Adapting graph visualization techniques for the visualization of RDF data

Abstract

The foundation language for the Semantic Web is the Resource Description Framework (RDF). RDF is intended to describe theWeb metadata so that theWeb content is not only machine readable but also machine understandable. In this way one can better support the interoperability ofWeb applications. RDF Schema (RDFS) is used to describe different RDF vocabularies (schemas), that is, the classes and properties associated to a particular application domain. An instantiation of these classes and properties form an RDF instance. It is important to note that both an RDF schema and an RDF instance have RDF graph representations. Realizing the advantages that RDF offers, in the last couple of years, many toolswere built in order to support the browsing and editing of RDF data. Among these tools we mention Prot́eǵe (Noy et al., 2001), OntoEdit (Sure et al., 2003), and RDF Instance Creator (RIC) (Grove, 2002). Most of the text-based environments are unable to cope with large amounts of data in the sense of presenting them in a way that is easy to understand and navigate (Card et al., 1999). The RDF data we have to deal with describes a large number ofWeb resources, and can thus easily reach tens of thousands of instances and attributes.We advocate the use of visual tools for browsing RDF data, as visual presentation and navigation enables users to effectively understand the complex structure and interrelationships of such data. Existing visualization tools for RDF data are: IsaViz (Pietriga, 2002), OntoRAMA (Eklund et al., 2002), and the Prot́eǵe visualization plug-ins like OntoViz (Sintek, 2004) and Jambalaya (Storey et al., 2001). The most popular textual RDF browser/editor is Prot́eǵe (Noy et al., 2001). The generic modeling primitives of Prot́eǵe enable the export of the built model in different data formats, among which is also RDF/XML. Prot́eǵe distinguishes between schema and instance information, allowing for an incremental view of the instances based on the selected schema elements. One of the disadvantages of Prot́eǵe is that it displays the information in a hierarchical way, that is, using a tree layout (Sugiyama et al., 1981), which makes it difficult to grasp the inherent graph structure of RDF data.