1. NKG Working Group for Geodynamics Copenhagen, 23 –24 April, 2008 1 AGrav - Establishing a Database for Absolute Gravity Observations Herbert Wilmes, Hartmut Wziontek, Reinhard Falk Federal Agency for Cartography and Geodesy, Frankfurt (BKG) Sylvain Bonvalot Bureau Gravimétrique International, Toulouse (BGI) NKG Working Group for Geodynamics Copenhagen, Denmark, 23 – 24 April 2008

2. NKG Working Group for Geodynamics Copenhagen, 23 –24 April, 2008 2 Motivation Experiences made with UNIGRACE, ECGN, NGOS and others Steadily growing number of absolute gravimeters and absolute gravity measurements Demand for an overview about existing locations, observations, instruments and institutions involved New questions from “GGOS”, “Global change” “Mass transports” etc. Cooperation and coordination of measurements Absolute gravity data from a period of about 20 years available and “potentially in danger”

3. NKG Working Group for Geodynamics Copenhagen, 23 –24 April, 2008 3 Growing number of absolute gravimeters Owners: Value of the data grows! Long-term availability of absolute gravity data ? Metrological, Geodetic, Geologic and Geophysical Institutions Complemented by other instrument types: e.g. JILA-G, A10, IMGC, GABL, Faller etc.

4. NKG Working Group for Geodynamics Copenhagen, 23 –24 April, 2008 4 AG observations by BKG, 1993 - 2006 Observations with FG5 absolute gravimeters Contribution to a global gravity network

5. NKG Working Group for Geodynamics Copenhagen, 23 –24 April, 2008 5 Absolute gravimetry network (NGA) (National Geospatial Intelligence Agency, USA) Need more co-located observatories (VLBI, Laser, GPS, abs.grav., …) Many regional initiatives - ECGN/Europe, NGOS/Nordic..).. Courtesy: S. Kenyon

6. NKG Working Group for Geodynamics Copenhagen, 23 –24 April, 2008 6 AGrav - Absolute Gravity database  Global database of absolute gravity measurements:  Definition of standards for data exchange  Free exchange of meta-data: Who measured where and when?  Provision of contact information  Sharing gravity values among contributing groups  Map based web-interface to access meta-data and/or processing results  Collaboration: BKG and BGI, 2 mirror sites: Frankfurt: http://agrav.bkg.bund.de Toulouse: http://bgi.dtp.obs-mip.frhttp://agrav.bkg.bund.dehttp://bgi.dtp.obs-mip.fr

7. NKG Working Group for Geodynamics Copenhagen, 23 –24 April, 2008 7 International Gravimetric Bureau Absolute Gravity Database BKG database prototype installed & tested at BGI (S. Bonvalot, visit of H. Wziontek – Oct. 2007) – Additional functionalities Worldwide data collection in progress at BGI Two mirror sites : BGI & BKG

8. NKG Working Group for Geodynamics Copenhagen, 23 –24 April, 2008 8 Agrav User Interface: Web-based Application (2) Access to meta-database or detailed processing results with one user interface: Complete data access: Access restricted to contributing groups Meta data and gravity values if available) complete processing details

9. NKG Working Group for Geodynamics Copenhagen, 23 –24 April, 2008 9 Data Exchange Format Definitions: Observation epoch: Period of an instrument set up and continuous data acquisition. Short interruptions due to technical demands are allowed. More than one processing result per observation epoch is possible and a final gravity value can be given outside automated processing schemes. Location: One or more gravimetric points/monuments in close spatial relationship with equal environmental (atmospheric, hydrologic) conditions. Usually the distance between such points should not be greater than a few 100m Required information: Meta-information - location: site name, coordinates - instrument: type, serial number and additional properties - period of the observation epoch Observation and processing details: - gravity value, error estimates - applied reduction models - vertical gravity gradient Simple structure ASCII format to be created without additional effort  For FG5 and A10 users project-files created with the “Absolute Gravity Data Acquisition and Processing Software” (g-Soft) by Micro-g/LaCoste already meet these requirements! Since most of the AG operators use g-Soft, the preparation is a by-product of standard processing, no additional is necessary! Other users can generate such a file easily. “g”-Software project file

10. NKG Working Group for Geodynamics Copenhagen, 23 –24 April, 2008 10 Data Upload (1) Insertion of new data by the user data under direct control of the user, the user decides whether only meta data should be made available or the gravity value no additional personnel necessary  Web-form was developed to upload new or update existing data. Project-files are processed by a Perl-script on the server. Step 1: Definition of the Location (Site) choice of an existing station from the list or definition of a new station Consistency check: Combination of country code / site code must be unique! Step 2: Definition of the Point/Monument at the specified Location (Site) choice of an existing point from the list or definition of a new point Now project files can be chosen for the particular point! Decision, to upload meta data or the complete data.

11. NKG Working Group for Geodynamics Copenhagen, 23 –24 April, 2008 11 Data Upload (2) The project files are uploaded to the server and information stored in the data-base. If meta-data was chosen, all gravity related information is deleted. Script output helps to identify possible inconsistencies. The data can be edited manually after upload. Problems of this approach: Assignment of consistent site codes in responsibility of the user alone! Inconsistencies must must be avoided! No conventions for site codes, teams use their own naming schemes! Site- and point-codes not distinguished in project file

12. NKG Working Group for Geodynamics Copenhagen, 23 –24 April, 2008 12 Conclusions Growing number of absolute gravity measurements Demand of an overview about existing locations, observations, instruments and institutions For both purposes, a relational database was designed and implemented and has reached the operational status now. Prospectively, the database will contribute to the realisation of an international gravity reference and serve as a basis for geophysical interpretation of absolute gravity on a global scale. Database can ensure the long-term availability of absolute gravity data for e.g. the maintenance of reference frames or the monitoring of global change processes. Database available for the NGOS community to include AG data and work with the database as individual institution or as a group. Comments and proposals are welcome!