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what we need or have components that we can use?” It finds that there are some
ontologies, largely micro-ontologies, that describe useful properties, and Merea
Maps decides to use these rather than reinvent the wheel.
These are the following:
Ontology
Description
Prefix
OGC (Open Geospatial
Consortium) Geometry
Contains definitions for the basic OGC geometry primitives
corresponding to points, lines, and polygons
OGC: 6
RCC8 Topology
Contains definitions of the RCC8 relations
RCC8:
Mereology
Contains basic part of and whole relations
GeoParts:
Geographic Names
Defines a class for geographic names and associated relations
GeoName:
Network Topology
Contains basic network topology classed (link and node)
along with standard network relations such as connects to
NetTopo:
Having selected some ontologies to reuse, Merea Maps now embarks on the main
task of constructing the ontology; the first part of this activity is usually the hardest
and takes up a disproportionate amount of time, especially for domain ontologies.
This is because the first phase is about establishing high-level classes and design
patterns or templates that can be applied to many of the classes. It is particularly true
of domain ontologies as these often contain many classes that can be grouped and
expressed using particular patterns.
10.4.4.1 Top-Level Classes
Establishing the top-level classes in an ontology can be deceptively easy, but if done
properly quite hard. This is certainly true for Merea Maps. Merea Maps already has
a Feature Type Catalogue that it uses to define the content of its maps, and this cata-
logue is arranged as a hierarchy starting at the top with “Map Feature” and going all
the way down to the individual surveyable objects such as buildings, roads, rivers,
factories, schools, and so on. The deceptively easy bit is simply to use this hierarchy
to establish the basic structure of the ontology. But rather than just doing this, Merea
Maps pauses and asks whether a hierarchy defined thirty years previously to meet
the needs of surveyors is suitable to be used as is by an organization wishing to pub-
lish data on the Semantic Web. The answer it comes up with is that while much of
the lower levels of the hierarchy can be reused, the top level requires serious rethink-
ing. The reason for this is twofold. First, while the lower levels of the hierarchy refer
to fairly concrete concepts such as Building, the levels above become progressively
more abstract and thus open to philosophical dispute. In the case of Merea Maps,
the hierarchy divides early on into two branches, one for natural features, the other
for artificial features. This split may seem fairly logical, but in truth it can be very
difficult to determine whether something is artificial. In Merea, there are very few
things that have not been affected or altered by the action of people. Even seemingly
complete natural features such as woods and rivers have been altered over the pas-
sage of time by the islanders; rivers have been straightened and rerouted, and all
the woods have been managed, Merea of the twenty-first century has no wild wood.
This leads to the second factor: In an ontology, everything must add value; the upper
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