Virtual Hubs for facilitating the use of geospatial datasets from heterogeneous data sources

The world of geoinformation is characterized by a great complexity with many actors involved including: Data (and information) producers who acquire observations (e.g. through sensors) or generate value-added information (e.g. through data processing); Data providers who distribute data, managing data centres, long-term preservation archives, Spatial Data Infrastructures, etc.; Overarching initiatives that influence the geoinformation world designing new solutions, building disciplinary or interdisciplinary systems of systems, managing high-level expert groups, etc.; Technology providers who develop and distribute technological solutions for geospatial data management and sharing; Application developers who make use of data to build applications for end-users; End-users who utilize data. In such a context, interoperability is perceived as a main issue even limiting to technological aspects. Indeed, all of actors have a substantial impact in terms of technological choices: Data (and information) producers are mostly focused on data and metadata models and formats defining specifications such as HDF, netCDF and GRIB for EO data, ESRI Shapefile or OGC GML for feature type information; Data providers are mainly focused on data sharing services defining and adopting solutions like OGC discovery and access standards, TDWG specifications, THREDDS data servers; Overarching initiatives influence technological aspects in several ways, in particular on data, data harmonization, and data sharing including policy; Technology providers contribute to the heterogeneity providing many different competing solutions for geospatial data sharing; Application developers increase the heterogeneity because they provide geospatial applications adopting different technologies, from operating systems and related ecosystems (e.g. Linux, Microsoft, Apple, Google Android), to development platforms (e.g. Java, Python, Javascript) and libraries. Lack of interoperability is then perceived as one of the main barriers to data sharing and use: a potential user, being he either an intermediate user like an application developer, or an end user, must be or become an expert in geospatial technologies to be able to discover, access and use geospatial data served by different systems and provided in different models and formats. In October 2014, the ENERGIC OD (European NEtwork for Redistributing Geospatial Information to user Communities - Open Data) innovation project, funded by the European Union under the Competitiveness and Innovation framework Programme (CIP) has started aiming at the "development of virtual hubs that facilitate the use of open (freely available) geographic data from different sources for the creation of innovative applications and services". ENERGIC OD designed and implemented a set of virtual hubs based on the innovative approach of brokered architectures firstly adopted in building large infrastructures in the context of the Global Earth Observation System of Systems (GEOSS) to overcome limitation of the federated approach. The notion of "System of Systems" (SoS) and "System of Systems Engineering" (SoSE) emerged in many fields of applications to address the common problem of integrating many independent, autonomous systems [1]. From a technical point-of-view, there are two general approaches for building a SoS: through federation and through brokering. In the federated approach, the participants agree on a common set of specification (federated model) rangin from the adoption of a suite of interfaces, metadata and data models, to a very strict approach imposing the adoption of the same software tools. To be successful, this approach requires: a) the definition of the common model, which can be very complex for a big multidisciplinary SoS; b) t