the semantic web and ontology
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The Semantic Web and Ontology. The Semantic Web. WWW: syntactic transmission of information only processible by human no semantic conservation of the information - PowerPoint PPT PresentationTRANSCRIPT
The Semantic Web and Ontology
The Semantic Web
• WWW: – syntactic transmission of information– only processible by human– no semantic conservation of the information– can not be processed by machine (e.g.. Machine does
not know whether a branch means a part of a tree or a part of an organization)
The Semantic Web
• Information transmitted conserves semantics
• information can be processed by machines
• enable intelligent services: search agents, information brokers, and information filters
What is information
• Factual data
• Meta Data about data
• plans and activities
• beliefs and doubts
• Reasoning
What is an Ontology?
• The conceptualization of a domain
• Represented as a set of terms and their relationships
• An example: • a car is a type of automobile
• it has engine, transmission, steering wheel, etc as parts
• its energy source can be gasoline or electricity
• and so on
• Term: a reference to real-world and abstract objects • Relationship: a named and typed set of links between objects• Reference: a label that names objects• Real-world object: an entity instance with a physical manifestation• Abstract object: a concept which refers to other objects
An Example Ontology
class-def animal
class-def plant
subclass-of NOT animal
class-def defined carnivore
subclass-of animal
slot-constraint eats
value-type animal
class-def lion
subclass-of animal
subclass-of defined carnivore
slot-constraint eats
value-type herbivore
Applications of Ontologies
• Define Terms used in System Construction to Enable Correctness in Understandingdesigners, implementers, users, maintainers
designers = implementers = users = maintainers
• Define Higher-level Abstractions Needed to Communicate in Large Contextsmanagers, decision-makers, systems in other domains
• Share the Cost of Knowledge Acquisition & Maintenancereuse encoded knowledge, remain up-to-date as domains change
Why ontology is crucial for the Semantic Web
• An existing technique to model the real world and information
• Describe semantics
• Understandable by different users
• Ontology representation languages needs to be encoded into a machine-processable way
Heavy-weight Ontology is needed for the Semantic Web
• Light-weight Ontology– concepts, atomic types– is-a hierarchy among concepts– associations between concepts
• Heavy-weight Ontology– cardinality constraints– taxonomy of relations– reified statements– Axioms / semantic entailments of various tastes
• expressiveness (DL, propositional, horn, or first order logic, higher order)
• inferences
XML document = labeled tree
course
teachertitle students
name http
<course date=“...”><title>...</title><teacher>...</teacher>
<name>...</name><http>...</http>
<students>...</students></course>
node = label + attr/values + contents
DTD: simple grammars to describe legal treesSo: why not use XML to represent ontologies?
Limitations of XML for Semantic Markup
• Multiple possibilities to code an ontology
• No commitment to domain-specific terms
• Lacks modeling primitives
• Requires pre-agreement between all users on a specific DTD
RDF: Resource Description Framework
• Intended for representation “meta-data”,basis for Web-based ontology-language
• W3C recommendation
– Supported by W3C – basis of $ 80M DAML program– Already embraced by some vendors
(e.g.Netscape)
• Object --Attribute-> Value tuples
• Objects are web-resources• Value is again an Object:
– data-model = graph
pers05 ISBN...Author-of
Author-ofpers05 ISBN... MIT
ISBN...
Publ-by
Author-of Publ-
by
RDF Schema• So, RDF :
– (very small) commitment to modeling primitives– but: no commitment to domain vocabulary
RDF Schema
• Define vocabulary for RDF
• Organize this vocabulary in a typed hierarchy–Class, SubClassOf, type–Property, subPropertyOf, –domain, range
RDF is Better than XML but Still Limited
• Provides more semantic interoperabilitythe object-attribute structure is natural semantic
units
easier mapping between two RDF descriptions
XML independent
• Lacks modeling primitives
• Lacks semantic support (Description Logic)
• provides syntax
Ontology Inference Layer (OIL)
• Sponsored by European Union IST programme for Information Society Technologies
• Frame-based system +
Description logic +
RDF
= OIL
Ontology Inference Layer (OIL)
• class-def• subclass-of• slot-def• subslot-of• domain• range
• class-def• subclass-of• slot-def• subslot-of• domain• range
• class-expressions• AND, OR, NOT
• slot-constraints• has-value, value-type• cardinality
• slot-properties• trans, symm
• class-expressions• AND, OR, NOT
• slot-constraints• has-value, value-type• cardinality
• slot-properties• trans, symm
RDF(S)OIL
OIL as RDFS extension
<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>
OIL: currently available tools• Definition of language
– semantics– XML encoding– RDF encoding
• Tools:– translators (XSL based)– OntoEdit
• case-studies– GIS ontology mapping– (KA)2 ontology– CIA world fact book
DARPA Agent Markup Language (DAML)
• Sponsored by the Defense Advance Research Project Agency
• Develop technologies to provide interoperability between agents in semantic manner
• First stage is DAML, similar to OIL
Conclusion
• OIL and DAML mark the first effort of using ontology for the Semantic Web
• Both needs to be further enhanced
• Tools to use these languages are in urgent need
• Ontology interoperativity is the next step
Future Directions
• Tools to build and use ontology using the standard Web ontology language
• Engineering tools to semantically integrate, migrate, reconciliate and share ontologies
• Deploy the technology to be used for intelligent services
References
• Decker et al.(2000) The Semantic Web: the roles of XML and RDF. IEEE Internet Computing 4:63-74.
• Bray et al. Extensible Markup Language (XML) 1.0, World Wide Web Consortium, 1998, current May 2000; www.w3.org/TR/REC-xml.
• Brickley and Guha (2000) Resource Description Framework (RDF) Schima Specification. W3C Candidate Recommendation. http://www.w3.org/TR/2000/CR-rdf-schema-20000327/
• OIL, http://www.ontoknowledge.org/oil
• DAML, http://www.daml.org