Database Reference
In-Depth Information
Admin Areas
Settlements
Landforms
Structures
Places
Road & Rail
FIGURE 10.1
Initial top level of the ontology.
But Merea Maps needs to be very sure that this is always true. Certainly, in the
case of Administrative areas and Settlements it is not only true, but also making
them mutually exclusive can help to detect confusions that may exist between them.
(A very common mistake is to believe that the settlement and the administrative
area that is responsible for the settlement are the same thing.) However, overuse of
disjoints can slow a reasoner, so caution should be exercised before use.
As we shall see, these top-level classes also provide common properties for their
subclasses. The development of the top-level classes is likely to be quite iterative,
especially in the early stages as design patterns are established.
Before we start to describe the next phase of developing design patterns and estab-
lishing lower-level classes, we stop for a moment to point out that an ontology is rarely
a simple hierarchy; the majority of it is a network. We have already encountered this
with the Footprint class. Footprint does not sit within the hierarchy of Features but is
nevertheless an important component of the ontology. Ontologies normally comprise
a number of interleaved hierarchies that form a network; establishing these hierar-
chies and the interrelatedness of them is an important part of the ontology authoring
process. One should not feel limited to one single hierarchy (a taxonomy).
As a general rule, it is good design practice to have each hierarchy as shallow
as possible. The more high-level classes there are, the more difficult it may be for
a third party to reuse the ontology. This is again because higher-level classes tend
to be more abstract and therefore more open to disagreement. It is also important
to note that hierarchical relationships are just that; they are strict trees and so can
only express one type of relationship: subsumption, “is a kind of.” The problem is
that if subsumption is the only tool you have, and in traditional hierarchical classi-
fication systems this is all you have, it is very easy to mix categories from different
hierarchies. For example, it is very common to see the following type of “hierarchy”:
Topographic Feature
Business & Industry
Factory
Office
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