Semantic Wiki

A semantic wiki extends a wiki by semantic technologies like RDF, OWL, Topic Map, or Conceptual Graphs. The main idea is to make the inherent structure of a wiki given by the strong linking between pages accessible to machines (agents, services) beyond mere navigation. This is generally done by annotating existing navigational links with symbols that describe their meaning. For example, a link from Mozart to Salzburg could be annotated with lived in or born in.

Such annotations are useful for many purposes, e.g. enhanced presentation by displaying contextual information, enhanced navigation by giving easy access to relevant related information, and enhanced semantic search that respects the context in addition to the content. Note that presentation, navigation, and search can be done in a rather generic manner, but often profit greatly from an adaptation to the represented context.

Semantic wikis exist in many different flavours (e.g. Se-MediaWiki, SemWiki, IkeWiki, PlatypusWiki). While for some the page content is still in the foreground and the annotations just optional added value, others require annotations sometimes to the extent where the annotation is more important than the page content itself. This reflects also that different systems have different purposes, e.g. extending existing content by annotations to allow for better navigation, collaborative ontology engineering, etc. Nonetheless, frequently found features are:

• Typing/Annotating of Links.

Virtually all semantic wikis allow to annotate links by giving them certain types. The idea behind this is that a link created by a user almost always carries meaning beyond mere navigation, as given in the example in the beginning of this section. The way link annotations are edited differs from system to system. Some semantic wikis include the annotations as part of the wiki syntax (e.g. Semantic MediaWiki), while others provide a separate editor for adding annotations (e.g. IkeWiki).

• Context-Aware Presentation.

Many semantic wikis can change the way content is presented based on semantic annotations. This can include enriching pages by displaying of semantically related pages in a separate link box, displaying of information that can be derived from the underlying knowledge base (e.g. a box with a graphical tree presentation for content belonging to a hierarchy, or license information), or even rendering the content of a page in a different manner that is more suitable for the context (e.g. multimedia content vs. text content).

Enhanced Navigation. Annotated/typed links provide more information for navigation. Whereas a traditional wiki only allows to follow a link, a semantic wiki offers additional information about the relation the link describes. Such information can be used to offer additional or more sophisticated navigation. For instance, links are more independent from the textual context they appear in and can be displayed e.g. in a separate “related information” box. The page describing Mozart could e.g. offer a separate box with references categorised by lived in, composed, etc.

• Semantic Search.

Most semantic wikis allow a semantic search on the underlying knowledge base. Usually, queries are expressed in the language SPARQL, a query language recently proposed as W3C recommendation for RDF querying. Using semantic search, users can ask queries like retrieve all pieces composed by Mozart or retrieve all documents where the license permits derivative works.

• Reasoning Support.

Reasoning means deriving additional, implicit knowledge from the facts entered into the system using predefined or user-defined rules in the knowledge base. For example, from the fact that Mozart composed Die Zauberfl¨ote, a system capable of reasoning could deduce that Mozart is a Composer. Although reasoning is an important feature, it is only supported by a small number of Wikis. The reasons for this might be that it is time-consuming, memory intensive, and can yield results that are not expected and/or traceable by the user.

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