The tidal observatories of the Geophysical Institute have been recording tidal data since 1952 (Pribram, later Rimov, Kasperske Hory, Lazec and Jezeri-Jezerka). In 1965-68 the first international comparative measurements of various models of tiltmeters took place in Pribram. The recorded data provide information on temporal variation of the Earth’s gravity field and on the deformation of the Earth’s body induced by the tide generating forces of the Moon, the Sun and the planets. The world-wide observations are promoted by the Earth Tide Commission of IAG (Intern. Assoc. of Geodesy) in order to obtain data for the assessment of local, regional and global deformation models of the Earth. In the 1990s two sites were selected for further observations with respect to favourable conditions, one of them for the research of tidal effects (Lazec – a part of the world network) and the other one for applied research at a coal mine gallery (Jezeri – observation of rock massif stability).

The tidal group under the leadership of Lumir Skalsky is located at the testing observatory in Pribram. At this site all the data are collected and proceeded.

1. The tidal observatory Lazec in South Bohemia [48^o50’12’’N; 14^o16’58’’E] has been established in a graphite mine gallery as a research station. Special chamber was cut for the installation of two perpendicular self-produced extensometers with 20 m long pure quartz tubes, tested for both capacity and inductance recording systems. Two tiltmeters ASNS with capacity sensors and resolution of 0.0001” record tilts in both N-S and E-W directions. Gravity meter BN27 (adjusted Askania GS11) with additional control of internal temperature records continuously the gravity acceleration. This equipment is comparable with similar observatories in Europe. All instruments are connected to digital recording system with modem data transfer to the processing centre in Příbram. Besides a number of in-house software, the Preterna and Eterna packages (Germany) are used for data processing and analysis.
2. At the observatory Jezeri 1 [50^o33’19’’N; 13^o30’19’’E] in the coal mining district of North Bohemia there are only two Ostrovsky’s tiltmeters with photoelectric registration located in a horizontal gallery in the marginal slope of an open pit mine. These instruments will be replaced by ASNS tiltmeters in the near future in order to provide digital data just as well. The second station Jezeri 2 is under construction with the aim of providing data from the basement-sediments contact zone. The two stations will act as part of the stability monitoring system of the mine.