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Comparisons of GNSS and Leveling-Derived Orthometric Heights Using GIS 

Comparisons of GNSS and Leveling-Derived Orthometric Heights Using GIS . Kevin M. Kelly, Esri Jay Satalich, Caltrans.

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Comparisons of GNSS and Leveling-Derived Orthometric Heights Using GIS 

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  1. Comparisons of GNSS and Leveling-Derived Orthometric Heights Using GIS Kevin M. Kelly, Esri Jay Satalich, Caltrans Kelly, K. M. and J. Satalich (2012). Comparisons of GNSS and leveling-derived orthometric heights using geographic information system software, AGU Fall Meeting, San Francisco, December 3-7, 2012, Abstract G13A-0940.

  2. Outline • NGS geoids • Problem statement • Data and results • Conclusions

  3. NGS Geoids • Six models since 1990 • GEOIDYY: YY = [90], [93], 96, 99, 03, {06}, 09, 12A • Hybrid geoids 96 through12A • intended to recover NAVD88 H from specific realizations of NAD83(YY) h • Corrector surface to the corresponding gravimetric geoid, USGGYYYY • This surface biases gravimetric geoid to fit NAD83(YY) ellipsoidal heights, h surveyed on NAVD88 benchmarks (GPSBM)

  4. Problem Statement Problem? In the Palos Verdes Peninsula of Southern California, discrepancies of up to 8 cm persist between published NAVD88 heights and those derived from GPS on benchmarks using the last four geoid models: GEOID99, GEOID03, GEOID09 and GEOID12A. Benefits? NAVD88 usage will continue even after the new datums. Geoid modeling and data quality implications/improvements for forthcoming models. Solutions for other problematic local areas. How well do NGS hybrid geoids perform locally? So What? Local accuracy is the workhorse of the geoid model; local projects exact high demands from geoid model performance. Solution Research and test potential causes Collect new gravity data to help verify existing holdings – GRAV-D Densify GPSBM data? Improve data screening and quality control

  5. Environmental Setting Key geophysical features influencing benchmark heights near the Palos Verdes Peninsula, California. • Palos Verdes Peninsula • Made of sandstone and basalt • Active landslides • Historic landslide areas • Los Angeles Basin • Made of alluvium • Wilmington Anticline • Wilmington/Torrance oil fields

  6. The Data Story • Steady improvement from GEOID99 to GEOID12A but ~8 cm discrepancies persist • Hybrid geoid surface lies consistently above the surface defined by NAVD88 • REDONDO – monument disturbance from construction • T1217, T1053 – 4 cm elevation differences between 1960 and 1994 • T1217 – ellipsoid height stable to 1 cm between 1997 and 2012 • The main purpose of this study is to show that these disagreements are not due to shortcomings in the control data – that is, the leveling and GPS.

  7. Benchmark stability over time (1) New-minus-old comparison of geodetic leveling shown as a component of error propagation ± numbers are a posteriori standard error for 1 km of single-run leveling in mm L25468/14 (1994) minus L17850 (1960) (±1.17 ⁄ ±2.31) New - Old: ±2.59 mm Error propagation: Error propagation:

  8. Benchmark stability over time (2) New-minus-old comparison of geodetic leveling shown as a component of error propagation ± numbers are a posteriori standard error for 1 km of single-run leveling in mm L25468/14 (1994) minus L24301/1 (1978) (±1.17 ⁄ ±0.93) New - Old: ±1.49 mm Error propagation:

  9. Benchmark stability over time (3) New-minus-old comparison of geodetic leveling shown as a component of error propagation ± numbers are a posteriori standard error for 1 km of single-run leveling in mm L25468/14 (1994) minus L25180 (1989) (±1.17 ⁄ ±0.99) New - Old: ±1.53 mm Error propagation:

  10. Benchmark Stability - Tabular New-minus-old comparison of orthometric heights in meters. Benchmarks shown with an asterisk indicate historic vertical movement beyond 2 cm between 1994 and 1960.

  11. GPS Ellipsoidal Height Repeatability New-minus-old comparison of ellipsoidal heights in meters

  12. Conclusions • Discrepancies of up to -8 cm persist between published NAVD88 heights and GPSBMfor all NGS hybrid geoids from 1999 to 2012 • Ellipsoidal heights exhibit repeatability at the 1 cm level over four epochs spanning 20 years • Differential leveling over four epochs spanning 34 years reveals that benchmarks are stable at ±2 cm or better level • The data support an apparent shortcoming in NGS geoid models in the Palos Verdes area

  13. A Real Possibility Fig. 12 The effect of significant gravity biases on the geoid Saleh, J., X. Li, Y.M. Wang, D.R. Roman and D.A. Smith. (2013). Error analysis of the NGS’ surface gravity database, J Geod 87:203-221.

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