Topics

A topic map consists of a collection of topics. A topic is a proxy for anything that the topic map author wishes to document in his or her topic map. As already indicated, the thing that a topic represents can be:

• An electronic resource that can be retrieved and processed by a computer.
• An electronic resource that is not accessible to the computer.
• A "real-world thing" such as a person or an object which cannot be retrieved or processed by the computer (although the computer may have information about how an actor in the real world could retrieve the thing).
• * A concept with no physical manifestation such as an emotion or a technical concept. A business entity such as a company or a department falls into this category too.
• A construct in a topic map - we will see later how this is useful for being able to add successive levels of detail to a topic map.

Figure 2, below, shows the distinction between a topic and the object or concept that the topic represents in the real world. An author has in his or her mind the subject to be documented and creates a topic. The topic is a machine representation of the subject which can now be stored, queried and manipulated by the computer.

Figure 2 - Representing the real world with topic maps

Topic Characteristics

Characteristics is a collective term used to describe the three properties which a topic may have which together form the collection of assertions that the topic map author has made about the subject that the topic represents.

• Names are labels for the topic that serve to identify it to the user of the topic map in some way. These labels can also be used by an application for sorting and display of topics. Names are string labels but each string label can have any number of alternate representations, including non-string forms such as graphics. For example a company may have a full name and a ticker symbol on the market on which it is listed. In addition, some names may be differentiated by their purpose such as sorting, iconic display and so on.
• Occurrences are identifiable resources that are in some way related to the topic. Note that an occurrence resource needs only to be identifiable to a topic map processor. It does not need to be retrievable, although typically a non-retrievable resource is not of much use to a topic map application. For topic representing a company, a retrievable occurrence might be its home page or a stock quote for the company. A non-retrievable occurrence might be the registration papers for the company.
  As well as resources external to the topic map, occurrences can also be used to specify additional information about the topic which is kept in the topic map itself.
  In Figure 1, occurrences are shown as the dashed lines connecting the topics to resources in the resource pool.
  
• Roles played in associations form the final class of characteristic of a topic. Associations are used to relate two or more topics to each other. In Figure 1 associations are shown as solid lines connecting topics together. We will look at associations in a moment; all that is important to know right now is that in any given association a topic will play some identifiable "role". For example an individual might own some shares in a company. In this case there is an association between individual and company with the individual playing the role of "shareholder" and the company playing the role of "share-holding". Note that the roles would be the same in the case of an organisation such as a financial institution holding shares in the company. In XTM syntax, the mark-up that declares what roles a topic plays in an association are part of the mark-up of the association, not of the topic, but conceptually the roles that a topic plays are considered to be a characteristic of the topic.

Basic Topic Syntax

The following example shows the representation of a simple topic map consisting of a single topic. Note that the <topicMap> element defines the default namespace as the namespace defined for XTM 1.0 and the namespace for XLink. In later examples I will omit the XML declaration and the namespace declaration to save space. To save space, I have not included the full text of the XTM DTD in this article, but it is available to online on the TopicMaps.Org site at http://www.topicmaps.org/xtm/1.0/

<?xml version="1.0"?>
<topicMap xmlns="http://www.topicmaps.org/xtm/1.0/"
	xmlns="http://www.w3.org/1999/xlink">

  <topic id="xzyyz">
    <baseName>
      <baseNameString>Redmond Computers Inc.</baseNameString>
    </baseName>
    <occurrence>
      <resourceRef xlink:href="http://www.redmondcomputers.com/"/>
    </occurrence>
    <occurrence>
      <resourceData>1977</resourceData>
    </occurrence>
  </topic>

</topicMap>
          

Sample 1 - A simple topic map

Some points to note about the XTM syntax introduced in this example:

• The <topic> element has an id attribute. I have deliberately used a value bearing no relation to the topic itself. The id attribute is simply a syntactical construct that will allow us to make references to the topic later; beyond this it has absolutely no meaning to a topic map processor. All elements defined by the XTM 1.0 DTD have an id attribute, enabling any part of the topic map document to be referenced if need be, but the id attribute value is only required for the <topic> element.
• The name label is contained within a <baseNameString> element, which is in turn contained within a <baseName> element. As you might expect, this indicates that there are further properties of a base name that we have not yet discussed.
• Similarly for the <occurrence> the reference to the occurrence resource itself is contained in a <resourceRef> element. The <resourceRef> element is declared in the XTM DTD as an XLink simple link. It is this use of XLink (and its use in all the other reference constructs in the XTM DTD) which requires the declaration of the XLink namespace. The second of the occurrences shows the use of the <resourceData> element to provide the occurrence data "in-line" with the topic map. This makes it somewhat easier to add meta-data to topics.

Up: Topic Map Basics
	Next: Types