Introduction to Layered Topic Maps

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 a 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. Internally Wandora doesn't perform a complete merge operation at any time so switching layer visibility is a relatively fast operation. This way user can easily focus on one topic map or view the entire merged information.

All different layers may be local topic maps in which case the layer paradigm makes it possible to easily separate different kinds of information. Some layers may be external online topic maps that are being provided by a third party. In this case the user can easily merge his own topic map with another topic map. The external topic map will most likely be provided read only so the user cannot modify it, only his own local topic map.