OVERVIEW EXAMPLE PRESENTATIONS PROBLEMS
& QUIZS
ERRATA LINKS
SW Vision
XML
RDF
OWL
Logic
Applications
Ontology Engineering
Conclusion




     Aditional Topics
Chapter 4 - OWL

The expresive power of RDF and RDF Schema is deliberatley very limited: RDF is (roughly) limited to binary ground predicates, and RDF Schema is (roughly) limited to subclass and subpropertiy hierarchies, with domain and range restrictions of properties.

There are a number of characteristic user-cases of the Semantic Web that would require more expresiveness. Such extensions include:

  • Disjointness of classes (e.g. one cannot be professor and admin staff at the same time).
  • Boolean combinations of classes (e.g. staff is the union of academics, admin staff and technical support staff)
  • Cardinality restrictions (e.g. a department can only have one head)
  • Special characteristics of proerties (e.g. "superior of" is transitive, "teaches" and "is taught by" are inverse properties)
  • Local scope of properties: rdf:range defines the range of a property, say "eats", for all classes. But sometimes we may want to restrict the range, depending on the class. For example, cows eat only plants, while other animals eat meat, too.

OWL was designed as a new standard Web ontoloy language. It is based on top RDF/S, and seeks to find a balance between expressive power and efficient reasoning support. Reasoning is important because it allows one to (a) check for consistency of an ontology and the knowledge; (b) check for unintended relationships between classes and (c) automatically classify instances in classes.