Show an overview of the data set at all times. It can be an
inset panel, as in the example at the top of the pattern. It
could also be a panel beside the detail view, or even another
window, in the case of a multiwindow application like
Photoshop (see the examples below).
On that overview, place a viewport. They're usually red boxes,
by convention, but they don't have to be -- they just need to
be visible at a glance, so consider the other colors used in
the overview panel. If the graphic is typically dark, make it
light; if the graphic is light, make it dark. Make the viewport
draggable with the pointer, so users can grab it and slide it
around the overview.
The detail view shows a magnified "projection" of what's inside
the viewport. The two should be synchronized. If the viewport
moves, the detail view changes accordingly; if the viewport is
made smaller, the magnification should increase. Likewise, if
the detail view has scrollbars or some other panning capability,
the viewport should move along with it. The response of one to
the other should be immediate, within a tenth of a second (the
standard response time for direct manipulation).