Geography Reference
In-Depth Information
desktop mapping system. A variation of this method is more widely used in the 3D display
of anatomical and other medical illustrations, creating an 'exploded view' (e.g. Bruckner
and Groller, 2006). However, the use of exploded views in statistical data visualization, as in
the case of 3D exploded pie graphs, includes potential interpretative errors into the display.
The third object displacement method involves the repositioning and spatial distortion of
contiguous (usually zonal) objects in geographical space, as in 2D area cartograms. A danger
with this family of techniques is that they render the user less likely to be able to interpret the
exact spatial relationships between objects that have been moved (often by some arbitrary
amount) from one another.
View distortion
By differentially distorting the overall geometry of a scene, objects near the user viewpoint
may be more clearly seen. Perhaps the best-known technique is the fish-eye view (Furnas,
1991), which selectively distorts a scene so as to enlarge objects near the point of interest.
One of the problems of this technique is that users may be unable to obtain a proper sense
of the spatial relationships between objects, though this is more likely to affect displays of
inherently spatial data than displays of purely statistical data, and it should be noted that
many geographers will be used to the spatial deformations used in map projections and
cartograms.
Rotation or viewer movement
One of the biggest advantages of interactive 3D data visualization over printed 3D images is
that users have the ability to resolve some of the occlusion problems interactively, either by
rotating objects within the scene or by moving their viewpoint with respect to the scene. This
induces the kinetic depth effect, which not only reduces symbol occlusion, but also enhances
the viewer's appreciation of the depth relations between objects in a scene (Ware and Franck,
1996). However, this may be accompanied by unpleasant user side effects, especially in an
immersive 3D environment and may also required adeptness in navigating within 3D scenes
which many analysts will not possess.
Symbol transparency
By displaying selected symbols in reduced opacity, occluded symbols may be seen through
foreground symbols. Several options are available: symbols in a scene may be drawn with
equally reduced opacity, or the transparency of selected occluding symbols near the focus
of the user's attention may be increased, leaving those further away at full or increasing
opacity. A somewhat different role for symbol transparency is to enable the comparison of
two datasets in a single scene by applying transparency to one set of symbols and rendering
the other set opaquely. An example is illustrated in Figure 10.5, which shows the comparative
distribution of cigar makers in the East End of London from two datasets: the population
census of 1881 (the green transparent symbols) and Booth's poverty survey of the late 1880s
(the red opaque symbols) (Shepherd, 2000).
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