G54A-01 INVITED
Concept for the Realization of a World Height System - Recent Findings of IAG Inter- commission Project 1.2 "Vertical Reference Frames"
Considerable progress is and will be attained in the conception and realization of a global vertical reference system from the data of the satellite gravity field missions (SGFM) CHAMP, GRACE and GOCE. Homogeneous satellite-only gravity models are key products for the unification of regional height systems. The SGFM will be used in future for a prototype realization of a global height reference system. A global unified vertical reference system can be realized at least by three components: (1) A global network of stations with coordinates in ITRF and geopotential numbers referring to a conventional global reference level. (2) A global reference level surface derived from a global gravity model (GGM) on the basis of a satellite- only gravity mission solution, in combination with tide gauge observations and a global sea surface height model (SSHM) from satellite altimetry; (3) In addition, local and regional gravity observations around the fundamental datum point are required (e.g., results from the Quasigeoid-D work package). Global gravity field models will be taken from the GOCE mission, which will allow substantial improvements compared to the GRACE models, as now an accuracy of 1 cm at 100 km resolution is envisaged. In addition, local and regional gravity observations shall be combined with the GOCE models, aiming at an accuracy of 1 cm for the complete spectrum. With regard to the unification of different national height systems, i.e. the determination of potential or height differences between corresponding reference surfaces, the GNSS/levelling approach as well as the tide gauge/mean sea surface approach shall be investigated. The results shall be compared with direct levelling observations or network adjustment results. The investigations will demonstrate the potential of GNSS and levelling together with SGFM-only solutions and regional quasigeoid models for the unification of height systems. It is anticipated that this application will strongly benefit from the GOCE mission results.
G54A-02 INVITED
Comparison of Gravimetric Geoid Height Models with Ocean Mean Dynamic Topography Models at Tidal Bench Marks around North America
Existing geoid height models are examined as a part of larger effort to develop a consistent regional vertical
datum. Such a datum will provide a consistent surface from which to estimate the impact of physical
phenomena influenced by heights such as storm surge, tsunamis, flood plain mapping, etc. Comparisons at
tide gages provide an alternative to existing methods that rely on comparisons at terrestrial bench marks.
Furthermore, tide-gage comparisons also enhance the transformations between oceanographic and
terrestrial datums desired as a part of NOAA's VDATUM project.
The working assumption is that tide-gage observed local mean sea levels (LMSL) can be compared with
geoid height models (N) in conjunction with ocean topography models (MDT); LMSL = N – MDT. Comparisons
were made at 86 tide gages along the shorelines of the East Coast, Gulf of Mexico, West Coast, and Alaska,
where an approximate 35 cm bias was determined – pointing to the potential need for selecting a new
geopotential surface for gravimetric geoid heights. This will likely determine a potential surface for the North
American Geoid but may have broader implications. Comparisons will be made to observations in territories
scattered farther overseas (Guam, American Samoa, etc.) as these become available as well as any
comparisons that can be made with other countries to determine the most optimal value.
http://www.ngs.noaa.gov/GEOID/NAG/NAG.html
G54A-03 INVITED
Transitioning to a Geoid-Based Vertical Datum: Challenges and Opportunities
The focus of this presentation is to identify and analyze the key aspects involved with transitioning from a conventional leveling-based vertical datum to one defined by a gravimetric geoid model. In particular, the practical implementation of such a geoid-based vertical datum in a large northern country such as Canada poses a number of challenges if one aims to achieve a consistent and accurate vertical reference surface over a number of years. The opportunities offered by low-Earth-orbiting satellite missions such as GRACE and GOCE will be analyzed as well as the need to continue to maintain a fundamental network of land-based benchmarks across the country. A method for optimally combining ellipsoidal, orthometric and geoid heights is presented with considerations for long-term geoid rate changes due to vertical crustal motion caused by on-going glacial isostatic adjustment (prevalent in Canada) and other phenomena.
G54A-04
GRAV-D Part I: Gravity for the Redefinition of the American Vertical Datum Project Ramps Up
The mission of NOAA's National Geodetic Survey (NGS) is to "define, maintain and provide access to the
National Spatial Reference System" (NSRS). NAVD 88 (North American Vertical Datum of 1988) provides the
vertical reference for the NSRS. Comparisons with the Gravity Recovery and Climate Experiment (GRACE)
satellite gravity data have demonstrated significant problems with NAVD 88. As repairing NAVD 88 through a
massive leveling effort is impractical, our approach will be to establish a gravimetric geoid as the vertical
reference. The linchpin in NGS's effort is the Gravity for the Redefinition of the American Vertical Datum
(GRAV-D) program, which will ultimately incorporate satellite, airborne and terrestrial gravity data to build the
1-2 cm geoid that the U.S. surveying public is demanding.
The GRAV-D program has two thrusts. First, a "high resolution snapshot" one-time measurement campaign
with dense spatial sampling but short temporal span would be used to repair and improve existing gravity
holdings. This campaign would involve airborne gravity surveys conducted over coastal areas first and
interior areas later for the entire US and its holdings. Second, a "low resolution movie" will track temporal
changes to the gravity field on a broad scale via a re-occurring survey with very coarse spatial coverage and
a long temporal span. This effort would involve time series of absolute and relative terrestrial gravity
measurements at regions of greatest temporal change to help update the geoid over time.
