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Integration of Future Geoid Models

Integration of Future Geoid Models. Dan Roman and Yan M. Wang NOAA/NGS Silver Spring, MD 20910 USA December 3-4, 2008. Overview. Introduction History of geoid modeling at NGS Tentative date of publication of GEOID09 Plan and specification of future of geoid models

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Integration of Future Geoid Models

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  1. Integration of Future Geoid Models Dan Roman and Yan M. Wang NOAA/NGS Silver Spring, MD 20910 USA December 3-4, 2008

  2. Overview • Introduction • History of geoid modeling at NGS • Tentative date of publication of GEOID09 • Plan and specification of future of geoid models • Interaction between the users community and NGS

  3. What is a geoid • An equilpotential surface which coincides with an idealized mean sea level (mean sea surface minus permanent ocean dynamic topography) • The reference ellipsoid (e.g. GRS80, or WGS84) is an approximation of the geoid • The geoid height is the vertical distance between geoid and reference ellipsoid

  4. Why we need it • Conversion surface between NAVD88 (orthometric) and GPS (ellipsoidal) heights • Defines next national vertical datum • Computation of deflections of the vertical • Surface Gravity Interpolation • Mean ocean dynamic topography? • Time varying geoid (gravity)?

  5. Why we need geoid (cont.) • Why not GPS (ellipsoidal) height? - historic convention: height above sea level: HAULOVER PIER, N. MIAMI BEACH h=-25.02 m H(NAVD88)=+2.972 m NEWPORT, NARRAGANSETT BAY h=-28.36 m H(NAVD88)=+1.612 m ……

  6. Why we need geoid (cont.) • Water flows usually from high to low orthometric height, not necessarily from high to low ellipsoidal height • Useful to identify inundation area, plans for storm surge, environmental change monitoring

  7. Geoid used in GPS campaign • Simple relationship: H=h-N h – GPS (ellipsoidal) height H – orthometric (e.g. NAVD88) height N – geoid (e.g. GEOID03) height This formula is accurate to a fraction of mm

  8. Geoid modeling History at NGS • Milbert, D. G., 1991b: GEOID90: A high-resolution geoid for the United States. Eos, 72(49), pp. 545-554. • Smith, D.A. and D.G. Milbert, 1999: The GEOID96 high resolution geoid height model for the United States, Journal of Geodesy, V. 73, N. 5, pp. 219-236. • Roman, D.R. and D.A. Smith, 2000, Recent investigations toward achieving a one centimeter geoid, presented at GGG2000 session 9 of the IAG Symposium in Banff, Alberta, Canada from July 31 - August 4, 2000. • Roman, D. R., Y. M. Wang, W. Henning, J. Hamilton, Assessment of the New National Geoid Height Model—GEOID03, Surveying and Land Information Science, Vol. 64, No. 3, 2004, pp. 153-162

  9. Geoid computation theories • Stokes theory – Using gravity measurements, requiring global coverage – For precise geoid, the Earth’s topography and its density need to be known

  10. Geoid computation theories (cont.) • Molodensky theory • Using gravity measurements, no need for knowledge of the density of topography • It does not provide geoid, rather a similar but non-equilpotential surface – the telluroid. • The distance between telluroid and the Earth’s surface is called the height anomaly

  11. Height anomaly and geoid height • Under the first order approximation • Conclusion: no matter which theory is used, the topography and its density are needed to be known for precise geoid computations

  12. Few words about NAVD88 • NAVD88 is the official vertical datum of the United States • It is based on leveling and gravity, very accurate locally • No satellite data used • Tied to Rimouski (zero point)

  13. Geoid Height DifferencesNAVD88-GGEM02S

  14. Discussions • The southeast to northwest trend • The most significant difference occurs in the Pacific Northwest region • 98cm bias between NAVD88 and GGM02S geoid surface • Father Point/Rimouski (zero point of NAVD88) has a bias of 80 cm above the global geoid of GGM02S

  15. Geoid in NGS 10 Yrs Plan • A gravimetric geoid will serve as the next National Vertical Datum • High accuracy of the geoid (2 cm) and high resolution (<= 1 arc minute) • Fixing the southeast to northwest tilt in the NAVD88 • Combing satellite data (models) and surface data (gravity, GPS/leveling, …) • Deciding and adopting a new W0 value for the US and establishing relationship with a global vertical datum

  16. How to reach cm accurate geoid • Theory and computation methods development • Develop and refine theories to the highest accuracy • Study the effect of the data spacing and data accuracy on geoid; provide guideline for data collection • Test and evaluate numerical computation methods to increase computation accuracy and speed up the computation

  17. How to reach cm accurate geoid (Cont.) • Data collection and processing • Collect surface gravity data with accurate positioning, absolute or relative • Airborne gravity – GRAV-D, data processing and filtering, downward continuation • Compiling and cleansing digital elevation models to the highest resolution • Acquire mass density of the topography • Re-compile and compute the potential number differences from NGS leveling database

  18. National and International cooperation • NGS visiting scientist program • IAG working group “North American Geoid” chaired by D. Roman • IAG study group “Towards cm-Accurate Geoid - Theories, Computational Methods and Validation” chaired by Y. Wang • NOAA internship • Cooperation between US, Canada, Mexico, Virgin islands

  19. National and International cooperation (cont.) Data issues: • Obtain DEM data from any available sources • Collection of mass density data • Data exchange between US government agencies • Data exchange between US, Canada, Mexico, Virgin island counties • Support and cooperate with State on Height Modernization Program (data collection and modeling)

  20. GEOID09 • GEOID09 will become available around ACSM09 • Data format and access method are the same as the GEOID03 • Interpolation program intg can be downloaded at NGS web site • Expect dm changes at some BMs

  21. Future Geoid modeling plan • One geoid model per year for internal and external evaluation and testing • Models will be published at NGS web site • Data format, grid size and coverage areas will be kept the same, except specially announced • Suggest (plan) a user discussion group hosted by NGS • Create and publish FAQ for geoid & related issues (simplify GEOID page?)

  22. Summary • Next national vertical datum will be a model based on satellite and surface data • Beta version of the models will be published at NGS web site • Expect a big change along the road • Welcome any feedback from the user community

  23. Point of contact By phone: Dr. Dan Roman: 1-301-713-3202 x 161 Dr. Yan-Ming Wang 1-301-713-3202 x127 By e-mail: Dr. Dan Roman: Dan.Roman@noaa.gov Dr. Yan Ming Wang Yan.Wang@noaa.gov

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