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Adding formal semantics to the Web

Explore how adding formal semantics to the Web using RDF Schema and OIL benefits semantic annotation and knowledge management. Learn about extending RDF(S), OIL, and the implications for the Semantic Web vision.

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Adding formal semantics to the Web

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  1. Adding formal semantics to the Web building on top of RDF Schema Jeen Broekstra, Michel Klein, Stefan Decker, Dieter Fensel, Ian Horrocks On-To-Knowledge project

  2. Context On-To-Knowledge IST project about content-driven knowledge management through evolving ontologies http://www.ontoknowledge.org/ OIL = Ontology Inference Layer http://www.ontoknowledge.org/oil

  3. Goal of this presentation • Show how we can apply ontologies to semi structured data • Show how defining OIL in an RDF syntax adds formal semantics to the Web

  4. Contents • Semantic annotation: why, and how? • how: XML? • how: RDF(S)? • how: The W3C vision • OIL • extending RDF Schema • Conclusion and summary

  5. Semantic annotation: why • Semantic annotations make the meaning machine-accessible: • Intelligent information brokering • Meaning-based searches • Prerequisite for many agent applications

  6. Is XML sufficient for semantic annotation? • XML: user definable and domain specific markup • XML document is a labeled tree • constraints on structure via DTD or XML Schema <animal><name>Tux</name><species>penguin</species> <eats>fish</eats>...</animal>

  7. Shortcomings of XML (for our purposes) XML makes no commitment on: • Domain specific ontological vocabulary • Ontological modeling primitives  requires pre-arranged agreement on  &  Only feasible for closed collaboration • agents in a small & stable community • pages on a small & stable intranet not for sharable Web-resources

  8. Is RDF(S) sufficient for semantic annotation? Author-of pers05 ISBN... • RDF provides metadata about Web resources • Object -> Attribute-> Value triples • RDF Schema • Defines vocabulary for RDF • Organizes this vocabulary in a typed hierarchy • Class, subClassOf, type • Property, subPropertyOf • domain, range

  9. Conclusions about RDF(S) • Next step up from plain XML: • (small) ontological commitment to modeling primitives • possible to define vocabulary • However: • no precisely described meaning • no inference model

  10. Semantic annotation: how Declarative Languages Logic layer Schema layer Data layer DC RDF Schema SMIL XHTML RDF HTML XML W3C’s vision: The Semantic Web

  11. OIL DL OIL WWW Frames • Based on standard frame languages (OKBC) • restricts & extends • formalized by DL style logical constructs • Still has frame “look and feel” • Can still function as a basic frame language • OIL language restricted: • to allow for reasoning support

  12. OIL (explained by example) class-def animal % animals are a class class-def plant % plants are a class subclass-of not animal% that is disjoint from animals class-def tree subclass-of plant % trees are a type of plants class-def branch slot-constraint is-part-of % branches are parts of some tree has-valuetree max-cardinality 1 class-def defined carnivore % carnivores are animals subclass-of animal slot-constraint eats % that eat any other animals value-typeanimal class-def defined herbivore % herbivores are animals subclass-of animal, not carnivore % that are not carnivores, and slot-constraint eats % they eat plants or parts of plants value-type plant or (slot-constraint is-part-of has-value plant)

  13. OIL has a formal semantics DL OIL WWW Frames • Defined by mapping to expressive DL • slot-constraint eats has-value meat, fish=eats:meat  eats:fish • Mapping is used to provide reasoning support from a DL system (e.g., FaCT)

  14. Extending RDF Schema Goal • make RDFS useable as ontology language • give RDF(S) precise semantics • extend RDF(S) with additional modeling primitives • to facilitate semantically grounded metadata Procedure • formulate ontology language as RDF Schema document • using existing primitives as much as possible • placing additional primitives in the hierarchy of RDFS primitives

  15. OIL as extension to RDFS (1) • part of the is-a hierarchy of RDFS extension • ontology language is defined in RDFS

  16. OIL as extension to RDFS (3) <rdfs:Class rdf:ID=”herbivore”> <rdf:type rdf:resource=”http://www.ontoknowledge.org/#DefinedClass”/> <rdfs:subClassOf rdf:resource=”#animal”/> <rdfs:subClassOf> <oil:NOT> <oil:hasOperand rdf:resource=”#carnivore”/> </oil:NOT> </rdfs:subClassOf> </rdfs:Class>

  17. Using the extension: three levels • OIL modeling primitivesslot-constraint, subclass-of, value-type,... • RDF-S document which extends RDF-S • a specific OIL ontologyanimal, plant, herbivore, leaf • RDF-S document (using ) • instances of this ontology“Mel the giraffe”, “Tux the penguin” • RDF expressions (uses&) • explicit metadata

  18. What did we gain? • Any RDF agent can process OIL instances • Any RDF-S agent can process OIL ontologies • Any OIL-aware agent can exploit semantics & reasoning(and materialize the OIL derivations for use by OIL-ignorant RDF agents)

  19. Summary • to enable intelligent information handling, machine-understandable semantics are needed • advantages of our approach • reuse of modeling primitives • conform W3C view of the world • added benefits (from OIL): • reasoning support • formal semantics • Full schema available • http://www.ontoknowledge.org/oil/rdf-schema

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