Database Reference
In-Depth Information
9. </ogc:asWKT>
10. </ogc:Polygon>
Lines 1 to 3 state that feature is an instance of
example:feature
with a
default geometry identified of
example:geometry
.
example:geometry
in
turn is an instance of the
ogc:Polygon
class that has the literal value defined by
the WKTLiteral (line 6 specifies that the coordinate reference system being used is
WGS84). Points and lines can be expressed in a similar fashion.
6.5 CLASSIFICATION
Another important aspect of a geographic feature is what it is, that is, how it is
classiied. We give a fuller description of how we build classification systems in the
chapters on ontologies but start with a brief introduction here.
As we said in a previous chapter, in RDF we can give a thing a classification using
the statement
rdf:type
:
Ash Fleet Farm is a Farm.
mm:181113 rdf:type mm:Farm.
which simply says that Ash Fleet Farm is an instance of a particular class called a
Farm. It does not, however, explain what a Farm
is
. We deal with how we provide
a description of a class when we discuss the development of ontologies in further
chapters. For now, it is sufficient to understand that once we have defined something as
a Farm we can ask questions about Farms and get results that include Ash Fleet Farm.
There is nothing to stop us from applying multiple classifications to an object; we
could, for example, also define Ash Fleet Farm as a heritage site and a conservation
area. There is nothing wrong in doing so, but we need to think carefully about why
we might do this. Often when we assign different classifications to the same thing,
we are mixing different contexts. In the example case, we are mixing its function
(it is a farm), its historic interest (it is a heritage site), and ecology (it is a conserva-
tion site). We need to carefully consider whether the whole of the farm is a heritage
site or conservation area. If not, we may be really dealing with different areas and
hence different things, even though they may contain some reference to Ash Fleet
Farm. For example, parts of Ash Fleet Farm, including the main farmhouse, are part
of a conservation area known as Ash Fleet Farm Conservation Area. However, since
only parts of the farm, and not the entire farm, are included in the conservation area,
defining the Ash Fleet Farm as a conservation area would be incorrect. Indeed, what
this tells us is that a new feature or resource exists: Ash Fleet Farm Conservation
Area, which should have its own URI and be treated independently from the farm.
6.6 TOPOLOGY AND MEREOLOGY
A geometrical description of a geographic object is obviously important, but in
Linked Data terms, topology (explicit spatial properties) and mereology (part-whole
properties) are probably more important.
