Global Drought Monitoring Service through the GEOSS Architecture ...

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Architectural Implementation Pilot, Phase 3 Version: 2.0 Global Drought Monitoring and European Drought Observatory-Water SBA Engineering Report Date: 11/Feb/2011 network registry. Such a specialized drought vocabulary or a water ontology would link to drought indicators and drought and water datasets used in common with the European Drought Observatory. The Simple Knowledge Organizing System (SKOS) had been used to express this GEMET data structure, so the water ontology, developed in AIP-3, would also be translated into SKOS data structures and linked to relevant terms in the reference thesauri as an AIP-4 activity. 5.4.1 Water Ontology-enablement within the DAC Semantics One possible candidate as a foundation water ontology was the USA Consortium of Universities for the Advancement of Hydrologic Sciences (CUAHSI) water ontology, version 1. The CUAHSI water ontology does list water stores of surface and subsurface (soil) water, thereby meeting some of the requirements needed in a hydrologic drought indicator. Fugazza has converted the CUAHSI Ontology Web Language (OWL) into SKOS, as an AIP-3 contribution (See AIP-3 Semantics Engineering Report). 5.5 How the EuroGEOSS Discover Augmentation Component supports semantic searches To conclude, the Discovery Augmentation Component enables semantics-aware discovery by matching the search patterns entered by the end user against a collection of multilingual, SDI-related thesauri: these are controlled vocabularies providing multiple textual representations for terms and organizing them according to specificity and relatedness. As a consequence the user’s query is first related to a set of language-neutral identifiers (URIs) (like the URIs used for geographic spatial entities, noted above in section 5.2.1.2). These URIs represent entities in a concept graph that the user may navigate for identifying related terms that are relevant to her search. These data structures are hosted by the GENESIS Vocabulary Service. These thesauri are provided in the Simple Knowledge Organizing System (SKOS) format, a lightweight ontology for expressing knowledge organization systems (such as taxonomies, classification schemes etc.), and have been harmonized in the context of the EuroGEOSS project by relating terms from distinct thesauri, thus allowing the user to move from one categorization to the other, i.e., one scientific discipline to another within a GEO SBA or from one SBA (water) to another SBA (agriculture). Once the user has identified an exhaustive set of terms that are relevant to her query, the broker translates the corresponding URIs back to a customizable set of languages and executes multiple queries against the catalogs it is federating (recalling the desired datasets). The EuroGEOSS Discovery Augmentation Component (DAC) implements a query expansion strategy deriving multiple traditional geospatial queries from a single semantic query. The DAC is able to accept a semantic query and, accessing a configurable set of external semantic services (e.g. controlled vocabularies, gazetteers, etc.), split it into several geospatial queries directed to a set of federated traditional/standard services for geospatial resources discovery. Results are finally combined in a meaningful way and sent back to the client. This framework realizes the Separation-of-Concerns pattern assigning specific tasks to Page 56
Architectural Implementation Pilot, Phase 3 Version: 2.0 Global Drought Monitoring and European Drought Observatory-Water SBA Engineering Report Date: 11/Feb/2011 different components, making the architecture flexible and scalable. Moreover, it does not affect existing geospatial service interfaces implementing a loosely-coupled solution in compliance with the GEOSS architectural principles. The system design for the DAC applies the well-known principle of Layered Architecture (ISO, 1994), as depicted in Figure 11. Functionalities are grouped and layered according to their abstraction level. Figure 11 shows the three layers of the proposed architecture, implementing each layer on a different distribution tier: • in the Presentation Layer we find components implementing graphic user interfaces (GUIs); • the Integrated Semantic Layer is composed of components which implement the business logic necessary to integrate semantic and geospatial services; • The Single Semantic and Geospatial Query Layer provides query functionalities towards a set of different services (geospatial, semantic, etc.). Figure 17 – System Architecture for the DAC System The DAC clearly falls into the integrated semantic layer and makes use of the services in the single semantic and query layer in order to implement the query expansion strategy. The choice of service interfaces was mainly driven by the need of being as compliant as possible with widely adopted catalog service specifications to be interoperable with existing systems. Thus, for the interaction between the DAC and the catalog service, the OGC CSW/ISO AP (Application Profile) interface is used. Among the present application profiles of the OGC CSW core specification; this is presently one of the most widely implemented. Moreover, this is the INSPIRE compliant catalog service interface. The access to the semantic service takes place through SPARQL (the Query Language for Resource Description Format (RDF) semantic Page 57
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