G33B-01
A New Vertical Reference System for South America
A main objective of a modern vertical reference system is to satisfy h = H + N world wide. It requires the definition and realisation of a unique physical reference surface, and the linkage of the local reference levels to the global one. Based on the gravity field missions and satellite altimetry data an accurate global reference level (W0) can be determined. Its realisation (the geoid) on marine areas becomes feasible by solving the classical boundary value problem (BVP). On continental areas, it is more complicated: besides the hypothesis to determine the geoid, the observables included in the BVP are affected by the inconsistencies of the height datums. In this way, four steps must be followed: 1) Solution of the BVP according to Molodenskii Theory, 2) Connection of the local levels with the global reference surface, 3) Recalculation of the height-related observables to iterate the BVP and, 4) Repetition of the procedure until getting a mm-level accuracy in normal heights and height anomalies. The central problem is to find the adequate strategy to unify the local height datums. Our proposal is: a) joint analysis of tide gauge and satellite altimetry data to determine the time variation of sea level at the reference tide gauges, b) continuously GNSS positioning at the same reference points to separate crustal movements from sea level rise, c) evaluation of physical and geometrical heights on a high-precise reference frame, and d) connection of the existing height datums. The unification of the South American classical vertical datums is presented as an example of the proposed methodology.
G33B-02
The Current Status of IGS Regional Network Associate Analysis Centre SIRGAS (IGS RNAAC SIR)
Since 1996 the German Geodetic Research Institute (DGFI) acts as the Regional Network Associate Analysis Centre for the Geocentric Reference System of the Americas (SIRGAS) within the International GNSS Service (IGS RNAAC SIR). It is processing all permanent GPS stations (IGS and regional stations) of South America, Central America, the Caribbean and surrounding areas. Each week a coordinate solution is delivered to the IGS data centres. After the decision of the IGS to introduce the new ITRF2005 (IGS05) and the implementation of absolute phase centre variations since GPS week 1400 the IGS RNAAC SIR at DGFI will reprocess all weekly solutions since GPS week 1860 (July 1996). A new position and velocity solution of IGS RNAAC SIR will be presented containing all reprocessed weeks so far. This paper reports about the station network, the used input data, and the strategy of processing. Comparisons with the latest ITRF2005 solution will be shown (coordinates and velocities). Some remarks will be given on future plans of IGS RNAAC SIR.
G33B-03
The Korean Datum Change to a World Geodetic System
The geodetic datum of Korea is superseded by the new Korean Geodetic Datum 2002 (KGD2002) which is a world geodetic system compliant with the ITRS from January 1, 2007. Based on the new definition of the geodetic datum, the geodetic network in Korea is re-adjusted to provide the new realization of the datum. The readjustment is carried out under the campaign of KGD2002 which the Korean Geographic Information Institute (KGII) was initiated in 1999. In this study, the summarized whole procedure of the network adjustment to realize KGD2002 is explained and described. Starting from the data arrangement in terms of regions and years, the quality check, the readjustment strategies such as local and nationwide adjustments are described in detail. As the result, the nationwide network of the datum focused on the horizontal one with its accuracy is presented. For general readers, a brief history of Korean geodetic network is also included. In addition, the future plans on the management/development of the Korean network are also described. It is expected that this study contributes in datum changes of other countries by providing the expected problems, strategies, and solutions.
G33B-04
Regional Geoid Modeling Compared to Ocean Surface Observations
Aerogravity over a limited coastal region of the northern Gulf of Mexico enhanced and rectified the local gravity field signal. In turn, these data improved the derived geoid height model based on comparison with dynamic ocean topography (DOT) and tide gage information at eleven stations. Additionally, lidar observations were analyzed along nearly 50 profiles to estimate the reliability of these models into the offshore region. The overall comparison shows dm-level agreement between the various geoid and DOT models and ocean surface observations. An approximate 30 cm bias must still be explained; however, the results of this study point to the potential for further cooperative studies between oceanographers and geodesists.
