Ontologies

What are ontologies?
Ontologies are structural frameworks for organizing information about some domain. They are machine-readable representations of knowledge; explicitly defining the concepts and relationships within some area of interest.

"An explicit specification of a conceptualization." - Tom Grüber.

Ontologies typically have distinct components:


 * Names for important concepts in the domain


 * Background knowledge/constraints on the domain


 * Relationships between concepts

Motivational factors for using ontologies include


 * the ability to share common understanding of the structure of information among people or software agents


 * the ability to reuse domain knowledge represented by ontologies


 * separating domain knowledge from operational knowledge of a system


 * the ability to analyze domain knowledge

Ontology Representation Languages
Alternatively just "ontology languages", means formal languages used to encode ontologies. There are many different languages, both proprietary and standards-based. Perhaps the most notable in recent years is the Web Ontology Language (OWL), being now a W3C recommendation (i.e., a standard.)

Web Ontology Language (OWL)
OWL was developed as a follow-on from RDF Schema, as well as the earlier OIL and DAML+OIL language projects.

OWL extends the vocabulary of RDFS and adds axioms.

The Web Ontology Language is intended to provide a language that can be used to describe the classes and relations between them that are inherent in Web documents and applications. The OWL language provides three increasingly expressive sublanguages.

OWL LITE: Classification hierarchy and simple constraint features.

OWL-DL: "Maximum expressiveness without losing computational completeness (all entailments are guaranteed to be computed) and decidability (all computations will finish in finite time) of reasoning system." OWL-DL is named as such due to its correspondence with description logics research.

OWL Full: "Maximum expressiveness and the syntactic freedom of RDF with no computational guarantees." OWL Full allows an ontology to augment the meaning of the pre-defined (RDF or OWL) vocabulary. It is unlikely that any reasoning software will be able to support every feature of OWL Full.

RDF Schema (RDFS)
Basically a lite version of OWL Lite, providing basic elements for the description of ontologies.

The RDFS vocabulary builds on the limited vocabulary of RDF.

Possibilities and difficulties
Ontologies allow us to describe the semantics of data. Ontologies gives different parties a uniform way to communicate semantic data.

How do ontologies support interoperability?
Ontologies empower semantic interoperability by defining a common, agreed-upon standard for describing information - and enables applications to exchange semantic data.

TODO: Elaborate.

Ontology Mapping
Ontology Mapping is the process whereby two ontologies are semantically related at conceptual level, and the source ontology instances are transformed into the target ontology entities according to those semantic relations.

There are three dimensions to ontology mapping:

Discovery: Manually, automatically or semi-automatically defining the relations between ontologies.

Representation: A language to represent the relations between the ontologies.

Execution: Changing instance of a source ontology to an instance of target ontology.

RDF
From w3schools:

RDF is a framework for describing Web resources, such as the title, author, modification date, content, and copyright information of a Web page.

From Wikipedia:

The Resource Description Framework (RDF) is a family of World Wide Web Consortium (W3C) specifications originally designed as a metadata data model. It has come to be used as a general method for conceptual description or modeling of information that is implemented in web resources, using a variety of syntax formats. Self:

RDF provides a way to express simple statements about resources, using named properties and values.