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THE VERTICAL DATUMS OF AUSTRALIA AND NEW ZEALAND. Sten Claessens 1 Will Featherstone 1 Matt Amos 2 Mick Filmer 1 Christian Hirt 1 Jon Kirby 1 1 The Western Australian Centre for Geodesy & The Institute for Geoscience Research Curtin University of Technology

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THE VERTICAL DATUMS OF AUSTRALIA AND NEW ZEALAND


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    1. THE VERTICAL DATUMS OF AUSTRALIA AND NEW ZEALAND Sten Claessens1 Will Featherstone1 Matt Amos2 Mick Filmer1 Christian Hirt1 Jon Kirby1 1 The Western Australian Centre for Geodesy & The Institute for Geoscience Research Curtin University of Technology 2 Land Information New Zealand REFAG2010, Marne-la-Vallée, France Session 5: Definition and Establishment of Vertical Reference Systems 6 October 2010

    2. INTRODUCTION (1/2) • Australia and New Zealand share strong ties in the development of geodetic infrastructure, e.g. through the joint Intergovernmental Committee on Surveying and Mapping (ICSM) and joint research • However, both countries have recently made different choices relating to their vertical datums • New Zealand has established a new vertical datum • Australia has decided to maintain its existing vertical datum for the foreseeable future

    3. INTRODUCTION (2/2) • Recent vertical datum activities: • New Zealand Vertical Datum (NZVD) 2009 based on gravimetric NZGeoid09 released • AUSGeoid09 combined gravimetric-geometric quasigeoid established – aids transformation from ITRF to Australian Height Datum (AHD) • Experimental version of new ‘scientific’ Australian vertical datum (AVD:Ev1) created through constrained re-adjustment of Australian National Levelling Network (ANLN)

    4. NEW ZEALAND VERTICAL DATUM 2009 (1/5) • Situation before 2009: • 13 major local vertical datums (LVDs) with one tide gauge each as origin • Origins do not represent current mean sea level • Levelling data was never adjusted nationally • The new New Zealand Vertical Datum (NZVD) 2009 is based on a new quasigeoid model, NZGeoid09

    5. NEW ZEALAND VERTICAL DATUM 2009 (2/5) Iterative quasigeoid datum unification scheme • Quasigeoid compared to GNSS-levelling in each LVD • LVD offsets applied to gravity anomalies • Quasigeoid recomputed

    6. NEW ZEALAND VERTICAL DATUM 2009 (3/5) NZGeoid09 specs • Data sources: • EGM2008 • DNSC08GRA • 40,737 land gravity observations • 56 m DEM • 1422 GNSS-levelling points (m) Residual quasigeoid heights with respect to EGM2008

    7. NEW ZEALAND VERTICAL DATUM 2009 (4/5) • Offsets between local vertical datums (LVDs) were determined through the iterative quasigeoid computation process • Range of offsets between NZGeoid09 and LVDs: -0.06 m to -0.49 m • Fit to LVD offsets from precise levelling: ±0.064 m

    8. NEW ZEALAND VERTICAL DATUM 2009 (5/5) • NZGeoid09 forms the reference surface of the New Zealand Vertical Datum (NZVD) 2009 • New Zealand was one of the first countries to adopt a geoid-based national vertical datum • NZVD2009 implemented by publication of NZVD2009 heights for existing marks • NZVD2009 is actively adopted by users and appears to be exceeding many users’ expectations • LINZ is currently considering improvements to NZVD2009 through collecting airborne gravity

    9. AUSGEOID09 / AHD (1/4) • Like New Zealand, Australia has recently established a new quasigeoid model: AUSGeoid09 • AUSGeoid09 will be officially released in the coming months • The ICSM has resolved to maintain the Australian Height Datum (AHD) for the foreseeable future, despite well-documented deficiencies • AUSGeoid09 contains a geometric component to model the offset between the gravimetric-only quasigeoid and the AHD

    10. AUSGEOID09 / AHD (2/4) AUSGeoid09 specs (m) Residual quasigeoid heights with respect to EGM2008

    11. AUSGEOID09 / AHD (3/4) Differences between gravimetric-only AUSGeoid09 and GNSS-AHD heights • ~ 1 m north-south trend in AHD (~23 mm/ 100 km) • Regional distortions up to ~ 0.5 m • Shows magnitude of geometric component of AUSGeoid09 (m)

