TripleGeo-CSW: A Middleware for Exposing Geospatial Catalogue Services on the Semantic Web

A wealth of data and services are available on the Web, and often have geographical context as well. But the vast quantity of offered geospatial information is rather difficult to explore, and its quality hard to assess, due to lack of sufficient metadata. Hence, the Open Geospatial Consortium has specified application profiles for publishing, accessing, and searching over collections of spatial metadata with standardized Catalogue Services for the Web (CSW). Unfortunately, existing spatial metadata remain largely unexploited by Semantic Web technologies. In this paper, we introduce TripleGeo-CSW, a middleware that can be used to discover metadata from existing CSWs through a virtual SPARQL endpoint. Acting as broker between a request (in SPARQL) and catalogue services (in XML/GML), this platform can provide on-the-fly information (as RDF triples) on available geodata according to multiple, user-specified criteria (e.g., area of interest, date of last update, keywords). As a proof of concept, we have set up an instance of this middleware against CSWs from public authorities across Europe, which involve datasets complying with the EU INSPIRE Directive. Our experience testifies that TripleGeo-CSW can assist stakeholders to repurpose existing CSWs with minimal overhead and readily expose spatial metadata on the Semantic Web.