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Ontologies
Mariana Damova, PhD
April, 2010
Outline
• Definition of ontologies
• History of the science of categorization
• Knowledge models
• Knowledge organization
• Use and Building of ontologies
• Ontology tools
• Ontologies on the Web
Ontologies
What Is An Ontology
• An ontology is an explicit description of a domain:– concepts– properties and attributes of concepts– constraints on properties and attributes– Individuals (often, but not always)
• An ontology defines – a common vocabulary– a shared understanding
Outline
• Definition of ontologies
• History of the science of categorization
• Knowledge models
• Knowledge organization
• Use and Building of ontologies
• Ontology tools
• Ontologies on the Web
Philosophy
• What exists?
• What is?
• What am I?
• What is describing this to me?
Philosophy
Greek etymology
Parmenides of Elea, ancient Greek philosopher (early 5th century BCE)
For never shall this prevail,
that things that are not are
Parmenides made the ontological argument against nothingness, essentially denying the possible existence of a void.
Philosophy
• Jacob Lorhard, German philosopher (1561 - 1609)
• 1607 - First occurrence of the word Ontology (lat. Ontologia) and the first published ontology
Lorhard‘s Ontology• Translation from: Peter Øhrstøm, Sara L. Uckelman; Henrik
Schärfe –• Historical and conceptual foundations of diagrammatical ontology
Ontologies and CS
• Tom Gruber, 1992
• An ontology is a specification of a conceptualization.
• An ontology defines
• Concepts
• Relationships
• Any other distinctions that are relevant for modeling a domain
Ontologies and CS
• To share common understanding of the structure of information among people or software agents
• To enable reuse of domain knowledge
• To make domain assumptions explicit
• To separate domain knowledge from the operational knowledge
• To analyze domain knowledge
Outline
• Definition of ontologies
• History of the science of categorization
• Knowledge models
• Knowledge organization
• Use and Building of ontologies
• Ontology tools
• Ontologies on the Web
Knowledge Models
Structured representations of knowledge using symbols torepresent pieces of knowledge and relationships betweenthem.
Different types of KM have different degrees of formality andlevels of expressivity.
A KM can include:Symbolic character-based languages, such as logicDiagrammatic representations, such as networks and laddersTabular representations, such as matricesStructured text, such as hypertext
Knowledge Models - Design
• Knowledge identification
- What?
• Knowledge specification
- How?
• Knowledge refinement
- Validation
Knowledge Models - Types
• Ladders: hierarchical (tree-like) diagrams
• Tables and Grids: tabular representations
• Network Diagrams: shows nodes connected by arrows
The most complex type of KM
Examples include semantic nets and conceptual graphs
Ladder Model Example – British Royal Family
Tabular Model Example – Stock Markets
Network Diagrams – Semantic Nets
• Nodes in the graph represent concepts
• Arcs represent binary relationships between concepts
• Any characteristic that links two concepts: isA, hasColour, hasAge,
LivesIn, etc.
Note the difference between this structure and the ladders.