    12. AUSGEOID09 / AHD (4/4) • The geometric component of AUSGeoid09: • Computed by Nicholas Brown and colleagues at Geoscience Australia • A posteriori fitting by least-squares collocation (LSC) in cross-validation mode based on extended dataset of 6,794 AHD-GNSS points • Gaussian analytical covariance function determined empirically; noise based on STD of post-processed GNSS ellipsoidal heights • Correlation length of 75 km provided smallest RMS misfit in cross-validation of ±30 mm

    13. AUSTRALIAN VERICAL DATUM: EV1 (1/6) • Despite the ICSM decision to retain the AHD as Australia’s vertical datum for the foreseeable future, research into an improved vertical datum for scientific purposes has continued • An experimental version of a new ‘scientific’ Australian vertical datum has been computed (AVD:Ev1) • AVD:Ev1 is based on a re-adjustment of the Australian National Levelling Network (ANLN), supplemented with: • The gravimetric-only version of AUSGeoid09 • GNSS data at ANLN junction points • CARS2006 SSTop model • Tide gauge data

    14. AUSTRALIAN VERICAL DATUM: EV1 (2/6) Australian National Levelling Network (ANLN) • >100,000 km of levelling, primarily of 3rd order • Many loops >1000 km • ~8% of 1366 loops exceed closure tolerance • Largest loop misclosure 0.93 m Yellow: first order Green: second order Red: third order one-way Purple: third order Blue: unspecified Dark green: fourth order

    15. AUSTRALIAN VERICAL DATUM: EV1 (3/6) Outlier detection and re-weighting (ODRW) • ODRW based on Baarda’s data snooping, MDB and loop closures • No improvement in fit to AUSGeoid09 before and after ODRW (m) Differences between re-adjusted MC ANLN before and after outlier detection and re-weighting (ODRW)

    16. AUSTRALIAN VERICAL DATUM: EV1 (4/6) Sea Surface Topography around Australia from CARS2006 • Most of the N-S trend in the AHD can be attributed to Sea Surface Topography (SSTop) • N-S trend (mm/100km): ANLN lev Figure 8.2. Filmer (m) CARS2006 SSTop

    17. AUSTRALIAN VERICAL DATUM: EV1 (5/6) • Adjustment strategy: • CARS2006 fixed to MSL at Albany tide gauge and subsequently corrected for bias with AUSGeoid09 (0.132 m) • Tide gauge/CARS2006 data introduced in adjustment with STD of ±0.005 m • AUSGeoid09 at 268 GNSS-levelling points (selected from set of 1042) introduced in adjustment with STD of ±0.010 m • ODRW process repeated (2 AUSGeoid09 points removed as outliers; 11 added to robustify areas with multiple levelling errors) • Result validated by comparison to GNSS-levelling points not used

    18. AUSTRALIAN VERICAL DATUM: EV1 (6/6) Validation of AVD:Ev1 • No trend is discernable in differences between AUSGeoid09 and AVD:Ev1 • AVD:Ev1 provides a substantial improvement over AHD (m) Differences between AHD and AVD:Ev1

    19. SUMMARY (1/1) • Australia and New Zealand have both recently computed a new quasigeoid model • New Zealand has adopted NZGeoid09 as the reference surface of its new vertical datum, NZVD2009 • Australia has ‘warped’ the gravimetric version of AUSGeoid09 to better fit the Australian Height Datum • An experimental version of a new Australian vertical datum has been created through a combined adjustment of the ANLN, gravimetric-only AUSGeoid09, GNSS, tide gauge and SSTop data

    20. PAPERS IN PRESS (1/1) • Claessens SJ, Hirt C, Amos MJ, Featherstone WE, Kirby JF (in press) The NZGeoid09 model of New Zealand. Survey Review. • Featherstone WE, Kirby JF, Hirt C, Filmer MS, Claessens SJ, Brown NJ, Hu G, Johnston GM (in press) The AUSGeoid09 model of the Australian Height Datum. Journal of Geodesy.