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Jean-Pierre Barriot
International Gravimetric Bureau, France
THE DATABASE OF THE INTERNATIONAL GRAVIMETRIC BUREAUAND ITS SERVICES (HTTP://BGI.CNES.FR)
БАЗА ДАННЫХ И УСЛУГИ МЕЖДУНАРОДНОГО ГРАВИМЕТРИЧЕСКОГО БЮРО
J.-P. Barriot, M. Sarrailh, B. Langellier, S. Pecquerie and T. Fayard
Key Words: gravimetry, database, validation
Summary: The International Gravimetric Bureau is a non-profit organization that is collecting, validating and redistributing gravimetric data all over the world, for academic and business purposes. We give in this paper a detailed account of all the Services provided by BGI.
THE INTERNATIONAL GRAVIMETRIC BUREAU: AN OVERVIEW The International Association of Geodesy established the International Gravimetric Bureau (BGI) in 1951. It is one of the Bureaus of the Federation of Astronomical and Geophysical data Analysis Services.
The main task of BGI is to collect, on a world-wide basis, all gravity measurements and pertinent information about the gravity field of the Earth, to compile them and store them in a computerized data base in order to redistribute them on request to a large variety of users for scientific purposes. Its functional diagram is given in Figure 1. The data consists of: gravimeter observations (mainly location - three co-ordinates, gravity value, corrections, anomalies...), mean free air gravity values, gravity maps, reference station descriptions, publications dealing with the Earth's gravity field. Other data types are sometimes used for data validation and geophysical analysis, such as satellite altimetry derived geoid height and gravity anomalies, digital terrain models, and spherical harmonic coefficients of current global geopotential models. The coverage of the current gravity database is given in Figure 2.
Interpretation
Users
A SCIENTIFIC DATA ANALYSES SERVICE
Figure 1: The functional diagram of BGI
13F 210* 240* 270* SOC? SSO* & SO* 60* 90* 120* Î501
Figure 2: Coverage of the current BGI gravimetric database (around 12 millions
data points)
BGI has been developing various algorithms and software for data validation (see Figure 3) and analysis, as well as its own data management system under Oracle.
Figure 3: Data validation and outliers identification at BGI (DIVA software)
All kinds of gravity data can be sent to BGI, with or without restrictions of redistribution to be specified by the contributors, sometimes in the form of a protocol of usage.
MAIN SERVICES PROVIDED BY BGI
The most frequent service BGI can provide is data retrieval over a limited area (see Figure 4). For security reasons, direct online electronic download is not allowed. Data are transferred electronically by ftp or sent on CDs. Data coverage plots may also be provided, usually over 20° * 20° areas. Cases of massive data retrieval requests may be considered; they are studied and may be processed in a specific way. The simplest way for users is to acquire the open files of the BGI database, which are on two CDs.
Other services include:
- Data screening.
- Provision of gravity base station information.
- Data evaluation and gridding.
- Computation of mean values.
- Contouring.
- Supply of, or information on existing maps.
Figure 4: Gravity data coverage visualized on the BGI server
The costs of the services have been established in view of the categories of users-mostly contributors of measurements and scientists, and also considering the large amount of our host organizations. The charging policy is explained online in detail on the BGI server.
Some of the services may be provided free of charge upon request, to data contributors, individuals working in universities, such as students, and generally to any person who can contribute to the BGI activities on a data or documentation exchange basis.
ANCILLARY SERVICES PROVIDED BY BGI
- Publication of the Newton’s Bulletin, a reviewed joint publication of BGI and of the International Geoid Service, available free of charge on the Web site.
- Computation of gravity maps (free-air or Bouguer anomalies). Satellite altimetry derived free-air anomalies are to be more and more frequently used to validate sea measurements.
- Set up of archiving formats (relative and absolute gravity measurements).
- Help to the International Gravity Field Service of IAG in data preparation in view of geoid model computations and geoid model evaluations.
- Promotion of satellites techniques to improve our global knowledge of the Earth's gravity field: satellite-to-satellite tracking, satellite gradiometry, etc.
- Bibliographic database (technical reports, grey literature).
- Gravimetric measurements (small surveys).
- Relative gravimeters data acquisition cards.
- Summer Schools on gravimetry.
PROVIDING DATA TO BGI
Essential quantities and information for gravity data submission are:
Position of the site with datum references:
- Latitude, longitude (to the best possible accuracy).
- Elevation or depth.
- For land data: elevation of the site (on the physical surface of the Earth).
- For water stations: water depth.
Measured (observed) gravity, corrected to eliminate the periodic gravitational effects of the Sun and Moon, and the instrument drift.
Reference (base) station(s) used. For each reference station (a site occupied in the survey where a previously determined gravity value is available and used to help establish datum and scale for the survey), give name, reference station number (if known), brief description of location of site, and the reference gravity value used for that station. Give the datum of the reference value; example: IGSN 71.
Give supplementary elevation data for measurements made on towers, on upper floor of buildings, inside of mines or tunnels, atop glacial ice. When applicable, specify whether gravity value applied to actual measurement site or it has been reduced to the Earth's physical surface (surface topography or water surface). Also give depth of actual measurement site below the water surface for underwater measurements.
For marine gravity stations, gravity value should be corrected to eliminate effects of ship motion, or this effect should be provided and clearly explained.
BGI is also collecting all available information about reference gravity stations, including photos and sketches.
BIBLIOGRAPHICAL NOTE
Jean-Pierre Barriot is the Director of BGI. He is currently working as a Senior Researcher at the Observatoire Midi-Pyrenees in France. He authored or co-authored more than fifty papers in the frame of Geodesy and Radio-Sciences. He is also Scientific Investigator in several space missions, and the General Secretary of the French Branch of the International Union of Geodesy and Geophysics. He is fellow of the International Association of Geodesy.
CONTACT
Bureau Gravimetrique International, Observatoire Midi-Pyrenees, 14, Av. E. Belin, 31400 Toulouse, France, Tel: 33561332894, Fax: 33561253098, email: bgi@cnes.fr, web site: http://bgi.cnes.fr
© Jean-Pierre Barriot, 2005
