BUREAU GRAVIMETRIQUE INTERNATIONAL (B.G.I.)

( = International Gravity Bureau )

Directeur : J-P Barriot (France)

OBJECTIVES AND MANDATE 

   Attributions of the BGI consist in collecting overall all the existing data of gravity and information available on the field of terrestrial gravity, to compile them and file in a data base in order to redistribute them with the request at a broad community of users to fine scientists. The data are various types: measurements with the gravimeter (localization of the site by three co-ordinates, value of gravity,…), average or specific values of the anomaly of gravity to the free air, gravimetric charts, description of station of reference, publications treating of terrestrial gravity. The BGI also lays out, via one of its organizations of support: grids of values geoid deduced from satellite altimetry, currently starting from missions Geosat, Topex-Poseidon, ERS1 and ERS2; series of coefficients (harmonic spherical) of the total models of the geopotential one; average values of topography. These data are sometimes used in an internal way for the validation of measurements and analyses geophysics.

  The BGI developed various algorithms and software of validation and analysis of data, like its own system of computerized management, based on the server of relational data bases ORACLE (actual version: 10G).

  Varied services are proposed to the users (see hereafter). All kinds of gravimetric data can be transferred to the BGI, with or without restriction of redistribution specified by the supplier, sometimes in the form of a protocol of use.

STRUCTURE AND MEMBERSHIP

  The BGI is one of the offices of the Federation of the Abstracting services of Data Astronomical and Geophysics (FAGS). It is also in a certain manner an executive organization of the International Gravimetric Commission (IGC) of the International association of Geodesy (IAG), which is one of seven associations constituting the Union of Geodesy and International Geophysics (IUGG). It profits from the support of 8 French organizations:

                              Le B.R.G.M , Bureau de Recherches Géologiques et Minière
  Le C.N.E.S , Centre National d'Etudes Spatiales

                           L'I.R.D , Institut pour la recherche et le Développement

                   Le C.N.R.S , Centre National de Recherche Scientifique
                                                           et l'I.N.S.U , Institut National des Sciences de l'Univers

                                                                                                       L'I.G.N , Institut Géographique National

                                         ESGT                 L'E.S.G.T , Ecole Supérieure des Géomètres Topographes

                                                                                                                   Le S.H.O.M , Service Hydrographique de la Marine

                                                                                          L'O.M.P , Observatoire Midi-Pyrénées
                                                                                            within the UPS, Université Paul Sabatier of Toulouse provides the lodging covered by the C.N.E.S within the framework of a
                                                                                            convention U.P.S-C.N.E.S

    A convention between these organizations guarantees their support for the BGI. 

   The BGI has a personnel made up of four people (two engineers full-time, an engineer part-time and an assistant engineer), of a secretary and the Director.

   The address of the BGI is: International Gravimetric office 18, Which occurred Edouard Belin 31400 Toulouse Cedex 4, France
   Telephones: (33) [0] 5 61 33 29 80 Faxes: (33) [0] 5 61 25 30 98  and e-mail: bgi@cnes.fr.

The NEWS BULLETIN OF the BGI

   The Office publishes in connection with the IGES (International Géoid Services in Milan) a News bulletin: Newton' S Bulletin once per annum generally in December. This one contains:
    * general information in the field,
    * the communications to scientific meetings referring to gravimetry.
    The bulletin is published jointly on the site of the BGI and the IGES and on paper. The first number goes back to December 2003.

   

TO PROVIDE DATA TO THE BGI

  Principal information and quantities to provide for measurements of gravity are as follows:

 (a) Position of the site:
   * latitude, longitude (with the best possible precision)
   * altitude (or depth). for the terrestrial data: altitudes of the site (on the physical surface of the Earth)
  for the marine data: depth of water.

 (b) measured (observed), corrected Gravity periodic gravitational effects of the Moon and Sun and instrumental drift.

 (c) Station (S) of reference or absolute used (S): such a station is a documented site, where a measurement of gravity was already taken (thus known) and is used to provide the
  reference and the scale of a whole of measurements which are attached to him. Give the name, the number of the station (if it is known), a short description of the site and the value
  of reference used in this station.
     Give the name of the network to which it belongs, for example: IGSN71.

     Give any information complementary to altitude for the measurements made on lathes, in top of buildings, inside ships or of tunnels, on the ice. If it is necessary, indicate if the
  value given is that observed at  the point of measurement or if it were already reduced on the physical surface of the Earth (ground or surface of water). For the measurements taken
  under water, give the depth to which the observations were made. For measurements on boat, the value of gravity should be corrected effects of movement, either this correction itself
  should be provided or clearly explained.
     Any additional  information is optional but welcome.

    The base of the BGI currently contains 10,5 marine measurement million (validated coarsely by tests of coherence by cruising) and 2,2 million terrestrial measurements
 (validated source by source, by a method of collocation) More than 10.000 stations of reference are presented on the site of which about half illustrated by shemes.
    Nearly 300 absolute stations are also described.

SERVICES

 The services most frequently required relate to the retrieval of data on a delimited area: the data are provided on CD-ROMS, are transferred by ftp or are sent by email. Visualizations showing the cover of the data can also be provided, usually for blocks of 20°*20°. Since the end of 1995, the BGI provides the whole of its nonconfidential, terrestrial and marine data, on CD-Rom, with the software of extraction (under Unix). The recordings contain essential information: geographical localization, value of G, type of measurement, anomalies to the free air, of Bouguer, correction of ground if calculated, date of measurement.

Among the other services, let us quote:

* the sampling of the data
* supply of information on stations of reference
* the evaluation of the data of the user and the constitution of grids
* the calculation of average values
* the cartography of the data by calculation and layout of isanomals
* supply of charts, or corresponding information (there is a catalogue)

  The cost of the services was fixed according to categories of users (generally of the scientists and the suppliers of data), and within sight of the important support of the French organizations. These costs are detailed in the news bulletin. Certain services can be provided free on request to the suppliers of data, with the academics, students, and in general with any person contributing in a certain manner to the activities of the Office, for example on a basis of information or data exchange.
For any request for data, contact: bgi@cnes.fr


FUTURE WORK PROGRAMME

* Continuation of the publication of Newton' S Bulletin
* Continuation of the data acquisition, filing and the distribution: the stress will be laid on the countries or not having little contributed to the base of the BGI. The priority will remain given to the precise evaluation of measurements. The validation of the data at sea, after some tests were led by employing methods of examination and minimization of the variations to the intersections of the ways, will be rather made by using the field of gravity to the free air deduced from satellite altimetry.
* Acceleration of the seizure, validation and put on line of the absolute gravimetric data as well as the publication of the localizations of the absolute stations.
* Organization of Schools of information and formation every approximately 2 years in the line of the Schools of summer of Leuwen the New one (2002) and of Lanzarote (2005).
* Continuation of the bonds with the Commission on the Geoid within the framework of the preparation and the evaluation of data having to be used by the International Service for the Geoid (IGeS).
* Regular update of the bibliographical data base and publication on the site.
* Participation in international programs of study (European geoid, Ligure project…).
 For further information, contact: bgi@cnes.fr