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11-11-2008 Commission 2 – Gravity Field (2007-2011) URL: http://www.iag-commission2.ch
President: Yoichi Fukuda (Japan)
Vice President: Pieter Visser (The Netherlands) Terms of Reference Accurate determination of the gravity field and its temporal variations is a prime target of modern geodesy. It is closely related to geophysics, geodynamics, navigation, metrology and other related disciplines including the Earth’s environmental issues as well. To this end, Commission 2 was established at the IUGG in Sapporo in summer 2003 for promoting, supporting, and stimulating the advancement of knowledge, technology, and international cooperation in the geodetic domain associated with Earth’s gravity field. In the last period of 2003-2007, Commission 2 has achieved its primary goals and has been ready for the next quadrennium. Moreover the major part of the associated scientific themes is of long-term interest so that the structure of Commission 2 essentially continues the one in the last period. Commission 2, at the start of the new period, consists of four sub-commissions, six projects, two study group and several inter-commission projects, working groups, study groups. The sub-commissions cover following science themes; terrestrial, airborne, ship borne gravimetry and relative/absolute gravity networks; spatial and temporal gravity field and geoid modelling; dedicated satellite gravity missions; and regional geoid determination. The projects are established to organize work on unique and exceptional areas of interest or particular problems requiring specific international cooperation. Commission 2 has strong links to sister commissions, ICCT, IGFS and other components of IAG. Connections to these components are created with inter-commission projects, working groups, study groups that provide a cross-disciplinary stimulus for work in several topics of interest to the commission. Commission 2 has been expected to involve important problems in the next four years; for instance, analysis of a new type of data from new satellite gravity mission (GOCE); more accurate modelling of the temporal gravity field variation to understand mass transports; new observation technologies. A new Study Group (SG2.2) has been established to look at forward gravity modelling techniques for high-resolution gravity field recovery to assist in processing data from current and future satellite gravity missions. Other Study Groups/ Working Groups focused on well-defined subjects will be established as the need arises. Objectives The main objectives of Commission 2 are:
Structure Sub-Commissions
Commission Projects
Study Groups
Inter-Commission Projects
Inter-Commission Study Groups
Inter-Commission Working Groups
Program of Activities The Gravity Field Commission fosters and encourages research in the areas of its sub-entities by facilitating the exchange of information and organizing Symposia, either independently or at major conferences in geodesy. The activities of its subentities, as described below, constitute the activities of the Commission, which will be co-ordinated by the Commission and summarized in annual reports to the IAG Bureau. The status of Commission 2, including its structure and membership, as well as links to the internet sites of its sub-entities and parent and sister organizations and services, will be updated regularly and can be viewed on the web site: http://www-kugi.kyoto-u.ac.jp/iag-commission2/ Steering Committee
Sub-CommissionsSC 2.1 Gravimetry and Gravity Networks President: Leonid F. Vitushkin (BIPM) Vice-President: Gerd Boedecker (Germany) Terms of Reference and Objectives Sub-commission 2.1 promotes scientific investigations of gravimetry, gravity measurements and gravity networks. It promotes the growth of the number of absolute gravity determination and of the number of the sites for such determinations. It provides the gravity community with the means to access the confidence in gravity measure-ments at the well-defined level of accuracy through orga-nizing, in cooperation with metrology community, the international comparisons of absolute gravimeters. The sub-commission proceeds from such point-wise gravi-metry to precise gravimetry/gradiometry which should cover, in particular, the land-sea border areas to resolve still existing problem of significant biases and errors in determination. The Sub-commission promotes such research and development by stimulating airborne and shipboard gravimetry and gradiometry. It encourages and promotes special absolute/relative gravity campaigns, techniques and procedures for the adjustment of the results of gravity surveys on a regional scale. Sub-commission encourages development of the Global Gravity Reference Network for GGOS. Through the inter-commission WG on Absolute Gravimetry the Sub-commission works on the standardization of absolute gravity data, software for absolute gravity measurement and appropriate information. The Sub-commission will encourage regional meetings or workshops dedicated to specific problems, where appropriate. Program of Activities To meet these goals, the Sub-commission sets up Special Study Group 2.1 on Comparisons of Absolute Gravi-meters, inter-commission Working Group on Absolute Gravimetry (inter SC 2.1 and IGFS) and Commission Projects CP2.1-2.7 Structure
