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navigation over these resources irrespective of the latter's form. Topic maps thus act as taxonomiesâ€”ways of describing, classifying, and indexing an information space consisting of Web and, as we'll see, non-Web objects. Whether or not Topic Maps can constitute full-fledged ontologies is subject to some dispute, and we will hold off on that discussion until the next chapter.
Topic Maps Standards
The development of Topic Maps began in the pre-XML and pre-WWW era when SGML (Standard Generalized Markup Language, a document composition language, of which a simpler subset became XML) reigned supreme. SGML was based on DTDs that later became the driving structural definition of early XML, now largely being superseded by XML Schema. So, the early Topic Maps standard was in fact based on SGML and used a non-XML syntax. The problem, then as now, is this: How do you characterize the semantics of your documents? How do you represent what your content meansâ€”in a way that a machine can use?
Topic Maps today, as defined by the International Standards Organization (ISO) 13250 standard (hereafter referred to as ISO 13250),8 are specified in terms of two different interchange syntaxes: a more recent one based on XML and an older one based on an SGML DTD that used the ISO 19744 HyTime standard (a standard for specifying hypertext that includes resource addressing and linking). To simplify the exposition, this chapter focuses only on the XML TM syntax, referred to as XTM.9
Figure 7.10 shows the components of the Topic Maps standard and their relationship to each other. The ISO 13250 components are on the left, and the OASIS Published Subject Indicator Technical Committees are on the right. Note that items marked with a * have yet to be fully definedâ€”though versions do exist. The Standard Application Model (SAM) defines the formal data model of Topic Maps and its semantics in natural language.10 The Reference Model is intended to be a more abstract model of Topic Maps than SAM and to enable Topic Maps to semantically interoperate with other knowledge representation formalisms and Semantic Web ontology languages.11 The Topic Map Query Language (TMQL) will be an SQL-like language for querying topic map information. The Topic Map Constraint Language (TMCL) will give a database schemalike capability to Topic Maps enabling constraints on the meaning to be defined for Topic Maps. Both TMQL and TMCL are dependent on the final elaboration of SAM, which is itself dependent on RM.12
8 For additional information on the various Topic Maps standards, see Biezunski et al., 2002.
9 Garshol and Moore (2002a).
10 Garshol and Moore (2002b).
11 See Newcomb and Biezunski (2002) for a view of what the RM might look like.
12 Biezunski et al. (2002) makes these relationships clear.
Standard Application Model
HyTime Syntax *Topic Map Query Language
XTM Syntax *Topic Map Constraint Language
Key: * - future
Figure 7.10 Components of the Topic Maps Standard.
The products of the OASIS technical committees are intended to be layered onto the ISO 13250 standard's products.13 The Published Subjects Technical Committee will define and manage published subjects (which will be discussed shortly), and establish usage requirements for these. The XML Vocabulary Technical Committee will define the vocabulary to enable Topic Maps to interact with existing and emerging XML standards and technologies; the vocabulary will be defined as published subjects according to the standards defined by the Published Subjects TC. Finally, the Geography and Languages Technical Committee will define geographical country, region, and language-based published subjects to ensure interoperability across geographical and linguistic boundaries. All of the OASIS technical committees are currently actively pursuing their objectives.
Listing 7.1 depicts a simple XTM topic map. We will refer to this example in the subsequent discussion of the important concepts of Topic Maps.14
13 See OASIS Topic Maps technical committees.
14 The left-hand side of Figure 7.10 is adapted from Biezunski et al. (2002).
<topic id="Front Royal">
<parameters><topicRef xlink:href="#display"/></parameters> <variantName>
<resourceData>Gateway to Skyline Drive</resourceData> </variantName>
<instanceOf><topicRef xlink:href="#portal"/></instanceOf> <resourceRef xlink:href="http://www.ci.front-royal.va.us/"/>
<instanceOf><topicRef xlink:href="#portal"/></instanceOf> <resourceRef xlink:href="http://www.ci.winchester.va.us/"/> </occurrence>