Initial data collection supporting GRAV-D was completed in July 2008. An airborne survey based out of
Anchorage, AK covered an area 500 x 400 km over Cook Inlet and Kachemak Bay in 24 flights and about
100 flight hours. Ground observations using absolute and relative gravity meters were made in Anchorage,
Palmer, Fairbanks, and Kodiak, Alaska. We will present both the project overview and the results of this first
phase of surveying.
http://www.ngs.noaa.gov/GRAV-D/
G54A-05
New Advances in Defining the Mongolian Gravimetric Geoid
The reference frame used by Mongolia for mapping has historically been based on Pulkovo 42 with a height system that utilized leveling extended from Kronstadt thru Russia. To modernize their mapping and infrastructure Mongolia recognized the need to improve their height system and vertical datum by the development of a new geoid model for the country. NGA and DNSC have partnered with Mongolia to improve their gravity field and geoid by a variety of cooperative activities over the past five years. An airborne gravity campaign in 2004-2005 covered a majority of the country and was incorporated into the new Earth Gravitational Model 2008 (EGM2008) released by NGA in April 2008. New ground surveys recently conducted to support mining and resource activities have also led to improvements in the accuracy of the gravity field for selected regions. The Shuttle Radar Topographic Model has also contributed to enhanced terrain corrections for the gravity field modeling in the country. To further improve the gravity field, regions lacking sufficient gravity coverage are currently being surveyed using a variety of techniques (helicopters, etc.) depending on their accessibility and difficulty. The development of a new Mongolian gravimetric geoid utilizing the data from these campaigns, including new collections in 2008, will be presented along with the remaining challenges to continue to improve the geoid. These include ongoing efforts to survey and improve areas of limited or no gravity coverage and utilizing the best available GPS/leveling to validate and assess the quality of the new gravimetric geoid model to be used to define the vertical datum for the country.
G54A-06
Selecting the optimal geoid computational method based on tests done in regions with access to high quality GNSS/leveling data.
At the Norwegian Mapping and Cadastre Authority development of Height Reference Surface Models have been ongoing for more than a decade. Utilizing an iterative approach new models are generated by adjusting the previous model to GNSS/leveling data in areas where improvements are needed. The initial model used in the adjustment procedure is the currently best available geoid model. The derived models allow GNSS users to determine heights in the National Height System better than 2 cm for most parts of Noway. Comparison with GNSS/leveling data has showed the importance of having access to high quality GNSS/leveling data in order to validate the computed geoid models and to intercompare different geoid computation methods. These data will assist in deciding which method to use when computing new geoid models. This is of particular importance for areas where no adjustment data are available like marine areas. A reliable estimate of the mean dynamic topography from satellite altimetry requires a high precision geoid model.
G54A-07
On the Geoid-Quasigeoid Separation in Mountain Areas
In modern precision height systems, two basic concepts of physical height definition are widely used: on the one hand the Stokes-Helmert concept of orthometric heights referred to the geoid, and on the other hand Molodensky's concept of normal heights to be used in combination with height anomalies or quasigeoid heights. The separation between the two reference surfaces - the geoid and the quasigeoid - is typically in the order of a few decimeters but can reach nearly 3 m in extreme cases. For a consistent use of both height concepts within a country, and in particular across borders of countries or regional vertical reference frames, the geoid-quasigeoid separation should be provided along with the vertical datum - with cm-accuracy and for every location (not only for benchmarks). The largest contribution to the geoid-quasigeoid separation is due to the distribution of topographic masses. We modeled the geoid-quasigeoid separation for test areas with very rough topography using a very fine grid resolution of 100 m and a rigorous treatment of topographic masses based on high-resolution digital terrain data. Results show that rigorous treatment of topographic masses leads to a rather small geoid-quasigeoid separation - only 30 cm at the highest summit - while results based on approximations are often larger by several decimeters. The accuracy of the topographic contribution to the geoid-quasigeoid separation is estimated to be 2-3 cm for areas with extreme topography if a modeling grid resolution of 200 m or less is used. We conclude that a consistent determination of the geoid and quasigeoid height reference surfaces within an accuracy of few centimeters is feasible even for areas with extreme topography, and that the concepts of orthometric height and normal height can be consistently realized and used within this level of accuracy.
G54A-08
One- and Two-Step Integral Solutions for Gravimetric Geoid - Revisited
Despite advances in global gravity field modelling through gravity-dedicated satellite missions (CHAMP, GRACE and GOCE) and the release of EGM08 by the National Geospatial-Intelligence Agency, ground and airborne gravity still remain irreplaceable sources of data to determine the fine resolution of the geoid model. Standard methods to model the geoid from ground gravity data are based on Green's surface integrals that represent solutions to boundary-value problems of potential theory. In the classical approach, reduced gravity is continued to a known reference surface prior to conversion into the disturbing potential. The disturbing potential is obtained by surface-integrated convolution of continued gravity with a respective Green's function. Thus, two integral equations must numerically be evaluated with one of them representing an inverse problem requiring sophisticated numerical methods. Besides stipulating the mean mass density within topography, they represent the major complication in determining the fine resolution of the geoid model. In this study, this classical approach is compared with an alternative solution that combines continuation and conversion of ground gravity in one integral equation. Both approaches are used for evaluation of the local geoid over a test region in Western Canada. Numerical computations of both approaches are compared in terms of consistency, stability and efficiency. The geoid models are also evaluated for external accuracy by using measured GPS heights at levelling benchmarks available over the test area.