French Activities in Ground Gravimetry During the Period 2003~2006
III. IMPROVEMENT OF THE NATIONAL GRAVITY NETWORK AND COVERAGE NETWORK AND COVERAGE
III.1 The integrated geodetic and gravimetric network
The first (CGF65) and second (RGF83) French gravity networks were established in 1965 and 1983 respectively [CNIG, 2000]. After the recommendations of the Comité National Français de Géodésie et
Géophysique (the French branch of the IUGG) [Louis, 1996] and of the Conseil National de l’Information Géographique [CNIG, 2000], the Institut Géographique National (IGN) begun systematic
measurements of gravity on 1020 points of the French GPS geodetic network (RBF), on the points of the permanent GNSS network, on the nodes of the national first order leveling network, on tide gauges and on points of the older gravity networks.
Conversely, since 1998, the Ecole et Observatoire des Sciences de la Terre as well as the Laboratoire de Dynamique de la Lithosphère of Montpellier University have undertaken absolute gravity measurements using Micro-g FG5 apparatus (see section “absolute gravity activities” below).
This work aims at:
renovating the old gravity network,
setting up an integrated geodetic and gravimetric network in accordance with the recommendations of EUREF,
verifying and improving the old gravity coverage in view of a more accurate geoid model.
Integrating the gravimetric and the geodetic network presents decisive advantages:
to better meet the scientific needs, mainly in geodesy and geodynamics,
to reduce the cost of
monumentation and maintenance.
An accuracy of 10 108 m s2 (10 µGal) is expected. IGN has planned to measure 200 points by absolute and relative gravimetry
and 900 points by relative gravimetry.
Relative measure-ments started from 2000 with Scintrex CG3M gravimeters owned by the Institut National des Sciences de l’Univers and the Royal Observatory of Belgium. In 2005, the Institut de Recherche pour le Développement (IRD), the Institut de Physique du globe de Paris (IPGP) and the IGN purchased together a Micro-g A10 gravimeter and absolute measurements begun immediately. The stations measured during the campaigns 2000~2006 are shown on Figure 3; see also [Gattacceca, 2005].
In order to assess the correspondence between old and new gravity networks and therefore insure a correct transition of the French Bouguer gravity map into the new Réseau et Carte Gravimétrique de la France, BRGM has been measuring since 2005 relative gravity ties between the RGF83 and the new gravity base stations [Martelet and Debeglia, 2006]. These ties also include the recently established and measured FG5 order-0 absolute reference sites: 7 new determinations were financed by BRGM during the period 1999-2006 and 4 are in project for 2007. In the same time, vertical gradient measurements were performed on these absolute sites.
In the current state of the measurements, most RGF83 and recent base stations have been found coherent,
with respect to their confidence level, except 3 of them, which differ from about 100 µGal. Whereas significant for metrologic applications, these local discrepancies have a negligible influence on the French gravity map.
In parallel, investigations were carried out at IGN to improve the procedures to correct and adjust the
measurements. Special attention was given to ocean loading correction, gravity gradient modeling and software development, see [Beilin, 2005] and [Duquenne, 2007a].
IGN plans to achieve this combined network and to compute a new model of the geoid by the end of 2008.
Figure 4: Bouguer anomalies prediction discrepancies: this represents the uncertainty of the French Bouguer map.
III.2 Quality control of the French gravity map and network
The two existing French gravity networks CGF65 and RGF83 and a large part of the national gravity coverage were established
under the supervision of BRGM. These data are now integrated in digital form, in the « Banque Gravimétrique de la France »
(BGF), under the responsibility of the BRGM, and can be accessed via the Infoterre Web portal (http://www.brgm.fr).
In order to improve its national geoid, and be able to address high accuracy gravity-related issues, France plans to update its national gravity network and gravity map by the end of year 2008. In this context, a careful validation and improvement of the existing
gravity data is necessary since the existing gravity stations (about 414000) and base stations (more than 1000) should be tied to the new gravity references.
Integration of new gravity network measurements performed by IGN since year 2000 and existing ones requires that both the gravity data and their position be controlled. The controls address possible distortions:
Figure 5: Limits of the respective areas covered with leveled GPS points, gravity data and DTM of the
“Alvernia” data set.
− between the new and old networks:
field gravity tie measurements are done,
− between the new network and the gravity coverage: gravity prediction is performed at the new network base locations using the surrounding gravity
measurements. For this purpose, the accuracy of both the gravity and position of the ~414000 gravity stations must be carefully verified.
The French gravity database (BGF) contains about 414000 gravity stations, divided into 173 surveys.
1. For each survey, in order to identify and correct existing systematic shifts of the gravity stations, methods minimizing the distance of gravity stations to a detailed DTM or to the roads were implemented [Martelet and Sarrailh, 2006] and allowed to rectify the positioning of 41 surveys, at the 1/25000e scale.
2. Once the modifications of positioning were applied, terrain corrections have been re-computed. Indeed, since the main sources of error of the terrain effect are erroneous altitude differences between the stations and the DTM, significant improvement of the terrain corrections was achieved
[Martelet and Debeglia, 2006].
3. In order to assess the overall accuracy of the French gravity database, prediction of the Bouguer anomaly for each station of the database, using the surrounding gravity stations, was performed [Martelet and Debeglia, 2006].
This allowed to: (i) point out about 300 erroneous Bouguer gravity stations (mainly located in the mountainous areas), (ii) quantify the internal accuracy of the French Bouguer map (Figure 4). For 94%
of the stations, Bouguer uncertainty is lower than 1 mGal, the mean uncertainty being around 0.3 mGal.
III.3 Quality control of geoid models
As mentioned above, IGN has planned to compute a new model of the geoid in 2008. This model should take advantage of the integrated geodetic and gravimetric network and of the improvement of the gravity coverage. In addition, IGN has begun a study on quality control of geoid models or other representations of the
gravity field. As a first stage, IGN has built the “Alvernia” data set to test methods and software for geoid modeling.
It is presently composed of gravity and terrain data and leveled GPS points, see [Duquenne, 2007b], Figure 5 and ftp://lareg.ensg.ign.fr/pub/duquenne/Test_
data_set/.