Introduction to Layered Topic Maps
(→Applications of topic map merging and layer paradigm)
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== Implicit duplication of information ==
== Implicit duplication of information ==
Revision as of 11:43, 21 September 2006
Merging of topics is a very powerfull feature of topic maps. However it also introduces some problems. Wandora uses a layered topic map paradigm to counter some of these problems and also to introduce new ways of using topic maps.
Problems with merging
Suppose we have two topic maps which both describe artists. They have been created from different points of view however, one deals mostly with biographical information about artists and the other has details about works the artists have created. They overlap mostly, but not completely, meaning that most of the artists in one topic map are present also in the other topic map but in some cases there isn't biographical information about an artist that has pictures in the other topic map or vica versa. Suppose also that both topic maps contain date of birth and date of death of most artists. In some cases these might have not been known and thus are not in the topic map.
We can merge these two topic maps to form a topic map with all the combined information. After merge it is however impossible to solve where some piece of information originated from. Any artist might have been in either or both of the topic maps just like a date of birth might have been in either or both. Suppose we find a mistake in the date of birth of some artist and want to fix it. There is no way to know which one of the original topic maps had the wrong date without inspecting the original topic maps. With associations and a greater number of merged topic maps things become even more complex.
In many cases it is also useful to edit small topic maps that focus on certain aspect but view the topic map with full information from many different sources. Merging of large topic maps can be computationally complex and require a long time to complete. Thus it may not be possible to perform a large merge operation after every small change to the original topic maps.
Layered topic maps in Wandora
Wandora uses a layered topic map paradigm. Topic maps are organized in layers, each layer containing one topic map. Each topic map can be turned visible or invisible. The topic map that the user sees is the merged topic map of all visible topic maps. However, internally Wandora does not perform a complete merge operation at any time so switching layer visibility is a relatively fast operation. Only the topics visible on the current page are merged. Because changing layer visibility is fast, the user can easily focus on one topic map or view the entire merged information.
The order of layers is used to decide which piece of information is visible when there are different values for some properties of topic. In the reduced topic maps that Wandora uses, topics can have only one base name, only one subject locator and only one occurrence for each scope. If two topics that will be merged have different values for one of these properties, we have to decide which of the possible values actually is used in the merged topic map. Value from the topmost layer is used, if there are several different values in topmost layer, one of them is chosen arbitrarily. Note that we wouldn't need to do this if Wandora used complete standard topic maps, because they may have several base names, subject locators and occurrences. We could simply have them all in the merged topic.
At all times, one of the layers is selected. All edit operations are done in the topic map of this selected layer. This may cause implicity duplication of topics when the visible topic is not in the selected layer. This is discussed in more detail below.
Note that the layers in Wandora resemble the layers paradigm used in many image manipulation programs. Just like in Wandora, these programs allow user to switch layer visibility and all edit operations are done in the layer user has selected. Also topmost layers overwrite information from bottom layers by drawing over them.
Applications of topic map merging and layer paradigm
Each layer can be a local topic map stored in memory or in database or they can be remote topic maps. Having several local topic maps allows you to keep different kinds of information separate but at the same time easily inspect the combined information.
You can also combine your own topic maps with topic maps provided by some other third party. This third party topic map may be provided as an online resource which can directly be used in Wandora or you may have downloaded it as an XTM file and use with your own topic map. Whichever is the case, you probably want to keep the third party topic map separate from your own topic map.
If the third party topic map is updated, it is easy to update just the one layer. It might even be done automatically if you use it as an online resource. Had you actually merged the third party topic map with your own topic map into one single file, it would be very hard to find out which parts of the file needs to be updated. Simply merging the new version again might leave deleted topics or erroneous information in the topic map.
The merging rules can be used make an adapter topic map that is used to combine two different topic maps. This is needed when two or more topic maps that have overlaping information but use different vocabulary and subject identifiers need to be merged. Because of the different vocabulary and subject identifiers, topics that represent same conepts would not be merged. An adapter topic map in a sense translates the indentifiers back and forth. This is done by creating topics that contain identifiers from both topic maps. This causes topics from both topic maps to merge with topics in the adapter topic map and thus the topics in the two original topic maps are merged together.
Middle circle represents the adapter topic that has two subject identifiers, one for both topics to be combined. This make the different topics in the other two layers to be combined with the adapter topic and thus all three are combined. The adapter topic doesn't contain any other information so it does not interfere in any way with the information contained in either of the other two topic maps.
Implicit duplication of information