Database Reference
In-Depth Information
layers must provide useful components that add descriptively to the lower levels or
that are useful to end users querying and interacting with the data. What benefit
does differentiating between natural and artificial features bring? The answer that
Merea Maps has arrived at is that it adds very little benefit; the classification is not
only difficult to unambiguously specify but also adds nothing that can be reused in
a concrete sense by the lower levels. As a result, they set about redesigning the top
level of their hierarchy such that it is now suitable to be used in an ontology. In fact,
the process of constructing this top level is not one that can usually be done simply
in a top-down manner. Some things are obvious—everything of primary interest to
Merea Maps has a location, so we can expect a high-level class. In the Merea Maps
case, Topographic Feature (or Feature for short) will specify a location property.
Merea Maps states that Features have a Footprint:
Every Feature has a Footprint.
Class: Feature
SubClassOf: hasFootprint some
Footprint
The Footprint class in turn comprises one or more OGC geometries that describe
areas, points, and lines related to Earth's surface.
Every Footprint has geometry
one or more of OGC Point, OGC
Line, or OGC Polygon.
Class: Footprint
SubClassOf: hasGeometry some
(OGCPoint or OGCLine or OGCPolygon)
This means that any number of geometries can be associated with a Feature. The
reason why more than one geometry may be necessary is that, in the case of Merea
Maps, it wants to be able to specify the geometry at different scales, including just a
point reference for very small scales.
Next, Merea Maps tries to see if there are other top-level classes that are immedi-
ate subclasses of Feature. To do this, the nature of the Features that they survey are
examined to see if there are any obvious groups and to see if these groups are useful
to the end users. They come up with a number of potential candidates: Administrative
Areas, Settlements, Landforms, Structures, and Places as shown in
Figure 10.1
.
Most of these classes are obvious; Place is less obvious and, as we shall see, is also
not the best choice. Although there are many different definitions for Place, Merea
Maps decides to use it in a very specific way: A place is somewhere (a Feature)
where there is a designed purpose (intent) for something to happen there. They then
use it to cover things such as hospitals, golf courses, and other complex areas where
some specific activity is intended to take place. At this point, one of the main things
that these classes do is to split the ontology into manageable chunks where it is rea-
sonably easy to assign subclasses in an unambiguous way. In fact, if Merea Maps
believes that it can unambiguously assign particular types of things such as farms
or towns to one and only one of these high-level classes, then it can explicitly make
these classes disjoint, for example:
Administrative Area and Settlement are
mutually exclusive.
Class: AdministrativeArea
DisjointWith: Settlement
