International Association of Geodesy's  Special Study Group 3.177 SYNTHETIC MODELLING OF THE EARTH'S GRAVITY FIELD
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Aims and Objectives

 The aims and objectives of IAG SSG3.177 include, but are not limited to:

• address the theoretical and practical basis of producing a synthetic model of the Earth's gravity field;
• study the effects of the various approximations currently made in physical geodesy;
• reach a consensus on the correct methods to test or compare the theories, approximations, approaches and software being developed, or in use, for physical geodesy;
• produce a gravity field model with realistic noise spectra from various sources (both correlated and un-correlated) to allow the study of the propagation of errors in current gravity field modelling methods;
• produce a model that generates data in various formats, so as to ensure its wide-spread application;
• supply the results (papers and software) to researchers in physical geodesy and geophysics world-wide.

 Associated with the production of synthetic Earth gravity models (SEGMs) are:

• recommendations for their implementation and use by the international community;
• identification of the correct methods of gravity field modelling and subsequent interpretation;
• procedures to validate software and their associated mathematical models (instead of relying only upon empirical accuracy estimates);
• validation of the software archive held by the International Geoid and Gravity Commission (IGGC).

This should allow groups of researchers around the world to provide a 'litmus test' of their theories and methodologies. This may also contribute to the resolution of some of the differences between groups.

Applications

 The synthetic gravity field can be used in several areas of physical geodesy. Examples include, but are not limited to:

• testing upward and downward continuation of gravity data;
• validating gravimetric geoid computation theories and software;
• simulating space-borne gravity gradiometry data; and
• simulating airborne gravity data.

 One suggested approach to constructing the SEGM is to model the broad/deep structure of the Earth's gravity field using spherical harmonics, point mass or digital density models (eg. REM). The gravitational effects of the Earth's crust and topography can be modelled using recent global digital density and terrain models and superimposed on the broad/deep structure component of the SEGM.

The detailed structure of the SEGM in specific areas can be refined using regional datasets, which avoids problems of data confidentiality and large computation time. These can be superimposed on the global field. In areas where detailed local data are not available, it will be necessary to simulate the gravity field completely artificaially (eg. by recursing the global data using fractal techniques).
 
 

• Some public domain global models of density and terrain are given here
• SSG member's specific views on the generation of the synthetic field are given here

 Most of the SSG's activities will be coordinated via the Internet (email and these web-pages). However, it is equally useful to have face to face meetings, the next of which will be at the IAG General Assembly, Budapest, Hungary, Sept 2 - Sept 7, 2001. The time, date and venue of this meeting is to be announced, but will probably coincide with technical sessions on the Earth's gravity field. Meetings of the SGG after this conference will be announced via this web-page.