Ontology alignment
Encyclopedia
Ontology alignment, or ontology matching, is the process of determining correspondences between concept
s. A set of correspondences is also called an alignment. The phrase takes on a slightly different meaning, in computer science
, cognitive science
or philosophy
.
s, concepts are expressed as labels for data. Historically, the need for ontology alignment arose out of the need to integrate
hetereogeneous database
s, ones developed independently and thus each having their own data vocabulary. In the Semantic Web
context involving many actors providing their own ontologies, ontology matching has taken a critical place for helping heterogeneous resources to interoperate. Ontology alignment tools find classes of data that are "semantically equivalent
," for example, "Truck" and "Lorry." The classes are not necessarily logically identical. According to Euzenat and Shvaiko (2007), there are three major dimensions for similarity: syntactic, external, and semantic. Coincidentally, they roughly correspond to the dimensions identified by Cognitive Scientists below. A number of tools and frameworks have been developed for aligning ontologies, some with inspiration from Cognitive Science and some independently.
Ontology alignment tools have generally been developed to operate on database schema
s, XML schema
s, taxonomies, formal language
s, entity-relationship model
s, dictionaries
, and other label frameworks. They are usually converted to a graph representation before being matched.
Since the emergence of the Semantic Web, such graphs can be represented in the Resource Description Framework
line of languages by triples of the form, as illustrated in the Notation 3
syntax.
In this context, aligning ontologies is sometimes referred to as "ontology matching".
The problem of Ontology Alignment has been tackled recently by trying to compute matching first and mapping (based on the matching) in an automatic fashion. Systems like DSSim
, X-SOM or COMA++ obtained at the moment very high precision and recall. The Ontology Alignment Evaluation Initiative aims to evaluate, compare and improve the different approaches.
More recently, a technique useful to minimize the effort in mapping validation and visualization has been presented which is based on Minimal Mappings
. Minimal mappings are high quality mappings such that i) all the other mappings can be computed from them in time linear in the size of the input graphs, and ii) none of them can be dropped without losing property i).
and 
we can define different type of (inter-ontology) relationships among their terms.
Such relationships will be called, all together, alignments and can be categorized among different dimensions:
Subsumption, atomic, homogeneous alignments are the building blocks to obtain richer alignments, and have a well defined semantics in every Description Logic.
Let's now introduce more formally ontology matching and mapping.
An atomic homogeneous matching is an alignment that carries a similarity degree
, describing the similarity of two terms of the input ontologies 
and 
.
Matching can be both computed, by means heuristic algorithms, or inferred
from other matchings.
Formally we can say that, a matching is a quadruple
, where 
and 
are homogeneous ontology terms, 
is the similarity degree of 
.
A (subsumption, homogeneous, atomic) mapping is defined as a pair
, where 
and 
are homogeneous ontology terms.
that reside in brains as "conceptual systems." The focal question is: if everyone has unique experiences and thus different semantic networks, then how can we ever understand each other? This question has been addressed by a model called ABSURDIST (Aligning Between Systems Using Relations Derived Inside Systems for Translation). Three major dimensions have been identified for similarity as equations for "internal similarity, external similarity, and mutual inhibition."
Ontology alignment is closely related to analogy formation, where "concepts" are variables in logic expressions.
.
Concept
The word concept is used in ordinary language as well as in almost all academic disciplines. Particularly in philosophy, psychology and cognitive sciences the term is much used and much discussed. WordNet defines concept: "conception, construct ". However, the meaning of the term concept is much...
s. A set of correspondences is also called an alignment. The phrase takes on a slightly different meaning, in computer science
Computer science
Computer science or computing science is the study of the theoretical foundations of information and computation and of practical techniques for their implementation and application in computer systems...
, cognitive science
Cognitive science
Cognitive science is the interdisciplinary scientific study of mind and its processes. It examines what cognition is, what it does and how it works. It includes research on how information is processed , represented, and transformed in behaviour, nervous system or machine...
or philosophy
Philosophy
Philosophy is the study of general and fundamental problems, such as those connected with existence, knowledge, values, reason, mind, and language. Philosophy is distinguished from other ways of addressing such problems by its critical, generally systematic approach and its reliance on rational...
.
