Travails in the Third Dimension: A Critical Evaluation of Three-dimensional Geographical Visualization - Geographic Visualization: Concepts, Tools and Applications - page 206

user. (Several examples may be seen in the images created using the OpenDX software, which
are available at www.opendx.org/highlights.php.) Where large parts of the globe are shown
in a 3D scene, the bounding box may need to be more complex, and where thematic layers
are stacked above the globe, each layer may also require its own local reference frame. Clearly,
the number of variables being displayed places limits on the effectiveness of this approach,
which will be considered again later. For the moment, it is worth noting that perceptual
research has revealed a possible distortion effect in the use of an enclosing frame, which may
flatten 3D scenes, and reduce perception of depth within them (Eby and Braunstein, 1995).
Reference and slicing planes
In this solution, a plane is drawn at a specific level within the view, thus acting as a visual plane
of reference for the analyst. The plane may be opaque or transparent, it may have grid lines
drawn across it, and it may be drawn on any of the three axes, though in most geographical
visualizations it will be positioned along the z -axis. Cutting planes are commonplace in
'slice and dice' voxel medical models, which have become more widely known through the
well-known 'Virtual Body' and 'Visible Human' projects. User-adjustable cutting planes are
also widely used for geophysical exploration (e.g. Frohlich et al. , 1999).
A simpler example is illustrated in Figure 10.2, in which a semi-transparent reference plane
is used to enable users to visually identify locations in part of north London where more
Figure 10.2 Three-dimensional view showing various types of commercial property in north
London, with a semi-transparent horizontal reference plane used to highlight locations with more
than five properties (source: author)