and tools

  24. OPUS-Projects for GNSS & Leveling 24

  25. OPUS-Projects for GNSS & Leveling 25

  26. Questions? Dan Gillins Daniel.Gillins@noaa.gov

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