The Visual Notation for OWL Ontologies (VOWL) is a well-specified visual language for the user-oriented representation of ontologies. It defines graphical depictions for most elements of the Web Ontology Language (OWL) that are combined to a force-directed graph layout visualizing the ontology. In contrast to related work, VOWL aims for an intuitive and comprehensive representation that is also understandable to users less familiar with ontologies. This article presents VOWL in detail and describes its implementation in two different tools: ProtégéVOWL and WebVOWL. The first is a plugin for the ontology editor Protégé, the second a standalone web application. Both tools demonstrate the applicability of VOWL by means of various ontologies. In addition, the results of three user studies that evaluate the comprehensibility and usability of VOWL are summarized. They are complemented by findings from an interview with experienced ontology users and from testing the visual scope and completeness of VOWL with a benchmark ontology. The evaluations helped to improve VOWL and confirm that it produces comparatively intuitive and comprehensible ontology visualizations.

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Abstract. Ontologies become increasingly important as a means to structure and organize information. This requires methods and tools that enable not only ontology experts but also other user groups to work with ontologies and related structures. We have developed VOWL, a comprehensive and well-specified visual language for the user-oriented visualization of ontologies, and conducted a comparative study on an initial version of VOWL. Based upon results from that study, as well as an extensive review of other ontology visualizations, we have reworked many parts of VOWL. In this paper, we present the new version VOWL 2 and describe how the initial definitions were used to systematically redefine the visual notation. Beside the novelties of the language definition, which is based on a well-defined set of graphical primitives and an abstract color scheme, we briefly describe two implementations of VOWL 2. To gather some insight into the user experience with the new version of VOWL, we have conducted a small qualitative evaluation. We report on the study and its results, which confirmed that not only the general ideas of VOWL but also various of our enhancements for VOWL 2 can be understood by casual ontology users.

Browsing ontologies and making sense of their concepts and relationships are important activities in knowledge engineering. They are supported by a large number of tools that often provide visual representations of the ontologies for better illustration. However, different notations are used to represent the ontologies which can be confusing when switching tools. Furthermore, many representations provide only basic overviews of the ontologies that are not sufficient to get a deeper understanding of the concepts and relationships. A unified visual notation for OWL ontologies would be most helpful to overcome these limitations. Having this goal in mind, we compare two different notations for OWL ontologies in this paper: the UML profile of the Ontology Definition Metamodel (ODM) and the Visual Notation for OWL Ontologies (VOWL). We report on a comparative user study of these notations and discuss benefits and limitations raised by the study participants. Based on these findings, we draw some general conclusions regarding the development of a unified visual notation for OWL ontologies.

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