Categories of objects as well as individual objects
are eliminated, others are replaced by a symbol of larger or smaller size, some are
displaced, their shape is simplified, topological relationships may change, groups
such as ???building blocks??? replace individual buildings where the density is too high,
and so on. In other words, the content of every map may lie, the measurements made
on every map may lie, and a topological relationship on every map may lie. Taking
this into account in a multiresolution spatial datacube goes beyond the traditional
topological concepts.
Figure 2 shows an example of the impact of map generalization with regard to the
spatial dimension of a datacube (a ???spatial aggregation-generalization mismatch???).
This introduces specific concerns for the interactive exploration of spatial data as
seen later in this chapter.
From a geomatics point of view, spatial and temporal data are typically considered
different from thematic data. Spatial and temporal data are reference data; they are
used to locate phenomena in space and time rather than describing them. They are
not intrinsic to a phenomenon like thematic data are. Since the human brain has builtin
capabilities to use space and time, users intuitively rely on spatial and temporal
data to integrate other data obtained from different sources. Such integration from
different sources is typically performed without a priori planning since space and
time are perceived by most to be universal reference systems.
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