VOWL distinguishes between three layers that may be visualized:
The notation is based on four types of graphical elements:
Table 1 provides an overview of the graphical representations for the different OWL elements (formatted as Scalable Vector Graphics (SVG)). Note that some of the elements are from RDF Schema (RDFS) – they are included in the notation, as they are reused in OWL.
|owl:Class||Circle with round label (rdfs:label) in the middle. Classes from imported ontologies are displayed entirely in gray (incl. the label background).|
|owl:Thing||Simple circle with label Thing.|
|Individuals, enumerations (e.g. owl:oneOf)||Individuals are visualized as sections of the circle that represents the class the individuals belong to.|
|owl:ObjectProperty, owl:DatatypeProperty||Line or arc annotated with the label of the property. The label's background color indicates the type of the property (object or datatype property).|
|rdfs:subClassOf, rdfs:subPropertyOf||Dotted line/arc with an unfilled arrow head at the end pointing to the superclass or superproperty. Subproperties having the same domain and range as their superproperties can alternatively be listed in square brackets within the label of the superproperty.|
|rdfs:domain, rdfs:range||Arrow starting at the domain and pointing to the range. Datatype properties do not need an arrow, as they are always directed to the literal or data value.|
|owl:inverseOf||Line/arc with arrow heads at both ends and annotated with the label of the property and its inverse counterpart. Filled arrow heads at the end of the labels indicate the direction of the properties (i.e. their range).|
|owl:disjointWith||Dashed line/arc with \\ symbol as label.|
|owl:FunctionalProperty, owl:InverseFunctionalProperty, owl:TransitiveProperty||Object property representation with an F for functional properties, InvF for inverse functional properties, and T for transitive properties on top of the property label.|
|owl:SymmetricProperty||Object property representation with identical domain and range, i.e. start and end point of the arrow is the same class.|
|owl:cardinality, owl:minCardinality, owl:maxCardinality||Exact, minimal, and/or maximal cardinality constraints are written above the property label.|
Colored ring around a class. The labels of equivalent classes are shown as comma-separated list in the label, each colored in the same font as the corresponding ring.
|owl:unionOf||Dashed line/arc with ∪ symbol as label.|
|owl:intersectionOf||Dashed line/arc with ∩ symbol as label.|
|owl:complementOf||Dashed line/arc with ¬ symbol as label.|
|Literals, data types and values||Rectangle containing the literal, data type or value.|
|owl:DeprecatedClass||Class representation colored in dark gray (with white label background).|
|owl:DeprecatedProperty||Object/datatype property representation colored in dark gray.|
|owl:equivalentProperty||Labels of equivalent properties are listed in curly brackets.|
|rdfs:comment, rdfs:seeAlso, rdfs:isDefinedBy||Inverted literal representation (i.e. black rectangle with white font).|
|owl:sameAs||Inverted literal representation listing the labels of the individuals connected with the keyword `sameAs'.|
|owl:imports, owl:versionInfo, owl:priorVersion, owl:BackwardCompatibleWith and owl:incompatibleWith||Text in an infobox complementing the visualization.|
The basic set operations owl:intersectionOf, owl:unionOf and owl:ComplementOf define new (anonymous) classes to describe the union, intersection, or complement of other classes, and are thus represented as circles. The circles contain the corresponding Description Logic (DL) symbols (∪,∩, ¬).
Elements such as owl:Ontology, owl:differentFrom, owl:AllDifferent, owl:distinctMembers, owl:Restriction, owl:onProperty, owl:allValuesFrom, owl:someValuesFrom, owl:hasValue, owl:AnnotationProperty, owl:OntologyProperty, and owl:DataRange (rdfs:datatype in OWL 2) serve as containers for other elements or define constraints for populating the ontology. They are not graphically represented by the visual notation.
The size of the circles is determined by the number of incoming and outgoind property relations (the more relations, the larger the circle). The class representation of owl:Thing has a fixed size, as it usually does not carry particular domain information. It is recommended to use multiple representations of owl:Thing to not give this element an importance in the visualization that it does not deserve.
If certain classes, properties, or individuals do not have an rdfs:label, it is recommended to display a part of the URI as label (e.g. the characters that follow the last slash or number sign).
Figure 2 visualizes the Integrated Layer of the Friend of a Friend (FOAF) vocabulary using the VOWL notation and some sample data. The sample data consists of 28 individuals that are distributed on the FOAF classes as follows: 5 x Person, 7 x Document, 5 x Image, 2 x Group, and 9 x Organization. Though all these individuals are represented as sections in the circles, the property relations of only one selected individual are shown (namely, "Jane Doe"). The FOAF profile of that individual in RDF/XML syntax is as follows:
<foaf:name>jane doe </foaf:name>
<foaf:nick> jane </foaf:nick>
<foaf:name>john doe </foaf:name>
This work was partially supported by the European Social Fund in Romania, under the responsibility of the Managing Authority for the Sectorial Operational Program for Human Resources Development 2007-2013 (grant POSDRU/107/1.5/S/78342).
Negru, S. and Lohmann, S.: A Visual Notation for the Integrated Representation of OWL Ontologies. Proceedings of the 9th International Conference on Web Information Systems and Technologies (WEBIST '13). SciTePress 2013; pp. 308-315 [BibTex]