A requirement for solving Earth rotation problems is the definition of a terrestrial reference system (or systems) and the determination of a terrestrial coordinate frame (or frames). In this context it is important to determine the positions of the observing sites and their temporal variations and establish the dependence of the observations on local conditions and observing methods.

In situ concurrent observations are therefore important. In this paper we report the results of a comparison between astronomical and geodetic coordinates derived from different observations at the Cagliari Observatory and the Carloforte latitude station.