Filters should adhere to the following principles:
They should work without the aid of concept maps, as they are useful in situations dealing only with the database structure, as well as in combinations with other sorts of concept presentations.
Their definition should not depend on any data format.
They should ultimately be able to produce several different aspect filtering and sorting systems:
One- or higher dimensional systems (list- or matrix-based aspect sorting), where the content is sorted into a grid of aspects. One axis could contain definition, location, use, history, etc., while the other contains the school level of the content.
Combinations with other type of filtering, corresponding to user profiles etc. that eliminates uninteresting content.