SC 2.2 Spatial and Temporal Gravity Field and Geoid Modelling President: Martin Vermeer (Finland) Terms of Reference The subjects of study that the Sub-commission supports and promotes can be summarized, without claim to com-pleteness, as follows. Research work in the spatial domain concentrates on:
Studies in the temporal domain of the gravity field include, among others, the following:
Program of Activities To meet these goals, the Sub-commission invites the establishment of Special Study Groups (SSG’s) on rele-vant topics, promotes and organizes special sessions at IAG Symposia and other conferences, and reports on the research work in these areas of interest. SC 2.3 Dedicated Satellite Gravity Mapping Missions President: Roland Pail (Austria) Terms of Reference The successful launches of the German CHAMP (2000) and US/German GRACE (2002) missions have led to a revolution in global gravity field mapping by space-borne observation techniques, and provided valuable contribu-tions to many geoscientific fields of application, such as geodesy, hydrology, oceanography/altimetry, glaciology, and solid Earth physics. These two missions have proven new concepts and technologies, such as space-borne accelerometry and low-low Satellite-to-Satellite Tracking (SST), in combination with more conventional observa-tion techniques, like GPS SST and Satellite Laser Ranging (SLR). CHAMP and GRACE have already produced consistent long- to medium-wavelength global gravity field models and its temporal changes, and have supported the preparation for the European Space Agency (ESA) GOCE dedicated gravity field mission, which will further revolutionize high-accuracy and high-resolution gravity field mapping employing for the first time in history the satellite gravity gradiometry (SGG) observations. Program of Activities The focus of this sub-commission will be to promote and stimulate the following activities:
SC 2.4 Regional Geoid Determination President: Urs Marti (Switzerland) Terms of Reference and Objectives Sub-Commission 2.4 is concerned with the following areas of investigation: regional geoid projects: data sets, involved institutions, comparison of methods and results, data exchange, comparison with global models, connec-tion of regional models gravimetric geoid modelling tech-niques and methods, available software GPS/levelling geoid determination: methods, comparisons, treating and interpretation of residuals common treatment of gravity and GPS/levelling for geoid determination geoid applica-tions: GPS heights, sea surface topography, integration of geoid models in GPS receivers, vertical datums. other topics: topographic effects, downward and upward con-tinuation of terrestrial, airborne, satellite data specifically as applied to geoid modelling. SC 2.5 Satellite Altimetry President: Cheinway Hwang (Taiwan) Terms of Reference Over the past two decades, satellite altimetry has con-tributed to our understanding of the ocean, marine geo-desy and geophysics, cryosphere, orbital science and solid earth geophysics. Recent technological advances in altimetry will further these understandings and applica-tions, and foster new discoveries in earth sciences. This IAG sub-commission will serve as an interface between altimeter data and their users and promote the visibility of IAG in altimetric science . Selected highlights are:
Program of activities This sub-commission will help to organize independent workshops or special sessions in major meetings to pro-mote altimetric applications in earth sciences, and to increase the visibility of IAG in altimetric science. Special study groups will be established to investigate important issues. Commission ProjectsCP 2.1 European Gravity and Geoid Project (EGGP) Chair: Heiner Denker (Germany) Terms of Reference and Objectives Within the first 4-year term of this project from 2003 – 2007, a new geoid and quasigeoid model EGG07 was developed. The EGG07 model is a complete update com-pared to the previous computation from 1997 (EGG97), employing all high-resolution gravity and terrain data available for Europe in mid-2007 as well as a GRACE based global geopotential model (EIGENGL04C). The computation procedure was based on the remove-restore technique, residual terrain model reductions and the spectral combination approach. The evaluation of the EGG07 model by independent GPS and levelling data proved significant improvements as compared to the pre-vious EGG97 model, indicating an accuracy potential of 0.03 – 0.05 m at continental scales and 0.01 – 0.02 m over shorter distances up to a few 100 km, provided that high quality and resolution input data are available. The aim of this project for the time frame 2007 – 2011 is to further refine the geoid and quasigeoid modelling in Europe, continuing and extending the present contacts and successful cooperation with the respective national and international agencies. The main topics of interest in the project include:
