Topic Maps

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Topic Maps is a standardized (ISO 13250) way to store knowledge. This page contains a very short introduction to topic maps. A more comprehensive documentation is for example The TAO of Topic Maps.

Wandora uses a slightly reduced version of the topic map standard. This page describes the general topic map standard. Some parts of it are not usable in Wandora.

Do you speak Finnish? Puhutko suomea? Suomenkielisen Topic Maps -esittelyn löydät täältä!



The most important concept in the topic map paradigm is topic. Topics are entities you want to say something about. A topic can represent a person, an image, a web page, a company or more abstract things such as a group of people acting together, a significant event in the life of a person or the act of being something. Everything that you want to say something about must be a topic.

For example, you could say that Wandora is a knowledge management suite. For this we need a topic representing Wandora and another topic for knowledge management suite. After that we can associate the two two topics with an association.

Topicmap example01.gif


Associations associate two or more topics. Continuing the previous example, we need to associate the topic Wandora and the topic knowledge management suite. We create an association between these two topics which we call players of the association.

The association also needs a type, otherwise we would only know that Wandora and knowledge management suite have something to do with eachother but we wouldn't know what the relation between the two is. We want to say that Wandora is a knowledge management suite. So the relation here is is a. is a is also a topic and represents the act of something being something. This topic is used as the association type in our association.

There is one more thing we need to add in our association. Players of association do not have any order so we don't know if Wandora is a knowledge management suite or if knowledge management suite is Wandora. We need to add roles for the players. The role of Wandora is instance because it is an instance of the category knowledge management suite. The role of knowledge management suite is category. The roles are also topics in the topic map. A role and it's player(s) are together called a member of the association.

So the association actually has five different topics in it:

  • The association type is a
  • The role instance
  • Wandora which is the player for the role instance
  • The role category
  • Knowledge management suite which is the player for the role category

Each association has exactly one association type and any number members. Each member has exactly one role and at least one player.

Topicmap example02.gif


Occurrences are used for two different purposes in topic maps. They can be used to link documents addressable with URI to the topic. These documents should have some relevance to the topic. They can for example be depictions of the topic, be articles that discuss the topic or they might have the textual contents of the topic if the topic represents a book. These occurrences are called resource reference occurrences.

Occurrences can also contain textual data instead of being a ling to some external (or in some cases internal)) resource. If the topic represents a poem, the occurrence might contain the full textual representation of the poem. These resource data occurrences can also be used to add short pieces of information about the topic such as the birth date of a person, dimensions of a picture etc.

As you have seen, occurrences can be used for many different purposes. Thus it is necessary for occurrences to have an occurrence type like associations. Occurrences do not however need roles because there is only one topic. The other part of occurrence is either textual data or a URI.

Other properties of topic

Topics also have other properties. One of the most important is the name of the topic. In fact, topics can have several different kinds of names. Base names are names that unambiguously identify the topic. For example a base name could be "Washington, Missouri, USA" to represent the city Washington in the state of Missouri in USA. As you can see, it is important to take care so that the topic is not confused with any of the other cities named Washington.

Topics can also have variant names. Variant names are other common ways used to refer to the topic. They need not be unique. A variant name for the city of Washington in Missouri could be just "Washington". Same variant name could be used for other cities.

Another very important property of topics is subject identifier. They are URIs that uniquely identify topics. What the identifying URI is, is application dependent. Some obvious choices would be the URI of the home page of a person, company or city the topic represents or URI to the original copy of an image.

Merging in topic maps

Topics in topic maps have a few properties that must be unique in topic map. In traditional databases, if you try to add a unique identifier that is already in use in the database, you would usually get an error message. Topic maps usually use a different philosophy.

Suppose that you have two different topics "Helsinki" and "Helsingfors" (Swedish for Helsinki). Now you add subject identifiers, the home page of city of Helsinki is and could be used as a subject identifier. Only one topic in the topic map may have this subject identifier because subject identifiers must be unique. Instead of getting an error message when you add the same subject identifier to both topics, the two topics will be merged. By using same subject identifier you have in effect stated that the two topics are in fact the same topic. Thus it is logical to merge the two topics into one topic. The new merged topic will contain all information from both of the unmerged topics.

Merging is a very powerful tool in topic maps. It can be used to merge two different topic maps. For example someone might have a topic map which describe artists and their works and maybe even have pictures of all different works. Another topic map might contain biographical information about the artists but not the pictures. These two topic maps can be merged together to form a topic map with all the artists, pictures and biographical information.

Merging of topics is the key concept behind the Layered Topic Map philosophy Wandora uses.

Topicmap subject.gif


Topic maps also allow use of scopes with occurrences, associations and names. Scopes can be used to say when some piece of information is relevant. For example, we could have variant names for different languages. Each is only relevant in the context of that language.

More about topic maps

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