9. Hierarchy
A meaningfully structured document can substantially assist in exploring and locating information. Hierarchical structuring, as employed frequently in technical documentation and manuals, is one of the most popular means of organizing large documents. Ideally a document is molded into a hierarchical structure during its creation and authoring phase. It is much harder to impose a hierarchical structure on an existing document. In the first section of the chapter we review existing approaches for the representation of hierarchically structured information. After briefly reviewing the electronic table of contents, we present the three conventional ways to lay out trees as identified by Knuth [Knu73]: "graph trees", "indentation", and "nested set notation". They will be discussed using the example system IGD [Fei88]. IGD can be used to organize arbitrary information into a hierarchical structure. We then present an newer tree visualization method called "tree-map" [Joh91] as well as tree-like structures called "multitrees" [Fur94]. We also briefly describe a technique of navigation by zooming in and out of trees. The final section in this chapter presents SuperBook, a system that automatically generates (hierarchical) structure based on the table of contents.