Ontology Standards

Ontologies written using the Web Ontology Language are built on a set of standards that are developed by an international consortium known as the World Wide Web Consortium (W3C). The W3C is made up of member organizations, paid staff, and interested members of the public, and has produced a large number of standards on Internet protocols. Each of the standards is open and publicly available. Below, we briefly discuss the relevant standards that are being developed, cover their current status, and explain their relationship to ontologies.

Extensible Markup Language (XML)

XML is a specification that allows people to create custom markup languages for specific needs. Roughly, it can be considered a programming language for creating other programming languages.

Another markup language, HTML, is used throughout the Internet and is loosely based on markup languages used in the publishing industry that indicate where text should be made bold, italic, or formatted when being set for printing. Similarly, XML provides a standardized way for anyone to create a specialized language for individual needs. An example of such a language in the marine sciences is SensorML, a language for "describing sensor and measurement processes."

When someone wants to create a language using XML, they need to define its schema. The schema works in conjunction with existing XML syntax and rules and helps to define the language being created. For example, SensorML's base schema includes things such as definitions for elements and groups, similar to individuals and classes in ontologies, which can be used by applications that implement the SensorML language to store, retrieve, and analyze data. Because SensorML conforms to the syntax and rules of XML, any application that knows how to read XML data will be able to work with the language. However, unless an application was written to specifically work with SensorML it won't be able to take advantage of the unique elements and groups that the language defines. This is because XML lacks semantics—something that is corrected with the implementation of the Resource Description Framework and Web Ontology Language.

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Resource Description Framework (RDF)

RDF is a data specification used to make statements about resources using subject-predicate-object statements called triples. Each of the subject, predicate, and object terms is typically a Web resource (though the object can also be a constant). In a triple, the predicate expresses the relationship between the subject and object.

To provide a plain language example, let's examine the statement: “The car has the color red.” “The car” is the subject, “has the color” is the predicate, and “red” is the object. Triples are a powerful model for describing resources.

RDF is a stable standard as the tasks of the W3C RDF Core Working Group were completed in 2004.

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Web Ontology Language (OWL)

OWL is a suite of knowledge representation languages that are used to construct ontologies. These languages include (from simpler to more complex): OWL Lite (rarely used), OWL DL, and OWL Full. All are based on the RDF/XML formats, described above. Though the languages are similar, the more advanced forms have features unavailable to the simpler ones, preventing full interoperability between the languages.

OWL ontologies can contain classes, which are used to categorize concepts with similar characteristics. These classes can be defined and restricted by axioms, statements that are considered true and which lay the groundwork for inferencing. OWL ontologies may also include individuals, sometimes referred to as instances, for example, a list of terms in a vocabulary. These instances are grouped together as class extensions, which are related to particular classes without defining them. Therefore, two or more classes could share a class extension, meaning that they would be related to the same group of instances.

Basic inferencing can be performed based on the characteristics of defined relations. For example, assume the relationship "is larger than" is defined as the inverse of the relationship "is smaller than." Given a statement such as, "A car is larger than a bicycle," computer software can use the inverse relationship to determine that a bicycle is smaller than a car. Transitivity and symmetry are other primary ontological characteristics that can be defined for relations.

More advanced inferencing is possible. For example, let's consider an ontology that describes human beings. If the class we use to group humans in this ontology is described with an axiom that indicates all humans have the property "hasParent" in combination with the property "hasMother," a computer could infer that every human being must have a parent and a mother.

OWL 1 has been finalized and is currently in use. In October 2007, a W3C working group was formed to extend OWL. The new version, OWL 2, is already in use in some applications.

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Suggested Citation

Alexander, P. 2011. "Ontology Standards." In The MMI Guides: Navigating the World of Marine Metadata. http://marinemetadata.org/guides/vocabs/ont/coretech/standards. Accessed September 26, 2020.