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.

Outline

• NGS geoids

• Problem statement

• Data and results

• Conclusions

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)

Problem Statement

How well do NGS hybrid geoids

perform locally?

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.

So What?Local accuracy is the

workhorse of the geoid model; local projects

exact high demands from geoid model performance.

SolutionResearch and test potential causes

Collect new gravity data to help verify existing holdings – GRAV-D

Densify GPSBM data?

Improve data screening and quality control

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.

Environmental Setting

Key geophysical features influencing benchmark heights near the Palos Verdes Peninsula, California.

• Palos Verdes Peninsulao Made of sandstone and

basalto Active landslideso Historic landslide areas

• Los Angeles Basino Made of alluviumo Wilmington Anticlineo Wilmington/Torrance oil

fields

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.

Benchmark stability over time (1)

L25468/14 (1994) minus L17850 (1960)(±1.17 ⁄ ±2.31)

New - Old: ±2.59 mm

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

2 2New Old New Old

Error propagation:

Error propagation:

Benchmark stability over time (2)

L25468/14 (1994) minus L24301/1 (1978)(±1.17 ⁄ ±0.93)

New - Old: ±1.49 mm

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

2 2New Old New Old

Error propagation:

Benchmark stability over time (3)

L25468/14 (1994) minus L25180 (1989)(±1.17 ⁄ ±0.99)

New - Old: ±1.53 mm

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

2 2New Old New Old

Error propagation:

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.

BENCHMARK PIDRUNNING DISTANCE

(km)

1994 minus 1989

1994 minus 1978

1994 minus 1960

941 0660 TIDAL 8 DY1083 0.00 FIXED FIXED FIXEDZ 1217 DY1118 8.95 0.00358 0.00138 noneT 1053* DY0994 13.30 0.01375 0.00814 0.04157Y 1312 DY2499 18.38 0.00912 0.00541 noneS 1053 DY1017 19.89 0.00432 0.00495 0.00038U 1217 DY1227 21.94 0.00208 -0.00321 noneE 169 DY1230 23.97 0.00421 -0.01158 0.00857T 1217* DY1231 24.66 -0.00595 -0.02714 none34 33 A DY9050 24.77 0.00115 -0.00636 noneT 1313 DY2498 24.97 0.00330 -0.01071 noneF 788 DY1233 25.54 -0.00270 -0.00917 -0.00741S 1217 DY1237 27.01 0.00368 -0.00131 none34 39 DY9045 29.08 -0.00103 -0.01846 noneTORRANCE F 10 RM 1* DY1217 30.12 -0.00447 -0.03458 -0.05578REDONDO* DY1212 31.04 -0.00178 -0.11205 -0.19149

GPS Ellipsoidal Height Repeatability

New-minus-old comparison of ellipsoidal heights in meters

GNSS STATION2012

minus 2011

2012 minus 1997

2012 minus 1994

2012 minus 1992

PALOS VERDES ARIES 7268 FIXED FIXED FIXED FIXED941 0660 TIDAL 8 0.007 none 0.014 0.012Y 1312 none none -0.003 noneS 1053 none 0.000 none noneU 1217 none 0.004 none noneT 1217 none 0.010 none noneT 1313 none 0.004 0.008 noneF 788 none -0.006 none none34 39 none 0.009 none noneTORRANCE F 10 RM 1 none -0.002 0.003 noneREDONDO* none 0.036 none none

Conclusions

• Discrepancies of up to -8 cm persist between published NAVD88 heights and GPSBM for 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

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.