Governments have a large number of basic data which can be of economic and social value to society. Along those lines, more and more European countries are developing policies to release this data as Open (Government) Data. In 2003, the European Union (EU) adopted legislation to foster the re-use of Public Data in Member States via the Public Sector Information (PSI) Directive 2003/98/EC, which was revised in 2013 (Directive 2013/37/EU). The main amendments are the adoption of the "open by default" principle, the breakaway from cost-based charging for PSI towards a marginal cost-oriented fee and increased transparency regarding calculation of the fees, the inclusion of certain cultural institutions as public sector bodies (previously outside the scope), and support to machine-readable and open formats.
In response to the requirements of the revised PSI directive, European public administrations have set up cross-domain and horizontal Open Data portals. Open data portals have significantly contributed to the establishment of the necessary foundation for a European open data ecosystem. But limitations appeared soon. In addition to the inherent political and cultural diversity in Europe and to multilingualism, the development of Open Data portals has not always been coordinated within or across countries. The use of different platforms and the lack of common semantics have resulted in a fragmented landscape of Open Data portals as disconnected information islands, making it hard to exchange metadata between them. This situation leads to duplication of information and inconsistencies, it hampers cross-portal search and limits the discovery of datasets.
Overcoming the limitations described above was only possible if the different portals with different descriptions of metadata would adhere to a common metadata language. This paper describes how such a common metadata language, DCAT-AP, was developed and implemented in national data portals and in the European Data Portal (EDP), how implementation challenges can be overcome and which benefits the DCAT-AP brings to data consumers, data providers, portal owners and society as a whole.