Network Diagrams – Conceptual Graphs
• Combination between the existential graphs and Semantic nets
• A conceptual graph consists of:
• Concept nodes – represented as rectangular boxes
• Relations nodes – represented as ovals
• One way connections between the nodes – represented as arrows
• Less intuitive then the Semantic Nets
Nemo the fish lives in water
Outline
• Definition of ontologies
• History of the science of categorization
• Knowledge models
• Knowledge organization
• Use and Building of ontologies
• Ontology tools
• Ontologies on the Web
Knowledge Organization
• Thesaurus
• Taxonomies
• Ontologies
Thesaurus
• Similar with dictionaries
• Provides synonyms and antonyms for words, and not definitions
E.g., WordNet
Taxonomies
• Hierarchical structures
• Subtype-supertype relationships, also called parent-child relationships
• Example: Whale is Mammal; Mammal is an Animal (not all Animals are Mammal, not all mammals are Whales)
Taxonomy Examples
• Taxonomies on the Web– Yahoo! Categories
• Catalogs for on-line shopping– Amazon.com product catalog
• Domain-specific standard terminology– Unified Medical Language System (UMLS)– UNSPSC - terminology for products and services
Yahoo
Amazon
Ontology
• More complexA formal definition of ontologies is provided in [Brewster andWilks, 2009]
O = (C,T,R,A,I,V,≤c, ≤t, σR, σA, IC, IT, IR, IA)
• Whereby:
C – Concepts V – ValuesT – Types ≤ – Partial Order on C and TR – Relations σ – FunctionsA – Attributes I – Partial Instantiation FunctionsI – Instances
Knowledge Organization
Differences in the degree of logical rigor, formality and the potential for reasoning over the data structure
Outline
• Definition of ontologies
• History of the science of categorization
• Knowledge models
• Knowledge organization
• Use and Building of ontologies
• Ontology tools
• Ontologies on the Web
How Ontologies can be used
Ontologies
Software
agentsProblem-
solving
methods
Domain-
independent
applications
DatabasesDeclarestructure
Knowledge
bases
Provide
domaindescription
Types of Ontologies
• Upper Ontology – model of the common objects that are applicable across a wide range of domain ontologies
• Domain Ontology – an ontology developed for a specific domain; conforms to an upper ontology
• Application Ontology – an ontology created for a specific application; may conform to a domain ontology
Types of Ontologies - Examples
Upper Ontologies:• Dublin Core• OpenCyc/ResearchCyc• SUMO• DOLCE• PROTON
• Domain Ontologies:• E-business : Rosetta-Net• Medical: UMLS• Engineering: EngMath
Methods for Building Ontologies
• From scratch – conceptual analysis
• Ontology acquisition
• Building on existing ontologies
Conceptual Schema
Sample Class Hierarchy
Sample Property Hierarchy
Modeling of an organization
DBPedia
http://mappings.dbpedia.org/server/ontology/classes
DBPedia
Proton Ontology
Person in DBPedia
Person in Proton
Outline
• Definition of ontologies
• History of the science of categorization
• Knowledge models
• Knowledge organization
• Use and Building of ontologies
• Ontology tools
• Ontologies on the Web
Protege
Protege
Class and Properties
Instances
TopBraid Composer - Diagram
TopBraid Composer - Graph
TopBraid Composer - Layout
Outline
• Definition of ontologies
• History of the science of categorization
• Knowledge models
• Knowledge organization
• Use and Building of ontologies
• Ontology tools
• Ontologies on the Web
OWL: Summary
The Web Ontology Language (OWL) was developed to provide for more expressive ontologies based on a decidable formal logic.
Three flavours of OWL have been specified: OWL Full for full
expressiveness without guarantees of decidability, OWL DL for a compromise expressiveness within the decidable fragment of Description Logic and OWL Lite as a subset of DL.
OWL provides for additional constructs not present in RDFS to define classes and properties. As a result, OWL is well suited to consistency checking and classification tasks.
OWL Lite
The complete language OWL Full has two sublanguages:
• OWL DL (Description Language)
• supports reasoning applications
• has restrictions on OWL Full constructs
• restrictions make reasoning systems decidable
• OWL Lite
• supports only a subset of OWL Full constructs
• provides a minimal set of features allowing the
development of ontologies without the encoding
of complex semantic relationships
OWL Lite - Classes
OWL classes define basic concepts.
A Simple Named Class is defined as follows:
– <owl:Class rdf:ID=„classname“/>
Ex. <owl:Class rdf:ID=„Restaurant“/>
Predefined OWL Classes (Extreme classes)
– Thing class (owl:Thing)
the most general class
every individual is member of this class
– Nothing class (owl:Nothing)
empty class with no member individuals
OWL Lite - Properties
There are four disjoint type of properties in OWL.
• Datatype properties (owl:DatatypeProperty)
• Object properties (owl:ObjectProperty)
• Annotationproperties (owl:AnnotationProperty)
• Ontology properties (owl:OntologyProperty)
OWL Lite – Classes and Properties
Linking Open Data
LDSR – reason-able view to LOD
http://ldsr.ontotext.com
http://linkedlifedata.com
Summary
• Definition of ontologies
• History of the science of categorization
• Knowledge models
• Knowledge organization
• Use and Building of ontologies
• Ontology tools
• Ontologies on the Web
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