The project reports to Sub-Commission 2.4. It is organ-ised by a steering committee and has national delegates (project members) from most of the countries in Europe. Steering Committee
CP 2.2 North American Geoid Chair: Dan Roman (USA) Terms of Reference and Objectives The primary objective of the Project is the development of a regional gravity field and geoid model for North America in order to achieve a common vertical datum. “North America" will encompass Iceland, Greenland, Canada, the U.S.A. (including Alaska and Hawaii), and Mexico as well as countries forming Central America, the Caribbean Sea, and the northern portions of South America. Both Canada and the U.S.A. are moving towards the definition of new vertical datums based on gravimetric geoids, and other countries may adopt similar approaches if a suitable model is developed that meets their respective national interests. The intent here is to ensure a suitable North American Geoid is developed to serve as a common datum for everyone as well as the basis for a forthcoming International Great Lakes Datum model in 2015 (IGLD 15). Likewise, all countries in the region would be served by having access to a common model for translating oceanographic effects to terrestrial datums for various scientific, commercial, engineering, a disaster preparedness applications. The achievement of a geoid model for North America will be accomplished by coordinating activities between agencies and universities with interest in geoid theory, gravity, gravity collection, gravity field change, geo-physical modelling, digital elevation models (DEM), digital density model (DDM), altimetry, dynamic ocean topography, levelling and vertical datums. Of particular interest will be relating geoid and ocean topography models to ocean topography and tidal bench marks. The determination of a geoid model for North America is not limited to a single agency, which will collect all neces-sary data from all countries. The Project encourages theoretical diversity in the determination of a geoid model between the agencies. Each agency takes respon-sibility or works in collaboration with neighbouring countries in the development of a geoid model for their respective country with an overlap (as large as possible) over adjacent countries. Each solution will be compared, the discrepancies will be analyzed, and the conclusions will be used to improve on the next model. Program of Activities The Project will support geoid activities in countries where geoid expertise is limited by encouraging more advanced members to contribute their own expertise and software. The Project will encourage training and educa-tion initiative of its members (e.g., International Geoid Service (IGeS geoid school and graduate studies). The chair of the Project will meet with the equivalent European and South American projects to discuss overlap regions and to work towards agreements to exchange data. Finally, the members of the Project will keep close contact with all related Study Groups of the IAG. The Project is open to all geodetic agencies and universities across North America with an interest in the development of a geoid model for North America. The meetings of the Commission Project 2.2 are open to everyone with interests in geodesy, geophysics, oceanography and other related topics. Members will communicate primarily using e-mails. However, members of the Project plan to arrange annual meetings. Preferably, these meetings will be held during international conferences where most members will be present; however, some meetings will be held in North America to minimize travel costs. Minutes of meeting will be prepared and sent to all members of the Project. Membership
CP 2.3 African Geoid Chair: Hussein Abd-Elmotaal (Egypt) Terms of Reference The African Geoid Project (AGP) is a project of Commission 2 of the International Association of Geodesy (IAG). The main goal of the African Geoid Project is to determine the most complete and precise geoid model for Africa that can be obtained from the available data sets. Secondary goals are to foster cooperation between African geodesists and to provide high-level training in geoid computation to African geodesists. Objectives The objectives of the project are summarized as follows:
Full Members
Associate Members
CP 2.4 Antarctic Geoid Chair: Mirko Scheinert (Germany) Terms of Reference and Objectives The main goal of AntGP is to work towards a continent-wide determination of the Antarctic geoid on basis of a completed Antarctic terrestrial gravity dataset. However, Antarctica remains the continent with the largest gaps in terrestrial gravity data coverage. A complete gravity dataset of the Antarctic will be needed in order to facili-tate the regional geoid determination, to contribute to the determination of the global gravity field in combination with data of the new gravity satellite missions GRACE and GOCE, but also to densify the satellite data. There-fore, the compilation of existing gravity data should be pursued and new observation campaigns should be pro-moted. The Antarctic Geoid Project (AntGP) should be a focus group for geodesists and geophysicists interested in gravity and geoid in Antarctica. Activities
