Geodetic activities in Finland 2006 International Association of Geodesy (IAG) Report for the IUGG National Committee, Compiled by Markku Poutanen Finnish Geodetic Institute (FGI) 1 Finnish Geodetic Institute 1.1 Reference frames and the permanent GPS network, FinnRef The Finnish Geodetic Institute hosts the permanent GPS network FinnRef, which consists of 13 permanent GPS stations. The network is the backbone of the Finnish realisation of the European reference system ETRS89. The National realisation of the reference frame is called EUREF-FIN. Connections to the neighboring countries and international networks, and maintaining of the reference frame are the responsibilities of the FGI. Four stations in the FinnRef network (Metsähovi, Vaasa, Joensuu, Sodankylä) belong to the EUREF permanent GNSS network (EPN), and one station (Metsähovi) belongs to the network of the International GNSS Service (IGS) of the IAG. Through these stations FinnRef creates a connection to the global reference frames and the stations are used for maintaining global reference frames and global geodetic studies. The data recorded in the EPN stations are transferred automatically hourly to the EPN regional data centres in Austria and Germany. The data are also transferred to the Onsala Space Observatory in Sweden for the BIFROST project. The FinnRef stations are also used for local studies on crustal movements as well as a reference for local and national GPS measurements. The absolute gravity observations were made at six GPS stations, viz. in Metsähovi, Vaasa, Joensuu, Sodankylä, Kuusamo and Kevo with FG5 gravimeter. Related to the IAGs project GGOS (Global Geodetic Observing System), the Nordic plan, NGOS (Nordic Geodetic Observing System) was continued as a task force of the Nordic Geodetic Commission. Markku Poutanen was the chairman of the task force, and a member of the GGOS steering committee. GGOS will be the most important geodetic effort in the coming years, and in NGOS the development of GGOS will be followed. Via GGOS geodetic data will be made available to the great public and researchers in a well controlled and uniform way. The Finnish Geodetic Institute (FGI) has participated in national committees and working groups in order to advance the usage and help in practical questions of the new reference frame, EUREF-FIN, in Finland. Also, several lectures and presentations were given. 1.2. Height systems The field work of the Third Precise Levelling of Finland, started in 1978, was completed in 2004. A total of more than 9000 km of double-run lines were measured, comprising more than 6000 benchmarks. Data were analysed, and a common Nordic adjustment was made on the network around the Baltic Sea to tie the national networks firmly into the European network. Based on the results of the adjustment, the new national height system N2000 was made following the guidelines of the European EVRS2000, and those agreed in the Nordic Geodetic Commission. N2000 will coincide with the new Swedish height system better than 2 mm at the border of Finland and Sweden in the valley of Tornio River. 1 The subcommission EUREF started in 2004 the project, which is called EUVN_DA (European Unified Vertical Network Densification Action). The purpose of the project is to densify the GPS/Levelling network in Europe so that the distance between the neighbouring stations will be less than 100 km. The minimum GPS observation time at a point should be at least 24 hours, and the point should be connected by precise levelling to the closest nodal point of the UELN network. A total of 30 GPS/levelling points were measured in Finland in 2005. The results were presented to the EUREF commission in the annual meeting in Riga and published in the series of the FGI. One connection to the Russian border was levelled in 2006 thus completing the project of measuring connections over the Finnish-Russian border. Joint publication of the project was published in the series of the FGI. 1.3. Gravimetric works 1.3.1 Absolute gravimetry In December-January 2005/2006, FGI performed absolute gravity measurements at the Finnish Antarctic base Aboa with the FG5-221. Previously, FGI had measured at Aboa in 1994, 2001 and 2004. Repeated measurements are used for geodynamical studies. Single occupations provide reference values for gravity surveys. Repeated measurements were made also at the South-African Sanae IV, and the Russian Station Novolazarevskaya. The Nordic Absolute Gravity Project was launched in 2003. It aims at producing a time series of absolute gravity measurements at about 20 Nordic sites. The time series will be compared with estimates of regional mantle inflow (due to the Fennoscandian postglacial rebound) obtained from the ongoing mission of the GRACE gravity satellite. As