G33B-05
Moving to a Modernized Height Reference System in Canada: Rationale, Status and Plans
A modern society depends on a common coordinate reference system through which geospatial information can be interrelated and exploited reliably. For height measurements this requires the ability to measure mean sea level elevations easily, accurately, and at the lowest possible cost. The current national reference system for elevations, the Canadian Geodetic Vertical Datum of 1928 (CGVD28), offers only partial geographic coverage of the Canadian territory and is affected by inaccuracies that are becoming more apparent as users move to space- based technologies such as GPS. Furthermore, the maintenance and expansion of the national vertical network using spirit-levelling, a costly, time consuming and labour intensive proposition, has only been minimally funded over the past decade. It is now generally accepted that the most sustainable alternative for the realization of a national vertical datum is a gravimetric geoid model. This approach defines the datum in relation to an ellipsoid, making it compatible with space-based technologies for positioning. While simplifying access to heights above mean sea level all across the Canadian territory, this approach imposes additional demands on the quality of the geoid model. These are being met by recent and upcoming space gravimetry missions that have and will be measuring the Earth`s gravity field with increasing and unprecedented accuracy. To maintain compatibility with the CGVD28 datum materialized at benchmarks, the current first-order levelling can be readjusted by constraining geoid heights at selected stations of the Canadian Base Network. The new reference would change CGVD28 heights of benchmarks by up to 1 m across Canada. However, local height differences between benchmarks would maintain a relative precision of a few cm or better. CGVD28 will co-exist with the new height reference as long as it will be required, but it will undoubtedly disappear as benchmarks are destroyed over time. The adoption of GNSS technologies for positioning should naturally move users to the new height reference and offer the possibility of transferring heights over longer distances, within the precision of the geoid model. This transition will also reduce user dependency on a dense network of benchmarks and offer the possibility for geodetic agencies to provide the reference frame with a reduced number of 3D control points. While the rationale for moving to a modernized height system is easily understood, the acceptance of the new system by users will only occur gradually as they adopt new technologies and procedures to access the height reference. A stakeholder consultation indicates user readiness and an implementation plan is starting to unfold. This presentation will look at the current state of the geoid model and control networks that will support the modernized height system. Results of the consultation and the recommendations regarding the roles and responsibilities of the various stakeholders involved in implementing the transition will also be reported.
G33B-06
Discussion about Formulas to Calculate Variation of Plumb-Line Deflection
Formula to calculate variation of plumb-line deflection by the way of repeated gravity measurement is discussed.
Two different are given:one is basic on time-variation of perturbation potential,the other is basic on time-variation
of gravity potential. The theroetic basis and deducing process of the latter formula are emphasized. By analysis it
is konwn that they are consistent in theory, but the latter is clearer in thought and simpler in form and less effect in
far area and lower precision than the one calculated by the former.
http:www.agu.org/liyuefeng
G33B-07
Study of a Gravity Precursor mode of the Lijiang Earthquake with Ms7.0
In order to analyses the earthquake pregnant process or the precursory behavior before the 1996 Lijiang earthquake with Ms7.0 occurredCin this paper, we make the best of the high precision repeat gravity observation data in the experiment field, western Yunnan, combine with the related geological survey and geophysical resultsCand take account of the gross errors caused by data observation and models difference, we first adopt the robust Bayefs least squares estimation and multi-fault dislocation models to invert and obtain the time slip distribution of the main active faults in the study region. The results show that the time changes of faults slip during 1990 to 1997 are good reflected the earthquake pregnant process of the 1996 Lijiang earthquake with Ms7.0, the picture of main precursor mode has the characteristic of main shock- after shock type and follow the mode of coupling movement between crust density and crust deformation (in a word, name it as DD mode of coupling movement). Keyword Lijiang earthquake precursor mode repeat gravity time changes of faults slip
G33B-08
The Error Analysis of Global Gravity Field Models according to the Earthquake Monitoring
Since the GRACE mission was launched in March 2002, there have been a lot of GRACE observational data and products released, including the time-variable global gravity field models with the maximum degree to 120, and 1 month time resolution. The purpose of this paper is how to use these global gravity field model products to analyze the earthquake monitoring according to the spatial resolution and the required precision. Firstly, this paper introduces a lot of global gravity field models, based CHAMPAGRACE satellite data, and the method of computing gravity anomaly error from these models. Secondly, the gravity anomaly errors of the above gravity field models are computed and compared. Thirdly, the feasibility of earthquake monitoring by low-Earth orbiting satellites according to these results is analyzed. Finally, the maximum degrees of these gravity field models are decided for the requirement of earthquake monitoring. Key wordsFgravity field model, error analysis, earthquake ,CHAMP , GRACE