Membership
Corresponding Members
CP 2.5 Gravity and Geoid in South America (GGSA) Chair: Maria Cristina Pacino (Argentina) Terms of Reference and Objectives: The project entitled Gravity and Geoid in South America, as part of the Commission II of IAG, was established as an attempt to coordinate efforts to establish a new Abso-lute Gravity Network in South America, to carry out gravity densification surveys, to derive a geoid model for the continent as a height reference and to support local organizations in the computation of detailed geoid models in different countries. Besides, a strong effort is being carried out in several countries in order to improve the distribution of gravity information, to organize the gravity measurements in the continent and to validate the available gravity measure-ments. The main objectives of the project are:
Links to be established in the Project
Coordinator María Cristina Pacino FCEIA – UNR Av. Pellegrini 250 2000 – Rosario – Argentina Phone: 54 – 341 – 4802649 (117) Fax: 54 – 341 – 4802654 E-mail: mpacino@fceia.unr.edu.ar Co-Coordinator: Denizar Blitzkow EPUSP - PTR Caixa Postal 61548 05424-970 São Paulo - SP - Brazil. Phone: (55) (11) 3091-5501 Fax: (55) (11) 3091-5716 E-mail: dblitzko@usp.br Membership
Corresponding Members
CP 2.6 South Asian and Australian Geoid Chair: Bill Kearsley (Australia) 1. Charter To promote cooperation in and knowledge of the geoid and related studies in the region of South East Asia. 2. Organisation 2.1 Target Membership Countries in and associated with ASEAN and countries in the region including The Philippines, Papua New Guinea, Indonesia, Malaysia, Singapore, Brunei, Thailand, Vietnam, Cambodia, Laos, Myanmar, and Australia and New Zealand. Because of the synergy which exists between the objectives of this Committee and those of the Geodesy Working Group of the UN Permanent Com-mittee For GIS Infrastructure for Asia and the Pacific (PCGIAP), it seems logical to extend the borders of the subject region to those covered by this UN Committee which have geographical connections with the above countries. 2.2 Sub-Commission Structure I feel that the executive should be small to ensure effi-ciency, and that the Committee should comprise one member from each participating country. Because of the need to carry national authority, the national member is logically the officer in the country’s National Geodetic Authority responsible for the National Geoid and/or National Height Datum matters. 3. Topics of Interest 3.1. Gravity and Related Data Explore ways in which we may
Clearly an important phase of this study is to identify and catalogue the gravity that exists – including the recently observed airborne campaigns. It is also important to establish a protocol for sharing the data. National authorities may be reluctant to give all the data available and at the precision available. It should be possible for geoid evaluation purposes, however, to decrease the resolution and accuracy of data shared along common borders without either comprising the precision of the geoid significantly, or the security of the national data shared. 3.2 Geoid Control Explore ways in which countries of the region may cooperate by
In such campaigns as these the activities of the PCGIAP group would be most relevant. 3.3 Education and Research Encourage and sponsor, for the region,
Study GroupsSG 2.1 Comparisons of Absolute Gravimeters Chair: Leonid F. Vitushkin (BIPM) Terms of Reference Absolute ballistic gravimeters have become the primary measurement standards in gravimetry in the field of the measurement of free-fall acceleration. Currently the only way to determine the level of accuracy of the absolute ballistic gravimeters and provide the uniformity in absolute measurements of free-fall acceleration is by a comparison of the results of their measurements. The principal task of the Special Study Group consists of organization (in collaboration with the BIPM (http://www.bipm.org), Working Group on Gravimetry of Con-sultative Committee on Mass and Related Quantities – CCM WGG) of the four-year period International Com-parisons of Absolute Gravimeters (ICAGs) at the BIPM and Regional International Comparisons of Absolute Gravimeters (RICAGs) at the sites selected on a conti-nental scale. The next Eighth ICAG should be held in 2009. The increasing demand for reliability and confidence in absolute gravity measurements requires further improve-ment of a technical protocol, developed for the first time for the Seventh ICAG in 2005, for the future ICAGs and RICAGs. Such a protocol should be developed according the rules of the international Mutual Recognition Arrangement for national measurement standards and for calibration and measurement certificates issued by National Metrology Institutes. The relevance to the SG is that its members are the spe-cialists from geodetic and geophysical communities, as well as the metrological community and this study group is more to participation than more official CCM WGG where the membership is related to the institutes respon-sible for the traceability in gravimetry. Such inter-communications within the Study Group as well as a linkage between this group and CCM WGG will make it possible to develop the ICAGs and RICAGs technical protocol accepted by both communities. The sites for regional comparisons of absolute gravi-meters (in America, Asia, Europe, and Africa) should be recommended by the geodetic-geophysical community and related to regional structures of metrology commu-nity (Regional Metrology Organization, for example, EURAMET – European Metrology Organization, SIM – Inter-American Metrology System, etc.). Objectives
