9.5 Zooming Into Hierarchically Structured Information

Once a document has been structured hierarchically, readers frequently need additional methods for navigating in the hierarchy. Girill and Luk suggest a different way of browsing that they have implemented in their system DOCUMENT [Gir92]. Their technique is based on zooming in and out of hierarchically structured documents. DOCUMENT is mainly used to store and retrieve technical information like on-line manuals that exhibit a hierarchical document structure. Girill and Luk motivate their work by identifying four weak aspects of browsing in tree-structured documents:

Order-dependency:

To reach a leaf-node, readers need to know the whole path from the root of the tree to the particular leaf.

Unbalanced concept discrimination:

The most discriminating concepts on the path towards a desired leaf-node are near the leaves, and not near the root. This means, that readers starting a path at the root have to deal with the most undiscriminating nodes first to find a very specific node at the end of the path.

Incomplete context:

To get complete navigation information, the siblings of a node are often essential. Unfortunately, the siblings are not included in the hierarchical path from the root to the leaf-node.

Semantic factoring:

Unfortunately, hierarchical paths are frequently not sufficient to offer enough information to find the wanted leaf-node. This problem has to be addressed by offering other navigation facilities like full-text search or keyword index.

Girill and Luk claim that the first three of the above defined problems can be solved by offering the functionality of zooming in and out. Order-dependency can be addressed by browsing the document in hypertext fashion and zooming into the hierarchy as soon as the context seems promising. The discrimination problem is also easily solved, because a zoomed-in view of the context of a node allows simple discrimination among other, similar nodes. The local and global context of a node is clarified by zooming in to and out from a particular node.

Zooming offers an simple solution for some navigation problems. Obviously, the concept is not limited to hierarchically structured information, but works with any graph. The zooming concept can be considered a special case of fish eye views. Fish eye views offer a general framework to display large amounts of information from a fish eye's perspective. Fish eye views are described in chapter 11 about mapping information.