Adapting graph visualization techniques for the visualization of RDF data

The foundation language for the Semantic Web is the Resource Description Framework (RDF). RDF is intended to describe the Web metadata so that the Web content is not only machine readable but also machine understandable. In this way one can better support the interoperability of Web applications. RDF Schema (RDFS) is used to describe different RDF vocabularies (schemas), i.e., 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 tools were built in order to support the browsing and editing of RDF data. Among these tools we mention Protégé (Noy et al. 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 of Web 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. The most popular textual RDF browser/editor is Protégé (Noy et al., 2001). The generic modelling primitives of Protégé enable the export of the built model in different data formats among which is also RDF/XML. Protégé 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 Protégé is that it displays the information in a hierarchical way, i.e., using a tree layout (Sugiyama et al., 1981), which makes it difficult to grasp the inherent graph structure of RDF data. In this paper, we advocate the use of a highly customizable, interactive visualization system for the understanding of different RDF data structures. We