Membership
SG 2.2 High-Resolution Forward Gravity Modelling for Improved Satellite Gravity Missions Results Chair: Michael Kuhn (Australia) Terms of Reference and Objectives With the launch of the new satellite gravity missions CHAMP, GRACE and in future GOCE a new era of modelling the static as well as time-variable Earth’s gravity field has begun. Currently, GRACE time-variable gravity observations attract enormous interest across all geo-sciences, which is due to the unprecedented spatial and temporal resolution and precision on a global and regional scale. However, local gravity field modelling still requires additional high-resolution information such as from terrestrial gravity observations or local topography data. Especially, for the latter high-resolution global, regional and local digital elevation models (DEMs) are available with resolutions down to about 100 m by 100 m on a global scale (e.g. 3-arc-sec by 3-arc-sec SRTM-derived terrain models) or well below the 100 meter-level for regional and local DEMs. Furthermore, the DEM data are complemented with an increasing number of geo-logical and geophysical information describing mass-density variations within the Earth’s interior. Today these data allow for the use of forward gravity modelling tech-niques (direct application of Newton’s integral) in order to recover high-resolution gravity field information on a local, regional and global scale. This IAG Study Group (SG) focuses on the application of forward gravity modelling techniques for high-resolution gravity field recovery with the specific aim to assist in processing data from current and future satellite gravity missions. This can be two-fold either introducing the high-resolution gravity information to results of the gravity missions or the removal of the same from satellite gravity observations. The SG will mostly focus on the following topics:
The SG can be seen as a logical continuation of IAG SSG 3.117 and IAG SG 2.2. While the former mostly looked into the construction of SEGMs the latter had a broad view on forward gravity modelling. The proposed SG will build on the experience of these study groups and focuses on a rather narrow application of forward gravity modelling, which is of great importance to the processing and analyzing of observations and results from current and future satellite gravity missions. The SG is open to researchers of any discipline who feel that are able to actively contribute to its aims. Further-more, members should be prepared to participate in special focus groups looking into particular aspects with-in the SG. It is aimed that the SG meets on a regular basis (e.g. once a year) during major conferences of the IAG or other related societies. Apart from these formal meetings communication within the SG will be mostly by e-mail and the setup of a designated webpage. Membership
Inter-Commission Working GroupsIC-WG 2.1 Absolute Gravimetry (Joint with IGFS) Chair: Herbert Wilmes (Germany) Terms of Reference and Objectives The Sub-Commission 2.1 “Gravimetry and Gravity Net-works”, promotes scientific investigations of gravimetry and gravity networks, terrestrial, airborne, shipboard and planetary gravity measurements. One of the outputs of the SC 2.1 activities is the result of gravity measure-ments, i.e. the gravity data. The International Gravity Field Service IGFS coordinates the servicing of the geodetic and geophysical community with gravity field-related data, software and information. The scientific community of IAG demands more detailed information about the Earth gravity field by precise terrestrial absolute gravity (AG) observations in an evenly distributed station network and with an improved timely coverage by repeated AG measurements and/or additional superconducting gravity (SG) observations. This is accentuated by the Global Geodetic Observing System GGOS. For consistent evaluation and combina-tion of gravimetric and geometric observations, the applied correction models, parameters and constants need to be compared and standardized. Currently the role of absolute gravimetry increases which is related to a growing number of absolute ballistic gravi-meters (ABG) and the rising number of AG measure-ments worldwide. The philosophy of gravity measure-ments is changing from the rare AG determinations at a few principal network stations to repeated absolute observations at hundreds of sites accompanied in some cases by measurements of temporal gravity variations using superconductive (relative) gravimeters. Absolute ballistic gravimetry is based on the primary method