Interoperating Geographic Information Systems (GIS) poses several challenges. First, despite OpenGIS Consortium recommendations, GML is an emerging standard. Second, each GIS provides its own proprietary format as well as its specific query language; while geographic resources are designed for a variety of different purposes. Finally, orthogonal directions in the design of geographic resources may affect the semantics of the data they contain and impair their integration. With the proliferation of GIS data and resources over the Internet, there is an increasing demand for robust geospatial information services that allow federation/interoperation of massive repositories of heterogeneous spatial data and metadata. The purpose of this paper is to show how mediation – a data integration technique – can help in building such a Web-based required geospatial service. This technique has been fully implemented in the context of a geographic mediation/wrapper system that provides an integrated view of the data together with a spatial query language. As a proof of concept, we deployed the service in building a prototype for an interoperability application involving several catalogues of satellite images.