An important reason for creating the open standards ontology is that it can be accessed by many different users and/or applications. The open standard OWL (Web Ontology Language) is used in my thesis and is explained by (Bechhofer and Carrol), and there are several markup languages that can be used to represent structured information.
McGuinness (2003) explains the role of markup languages in defining content to be machine readable, McGuinness cites a diagram from a presentation by Berners-Lee (2000) that contains a diagrammatic representation of the place of each language in a stacked representation alongside the purpose of the language. This is shown in below.
Architecture, sourced from Berners-Lee (2000) - http://www.w3.org/2000/Talks/1206-xml2k-tbl/slide10-0.html.
XML may not be sufficient on its own for defining ontologies. The XML syntax defines relationships by their position within the text file. Thus XML syntax always implies a sequence whereas in reality the order of items may be unimportant, also there is no explicit way of representing associations between items, or differentiating between an Inheritance and a Contains relationship. XML schemas and DTDs (Document Type Definitions) can be helpful in defining these relationships, but there is then scope for differences in the way they are defined. RDF has provided a layer of standardised semantics which overlays the basic XML. The RDF text can be embedded within XML.
I have chosen to use RDF represented using RDF/XML as this allows me to continue using XML tools for visualising or searching the RDF/XML while also allowing the use of tools available for representing RDF. Vehicle'. RDF consists of a resource, a property, and a property value. This 'Triple' corresponds to 'Subject', 'Predicate', and 'Object' in logic. Each RDF triple represents a fact. So RDF structures information into individual facts that link as a graph, each fact is a triple. This can be thought of as a sentence representing a fact such as 'Aircraft is a Vehicle'. An example of an RDF graph is shown in the figure below and the table illustrates rows of facts that make up the graph. More information is available here - Quick Intro to RDF.
RDF Graph Example, RDF Description of Aircraft
| Resource | Property | Value | |
| Subject | Predicate | Object | |
| Fact | Aircraft | Is a | Vehicle |
| Fact | Aircraft | Flown by | Pilot |
| Fact | Aircraft | Has | Engine |
| Fact | Plane | Is an | Aircraft |
| Fact | Plane | Has | Wings |
| Uniform Resource Identifier (URI) |
A Resource is anything that can have a URI (uniform resource identifier). A URI can look like a web address and can actually be a web address, but this is not always the case, it is a way of representing an entity. A URI consists of the name and location of the entity. An RDF Resource is described through a collection of properties and property values called an RDF Description. RDF provides a mechanism for describing collections, which are special kinds of resources, and a sequence is an ordered collection. A collection does not have to possess its own URI but it can.
RDF/XML has provided a layer of standardised semantics which overlays the basic XML. RDF does not have to be based on XML there is also a format called N3 http://rdfabout.com/quickintro.xpd. RDF extends the XML model and syntax to be specific for describing resources. For example Engine Ring Manufacture sequence can be represented as a sequence of groups of sequential operations as in this example. RDF is a W3C (World Wide Web Consortium) Recommendation, this means it is a stable specification and therefore a standard. Because a resource can represent anything, knowledge from any domain can theoretically be represented in RDF, this together with it's standardised syntax that allows it to be machine understandable are the reasons why RDF is such a useful and important technology for the Semantic Web.
RDF/XML Web pages can be linked to each other indefinitely, which is why it is such an important technology for the Semantic Web. If a web page exists for a URI there could be further information possibly represented using RDF on this web page. This allows resources to be linked to each other, which is why it is such an important technology for the semantic web. Because it is XML based, an RDF/XML Web page can be linked to an XSL stylesheet to produce a visual representation of the structure as in this example. This is also explained in this paper http://www.cems.uwe.ac.uk/~phale/ISPECE2003.htm, and by (Cayzer, 2004) who uses RDF to provide structure for Semantic blogging. Oren et al (2006) also use this approach of combining RDF and Semantic Web use with ease of editing in a Semantic Wiki.
SPARQL Simple Protocol and RDF Query Language is a query language and protocol for RDF being recommended to the W3C.
RDF/XML can be used to encode an ontology. (Fensel et al. 1998) and (Fensel et al. 2001) describe Ontobroker and the use of XML and RDF within this ontology tool. The use of ontologies is being driven by e-commerce and e-procurement where trading is online (UN/CEFACT and ebXML 2007).
References
Bechhofer S, Carrol J (2004) Parsing owl dl: trees or triples? Proceedings of the 13th international conference on World Wide Web, NY, USA pp 266 - 275.
Berners-Lee, T., 2000. Semantic Web on XML http://www.w3.org/2000/Talks/1206-xml2k-tbl/slide1-0.html.
ebXML 2002, ebXML Enabling a Global Electronic Market. http://www.ebxml.org/, OASIS & UN/CEFAC, accessed on 9th January 2007.
Cayzer, S. 2004. Semantic Blogging and Decentralized knowledge Management. Communications of the ACM. Vol. 47, No. 12, Dec 2004, pp. 47-52. ACM Press.
Fensel, D. & Angele, J. & Decker, S. & Erdmann, M. & Shnurr, H. & Studer, R. & Witt, A. 1998. On2broker: Lessons Learned from Applying AI to the Web. http://www.aifb.uni-karlsruhe.de/WBS/Publ/2000/llfaattw_dfeetal_2000.pdf, accessed on 9th January 2007.
Fensel, D. Van Harmelen, F. Horrocks, I. McGuinness, D. Patel-Schneider, P. F., 2001. OIL: An ontology infrastructure for the semantic web. IEEE Intelligent Systems, 16(2), pp 38-45. - http://www.cs.man.ac.uk/~horrocks/Publications/download/2001/IEEE-IS01.pdf.
McGuinness D. L., 2003. Ontologies Come of Age. http://www-ksl.stanford.edu/people/dlm/papers/ontologies-come-of-age-mit-press-(with-citation).htm In: Dieter Fensel, Jim Hendler, Henry Lieberman, and Wolfgang Wahlster, ed. Spinning the Semantic Web: Bringing the World Wide Web to Its Full Potential. MIT Press, 2003.
Oren, E., Breslin, J. G., Decker, S., 2006. How Semantics Make Better Wikis - WWW 2006, May 23-26, 2006, Edinburgh, Scotland.
Relevant Web Pages
Semantic Web Page - http://www.cems.uwe.ac.uk/amrc/seeds/PeterHale/RDF/RDF.htm
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