a by- product, a highly accurate gravity reference network with estimates of gravity change rates will be produced. The absolute measurements are performed by IfE (Institut für Erdmessung, University of Hannover), Norwegian University of Life Sciences, BKG (Bundesamt für Kartografie und Geodäsie, Frankfurt), and FGI, in cooperation with the Danish Space Center, the Norwegian Mapping Authority, and Lantmäteriet (Sweden). In 2006 FGI observed at 7 sites. The project is coordinated by the Working Group for Geodynamics of the Nordic Geodetic Commission (NKG), as a part of the Nordic Geodetic Observing System (NGOS) of the NKG. A comparison between IfE (Institute für Erdmessung, University of Hannover, Germany) and FGI gravimeters took place in Metsähovi. At Metsähovi the gravity is measured regularly, and fhe absolute gravity has been observed in Metsähovi more than 80 times since 1988. 1.3.2 Relative gravimetry Studies on the Keurusselkä impact structure in Central-West Finland were continued by detailed analyse of the 2005 gravity survey. Measured four gravity profiles (NS, EW, NE- SW,NW-SE) and densification of the gravity net of the Finnish Geodetic Institute reveal a Bouguer anomaly minimum in the impact area and there exists a small local anomaly maximum in the shatter cone area. The size of impact structure is still unknown and new gravity measurements are needed for investigation of the structure details. The studies on the Keurusselkä impact structure continue also by comparing gravity anomaly result with other geophysical investigations (FIRE-seismic, magnetic, petrophysical, topography etc.) in cooperation with the Solid Earth Geophysics section of the Department of Physical Sciences in the University of Helsinki. 2 1.3.3 Superconducting gravimetry The superconducting gravimeter GWR T020 at Metsähovi has been operational since August 1994. It participates in the GGP (Global geodynamics project), where 20 SG’s are deployed worldwide. In addition to the gravity data, the recordings consist of air pressure, groundwater level and precipitation. Studies of gravity data comprise the gravity spectrum from microseism to Chandlerian period. The research work has been focused on loading effects from air pressure field and the Baltic Sea. Use of the superconducting gravimeter for hydrological studies is investigated in cooperation with the Helsinki University of Technology (Department of Rock Engineering), Finnish Environment Institute, and the Geological Survey of Finland. Modelling of the Influence of Atmospheric Masses and Baltic Sea Level on Gravity in cooperation with the Finnish Meteorological Institute and the Finnish Institute of Marine Research continued. The detailed HIRLAM (High Resolution Limited Area Model) air pressure grid gives gravity correction models that during special events like the passage of strong anticyclones differ up to 10 nm s –2 from simple regression models. This improves the detection and modelling of signals correlated with air pressure, e.g. Baltic Sea level variations. Hydrological loading effects were calculated using both the high accurate model for Finland (Watershed Simulation and forecasting System, WSFS, by the Finnish Environment Institute) and the Climate Prediction Center global soil moisture data set (CPC). Results were compared and evaluated with SG and local permanent GPS observations. FGI participated in global and European studies where the variation in surface gravity observed point wise with the SGs is correlated with variation in regional gravity observed with the satellites CHAMP and GRACE, and with the variation in regional gravity predicted from global models of terrestrial water storage. One Doctoral Thesis on the topic was completed in 2006. 1.3.4 Satellite gravimetry GRACE time series were analysed and compared with time series from the superconducting gravimeter in Metsähovi. Furthermore, surface mass variations were estimated from the GRACE time series and they were compared with variations in global watershed models and the Finnish Watershed Simulation and Forecasting System of the Finnish Environment Institute. Good correlations were found. 1.3.5 Geoid models Work was done to calculate two new geoid models for Finland based on the new Nordic geoid model, NKG2004, and a new network of GPS and levelling data. The first model is for use with the old height system and the second with the new height system. Both models will be published in 2007 when the new height system for Finland is introduced. 1.4. The Metsähovi Research station The Metsähovi research station was founded in 1978 and it has through the years become an essential part of the activities of the FGI. Currently, it is globally one of the most versatile space geodetic station. The instrumentation