of measurement of free-fall acceleration. The accuracy of ABGs is determined in the international comparisons. The set of ABG with the determined un-certainty in the measurements provides the can we use valuable means to establish a gravity network of global scale with well-maintained stations where the gravity field variations can be monitored. Several ABGs with the determined metrological characteristics may be used for monitoring the gravity variations related, in particular, to changes of the solid Earth’s geometry and mass distribu-tion, hydrology or atmosphere variations, etc. Considering the role of absolute gravity measurements for the knowledge of the gravity field and its temporal variations, the requests from IGFS and the results of the discussions at the round table during the IGFS Sympo-sium in Istanbul (2006) and at the 2nd (2006) and 3rd (2007) Joint Meetings of CCM Working Group on Gravi-metry and SC 2.1 Study Group 2.1.1 on Comparisons of Absolute Gravimeters the President of Sub-Commission 2.1 “Gravimetry and Gravity Networks” lead to the deci-sion to establish a new “Working Group on Absolute Gravimetry”. This group will be the inter Sub-Commission 2.1 and IGFS working group which will report to SC 2.1 and IGFS. In reality, this WG 2.1.1 should be even more inte-grating because of the wide requests for the absolute gravity data from other sub-commissions and projects of Commission 2. The proposed Working Group on Absolute Gravimetry will have following objectives and tasks:
IC-WG 2.2 Evaluation of Global Earth Gravity Models (Joint with IGFS) Chair: Jianliang Huang (Canada) Terms of Reference and Objectives The US/German GRACE satellite gravity mission, launched in 2002, has succeeded in determining the Earth’s gravity field with an average accuracy of tens microGals, equivalent to a few centimetres in the geoid height signal, at a spatial resolution of about 250 km. In the meantime, the coverage and quality of surface gravity and elevation data over land and ocean areas has been also improving significantly with a number of airborne gravity campaigns in Arctic and Asia, several ongoing terrestrial gravity surveys worldwide, and a US space shuttle mission for the past decade. Due to these advances, the National Geospatial-Intelligence Agency, US, has been undertaking a revolutionary project to up-grade its benchmark Earth Gravity Model (EGM96) to a new improved version EGM07, with an expected average geoid accuracy of better than 20 cm and a spatial resolu-tion of 10 km, in order to meet the requirements from various scientific and industrial sectors. The upcoming European GOCE mission will be mapping the Earth’s gravity field with the same level of accuracy and a spatial resolution of about 100 km. A series of GOCE-based global gravity models are expected to be available in the next few years, contributing additional and valuable knowledge to global gravity field mapping. The evaluation of such EGMs is commonly based on comparisons with other “external” data sets that depend on the same gravity field. The EGM07 development team and various other centres responsible for the development of global gravity field models routinely perform such comparisons using a variety of validation data sets, such as geoid heights from GPS and spirit levelled heights, airborne and surface gravity measurements, marine geoid heights from mean oceanographic sea surface topography models and altimetry observations, orbits from other geodetic and altimetry satellites etc. In response to the call of having an independent, coordi-nated and inclusive team for the evaluation of the new EGMs, a Joint Working Group (JWG) was established between IGFS and the IAG Commission 2 in 2005. The main objective of the JWG is to develop standard valida-tion/calibration procedures, and to perform the quality assessment of GRACE-, CHAMP- and GOCE-based satellite-only and combined solutions for the static Earth’s gravity field, especially EGM07. For the past three years, many members of this group had conducted intensive evaluations for the CHAMP- and GRACE-based released models. Their contribution to the quality analysis and improvement of these models has been well recognized in the international geodetic community. Sig-nificant progress has been made in developing new vali-dation/calibration methods as well. Due to the ongoing demand for most of its objectives, the group will continue to work during the next four years towards the evaluation of global Earth gravity models, using existing and new validation procedures. Another equally important EGM evaluation topic is with respect to the temporal variation of the Earth’s gravity field features derived from GRACE monthly solutions. The repeated absolute/relative gravity measurements and super-conducting gravity observations provide the most accurate temporal variation on the ground level. As part of the initiative, validation/calibration methods for temporal gravity variation will be explored. Program of Activities
The Joint Working Group reports to IGFS and the Com-mission 2. Membership
Corresponding Members
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