Ontology Building: A Survey of Editing Tools
by Michael Denny |

Survey of Ontology Editors

This survey covers software tools that have ontology editing capabilities and are in use today. The tools may be useful for building ontology schemas (terminological component) alone or together with instance data. Ontology browsers without an editing focus and other types of ontology building tools are not included. Otherwise, the objective was to identify as broad a cross-section of editing software as possible. The editing tools are not necessarily production level development tools, and some may offer only limited functionality and user support.

Concise descriptions of each software tool were compiled and then reviewed by the organization currently providing the software for commercial, open, or restricted distribution. The descriptions are factored into a dozen different categories covering important functions and features of the software. These categories appear in Table 1 summarizing the results. (When possibly subtle distinctions in meaning or approach arose in these descriptions, we elected to retain the words of the tool provider.)

Despite the immaturity of the field, or perhaps because of it, we were able to identify a surprising number of ontology editors -- more than 50 overall.

Application

Commercial products include standalone editors designed exclusively for building ontologies in any domain, and editors that are part of commercial software suites designed to deliver broad enterprise integration solutions. Other editing software is the outcome of academic and government funded projects investigating the technical application of ontologies. Some editors are intended for building ontologies in a specific domain but still capable of general-purpose ontology building regardless of content focus. These ontology editors may have enhanced support for information standards unique to their target domain. An example in medicine is the OpenKnoMe editor's support of the GALEN reference medical terminology. Editors may also specifically support a broad upper level ontology, as in the case of the editing environment that has grown up around the unique Cyc ontology and is being released under the OpenCyc initiative.

The enterprise-oriented products have mostly started out as data integration tools like those from Unicorn Solutions and Modulant or as content management tools like Applied Semantics' offering. These latter products are more likely to include linguistic classification and stochastic analysis capabilities to aid in information extraction from unstructured content. This information can potentially become instance data or extend the ontology itself.

A few ontology editors included in the survey are actually software specification tools that are sufficiently general purpose to allow construction of domain ontologies. These tools, like Microsoft's Visio for Enterprise Architects, use an object-oriented specification language to model an information domain (in this case, the Object Role Modeling language). These tools presently lack useful export capabilities, although independent tools to convert between UML and ontology languages like DAML+OIL are under development.

Methodology

When ontology technologies emerged in the 1990s, the focus on knowledge acquisition influenced the way new capabilities were put to use in the field. Early ontology editors, for example, adopted the popular KADS method for developing knowledge bases. This orientation is not as evident in today's tools. Indeed, explicit support for a particular knowledge engineering methodology is not common. A few exceptions include Ontology Works' IODE and the Technical University of Madrid's WebODE, both with support for specific ontology organization approaches. There is also increasing support for common upper level ontologies like WordNet, Cyc, and others.

Interoperability

Ontology building today is a fragmented practice. The situation, in part, is a result of the proliferation of logic languages and information models that have combined to yield even more ontology forms and editing environments. These tools and methodologies, along with the ontologies built with them, generally exist without proven interoperability. This is one of the challenges facing the practice along with establishing methods to integrate ontology components with enterprise information systems and standards.

Ontologies are for sharing. They are intended to serve as consensual rallying points to exchange and interpret information. Clearly, the wider the range of applications and other ontologies that can use an ontology, the greater its utility and the mutual utility of the interrelating ontologies. This requires formal compatibility on syntactic levels as well as semantic levels. One consideration in the enterprise realm, for example, is the ability of a domain ontology to accommodate specialized XML languages and controlled vocabularies being adopted as standards in various industries. None of the current ontology editors address this capability fully, however vendors like Modulant and Unicorn are moving in this direction.

Interoperability, instead, is being addressed simply through an editor's ability to import and export ontologies in different language serializations. Some tools like Stanford Knowledge Systems Lab's Ontolingua offer a wide range of translations, while most are limited. Importing or exporting ontologies in the newer languages like DAML+OIL and OWL usually means that the translation is only partial and expressiveness is lost. A few editors like Web ODE also offer heterogeneous ontology merging capabilities.

Usability

In addition to the features already mentioned, ontology editors vary considerably in their overall feel to the user. The present survey did not attempt to compare editors under use, but a few general observations can be put forward. In terms of breadth and variety of features, especially as they relate to interfacing with other information system components, Protégé 2000 from Stanford Medical Informatics offers an editing environment with several third party plug-ins. From a strict ontology language point of view, Ontolingua and OpenCyc offer, or will offer, development environments affording highly expressive and complete ontology specifications. OpenCyc also provides native access to the most complete upper level ontology available (Cyc). Of the editors supporting DAML+OIL, as an important newer language, OilEd appears to offer strong support for composing description logic expressions.

The ability to organize and manage an emerging ontology is key to an editor's usability. Convenient and intuitive presentations and manipulations of an ontology's interlinking concepts and relations are essential. Because many ontology models support multiple inheritance in the concept hierarchies and relation hierarchies, keeping the associations straight is a challenge. The standard approach is the use of multiple tree views with expanding and contracting levels. A graph presentation is less common, although it can be quite useful for actual ontology editing functions that change concepts and relations. The more effective graph views provide local magnification to facilitate browsing ontologies of any appreciable size. The hyperbolic viewer included with the Applied Semantics product, for example, magnifies the center of focus on the graph of concepts (without labeled relations). Other approaches like the Jambalaya plug-in for Protégé-2000 achieve a kind of graphical zooming that nests child concepts inside their parents and allow the user to follow relations by jumping to related concepts. Some practitioners however, such as GALEN users, indicate a preference for non-graphic views for complex ontologies.

Finally, it is worth considering the inferencing support afforded by the ontology editor (beyond classification in description logic editors). While ontologies themselves can be treated as standalone specifications, they are ultimately used to help answer queries about a body of information. Some editors incorporate the ability to add additional axioms and deductive rules to the ontology for evaluation within the defined target of the development environment. For now, rule extensions are mostly proprietary in that standard rule languages able to reference ontology terms and structures directly are not available. A likely candidate to be supported in future ontology editors is RuleML.

Prev  [1] [2]