Computer Science
For computer scientistComputer scientist
A computer scientist is a scientist who has acquired knowledge of computer science, the study of the theoretical foundations of information and computation and their application in computer systems....
s, concepts are expressed as labels for data. Historically, the need for ontology alignment arose out of the need to integrate
Data integration
Data integration involves combining data residing in different sources and providing users with a unified view of these data.This process becomes significant in a variety of situations, which include both commercial and scientific domains...
hetereogeneous database
Database
A database is an organized collection of data for one or more purposes, usually in digital form. The data are typically organized to model relevant aspects of reality , in a way that supports processes requiring this information...
s, ones developed independently and thus each having their own data vocabulary. In the Semantic Web
Semantic Web
The Semantic Web is a collaborative movement led by the World Wide Web Consortium that promotes common formats for data on the World Wide Web. By encouraging the inclusion of semantic content in web pages, the Semantic Web aims at converting the current web of unstructured documents into a "web of...
context involving many actors providing their own ontologies, ontology matching has taken a critical place for helping heterogeneous resources to interoperate. Ontology alignment tools find classes of data that are "semantically equivalent
Semantic equivalence
In computer metadata, semantic equivalence is a declaration that two data elements from different vocabularies contain data that has similar meaning...
," for example, "Truck" and "Lorry." The classes are not necessarily logically identical. According to Euzenat and Shvaiko (2007), there are three major dimensions for similarity: syntactic, external, and semantic. Coincidentally, they roughly correspond to the dimensions identified by Cognitive Scientists below. A number of tools and frameworks have been developed for aligning ontologies, some with inspiration from Cognitive Science and some independently.
Ontology alignment tools have generally been developed to operate on database schema
Database schema
A database schema of a database system is its structure described in a formal language supported by the database management system and refers to the organization of data to create a blueprint of how a database will be constructed...
s, XML schema
XML schema
An XML schema is a description of a type of XML document, typically expressed in terms of constraints on the structure and content of documents of that type, above and beyond the basic syntactical constraints imposed by XML itself...
s, taxonomies, formal language
Formal language
A formal language is a set of words—that is, finite strings of letters, symbols, or tokens that are defined in the language. The set from which these letters are taken is the alphabet over which the language is defined. A formal language is often defined by means of a formal grammar...
s, entity-relationship model
Entity-relationship model
In software engineering, an entity-relationship model is an abstract and conceptual representation of data. Entity-relationship modeling is a database modeling method, used to produce a type of conceptual schema or semantic data model of a system, often a relational database, and its requirements...
s, dictionaries
Dictionary
A dictionary is a collection of words in one or more specific languages, often listed alphabetically, with usage information, definitions, etymologies, phonetics, pronunciations, and other information; or a book of words in one language with their equivalents in another, also known as a lexicon...
, and other label frameworks. They are usually converted to a graph representation before being matched.
Since the emergence of the Semantic Web, such graphs can be represented in the Resource Description Framework
Resource Description Framework
The Resource Description Framework is a family of World Wide Web Consortium specifications originally designed as a metadata data model...
line of languages by triples of the form
Notation 3
Notation3, or N3 as it is more commonly known, is a shorthand non-XML serialization of Resource Description Framework models, designed with human-readability in mind: N3 is much more compact and readable than XML RDF notation...
syntax.
In this context, aligning ontologies is sometimes referred to as "ontology matching".
The problem of Ontology Alignment has been tackled recently by trying to compute matching first and mapping (based on the matching) in an automatic fashion. Systems like DSSim
DSSim
DSSim is an ontology mapping system, that has been conceived to achieve a certain level of the envisioned machine intelligence on the Semantic Web. The main driving factors behind its development was to provide an alternative to the existing heuristics or machine learning based approaches with a...
, X-SOM or COMA++ obtained at the moment very high precision and recall. The Ontology Alignment Evaluation Initiative aims to evaluate, compare and improve the different approaches.
More recently, a technique useful to minimize the effort in mapping validation and visualization has been presented which is based on Minimal Mappings
Minimal mappings
Minimal mappings are the result of an advanced technique of semantic matching, a technique used in Computer Science to identify information which is semantically related....
. Minimal mappings are high quality mappings such that i) all the other mappings can be computed from them in time linear in the size of the input graphs, and ii) none of them can be dropped without losing property i).
Formal Definition
Given two ontologies and we can define different type of (inter-ontology) relationships among their terms.Such relationships will be called, all together, alignments and can be categorized among different dimensions:
- similarity vs logic: this is the difference between matchings (predicating about the similaritySemantic similaritySemantic similarity or semantic relatedness is a concept whereby a set of documents or terms within term lists are assigned a metric based on the likeness of their meaning / semantic content....
of ontology terms), and mappings (logical axioms, typically expressing logical equivalenceLogical equivalenceIn logic, statements p and q are logically equivalent if they have the same logical content.Syntactically, p and q are equivalent if each can be proved from the other...
or inclusion among ontology terms) - atomic vs complex: whether the alignments we considered are one-to-one, or can involve more terms in a query-like formulation (e.g., LAV/GAVData integrationData integration involves combining data residing in different sources and providing users with a unified view of these data.This process becomes significant in a variety of situations, which include both commercial and scientific domains...
mapping) - homogeneous vs heterogeneous: do the alignments predicate on terms of the same type (e.g., classes are related only to classes, individuals to individuals, etc.) or we allow heterogeneity in the relationship?
