Problem: How can you present this large amount of content so that a user can explore it at their own pace, in a way which is comprehensible and engaging to the user?
Keep the overview and detail areas spatially adjacent, so that the user can easily glance back and forth, using the overview area to drive the decisions about what to look at next. If you use Tiled Working Surfaces to relate them to each other, the user doesn't need to mentally "context switch" or make any extra gestures to go from one area to the other (such as raising one window above another). This enables a sense of flow.
Resulting Context: You have to decide how the user selects the part of the overview that they're interested in. The kind of data you're working with should suggest a graceful solution: a set of objects can be selected one at at time (or perhaps several contiguous ones), while a continuous data set like a geographic map may provide a freely movable "thumbnail," drawn on top of the overview, that shows the current selection.
A set of high-level objects or categories can be linear, hierarchical, or otherwise organized by whatever principle is appropriate -- see Hierarchical Set, Tabular Set, Editable Collection, Personal Object Space, and whatever other patterns you find appropriate.
Sometimes you just can't easily partition the content into only two levels of abstraction at a time. You should always retain the high-level overview, but you could then take the detail view and make it act like a second-level overview, so that the user can drill down one more level to see yet more detail. This recursion then gives you a total of three views, and you can continue the recursion for as many levels of depth as you need to. Netscape Messenger does this to show email and news, by the way: an upper-left panel shows the mailbox or newsgroup hierarchy, an upper-right panel shows the list of messages in the currently-selected mailbox or newsgroup, and a large bottom panel shows the message currently selected in that upper-right panel. Stuart Card et. al. recommend that for zooming in on a continuous data set, use a zoom factor between 3 and 30; more than that becomes disorienting for the user.
Sophisticated users may find it too constraining to see only one detail view at a time; if so, consider allowing multiple detail views simultaneously, possibly using Pile of Working Surfaces. Just keep in mind the increased complexity of the artifact when you do that -- users may get confused about where they are. I have rarely found it necessary to do this, however.
Notes: This pattern has also been noted, though not in pattern terminology, by Stuart Card, et. al. in their book Readings in Information Visualization: Using Vision to Think. A two-paper section called "Overview Plus Detail" is devoted to this concept; in the section introduction, they explain their reasoning:
Having an overview is very important. It reduces search, allows
the detection of overall patterns [in the data], and aids the user in choosing
the next move. But it is also necessary for the user to access details
rapidly. One solution is overview plus detail.
Copyright (c) 1999 by Jenifer Tidwell. All rights reserved.