- type of alignment: the semantics associated to an alignment. It can be subsumptionSubsumptionSubsumption may refer to:* A minor premise in symbolic logic * The Liskov substitution principle in object-oriented programming* Subsumption architecture in robotics...
, equivalence, disjointness, part-of or any user-specify relationship.
Subsumption, atomic, homogeneous alignments are the building blocks to obtain richer alignments, and have a well defined semantics in every Description Logic.
Let's now introduce more formally ontology matching and mapping.
An atomic homogeneous matching is an alignment that carries a similarity degree
, describing the similarity of two terms of the input ontologies and .Matching can be both computed, by means heuristic algorithms, or inferred
Inference
Inference is the act or process of deriving logical conclusions from premises known or assumed to be true. The conclusion drawn is also called an idiomatic. The laws of valid inference are studied in the field of logic.Human inference Inference is the act or process of deriving logical conclusions...
from other matchings.
Formally we can say that, a matching is a quadruple
, where and are homogeneous ontology terms, is the similarity degree of .A (subsumption, homogeneous, atomic) mapping is defined as a pair
, where and are homogeneous ontology terms.Cognitive Science
For cognitive scientists interested in ontology alignment, the "concepts" are nodes in a semantic networkSemantic network
A semantic network is a network which represents semantic relations among concepts. This is often used as a form of knowledge representation. It is a directed or undirected graph consisting of vertices, which represent concepts, and edges.- History :...
that reside in brains as "conceptual systems." The focal question is: if everyone has unique experiences and thus different semantic networks, then how can we ever understand each other? This question has been addressed by a model called ABSURDIST (Aligning Between Systems Using Relations Derived Inside Systems for Translation). Three major dimensions have been identified for similarity as equations for "internal similarity, external similarity, and mutual inhibition."
Ontology alignment is closely related to analogy formation, where "concepts" are variables in logic expressions.
Philosophy
For philosophers, much like cognitive scientists, the interest is in the nature of "understanding." The roots of discourse, however, may be traced to radical interpretationRadical interpretation
Radical interpretation is interpretation of a speaker, including attributing beliefs and desires to them and meanings to their words, from scratch—that is, without relying on translators, dictionaries, or specific prior knowledge of their mental states. The term was introduced by American...
.
Visualization Tools
- Optima: A Visual Ontology Alignment Tool
- CogZ: Cognitive Support and Visualization for Human-Guided Mapping Systems
- AgreementMaker: Efficient Matching for Large Real-World Schemas and Ontologies
See also
- Ontology (computer science)Ontology (computer science)In computer science and information science, an ontology formally represents knowledge as a set of concepts within a domain, and the relationships between those concepts. It can be used to reason about the entities within that domain and may be used to describe the domain.In theory, an ontology is...
- Rule Interchange FormatRule Interchange FormatThe Rule Interchange Format is a W3C Recommendation. RIF is part of the infrastructure for the semantic web, along with SPARQL, RDF and OWL...
(RIF) - Data conversionData conversionData conversion is the conversion of computer data from one format to another. Throughout a computer environment, data is encoded in a variety of ways. For example, computer hardware is built on the basis of certain standards, which requires that data contains, for example, parity bit checks....
- Semantic IntegrationSemantic integrationSemantic integration is the process of interrelating information from diverse sources, for example calendars and to do lists; email archives; physical, psychological, and social presence information; documents of all sorts; contacts ; search results; and advertising and marketing relevance derived...
- Semantic matchingSemantic matchingSemantic matching is a technique used in Computer Science to identify information which is semantically related.Given any two graph-like structures, e.g. classifications, database or XML schemas and ontologies, matching is an operator which identifies those nodes in the two structures which...
- Minimal MappingsMinimal mappingsMinimal mappings are the result of an advanced technique of semantic matching, a technique used in Computer Science to identify information which is semantically related....
- InterpretationSemanticsSemantics is the study of meaning. It focuses on the relation between signifiers, such as words, phrases, signs and symbols, and what they stand for, their denotata....
"An interpretation can be the part of a presentation or portrayal of information altered in order to conform to a specific set of symbols." - Graph isomorphismGraph isomorphismIn graph theory, an isomorphism of graphs G and H is a bijection between the vertex sets of G and H f \colon V \to V \,\!such that any two vertices u and v of G are adjacent in G if and only if ƒ and ƒ are adjacent in H...