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demonstration of this. In English, there is a general differentiation between rivers
and streams since rivers are considered to be larger than streams. It is also clear
that there are many instances of things named as streams that are larger than things
named as rivers, and although large watercourses are always called rivers and very
small watercourses are always streams, there is a blurring between large streams and
small rivers. There is no established convention to distinguish the two; there are no
rules to say that a watercourse above a particular length, width, or rate of flow is a
river and less than that it is a stream. The first thing that Merea Maps asks of itself is,
“Is it important?” Even the answer to this question is ambiguous because in one sense
it really does not matter—at a physical level, there really is no difference: Both rivers
and streams are watercourses; they both transport water as natural flows through
channels cut through the landscape. Differences in naming are entirely a linguistic
artifact. So, from this perspective it really does not matter. However, because both
terms are very well established and because there is a general, although imprecise
and undefined, differentiation in terms of size, it would be a strange Topographic
Ontology that did not include the terms
river
and
stream
or include one but not the
other. Merea Maps has a number of options. First, it can provide its own definition
that precisely distinguishes between the two; for example, any watercourse with a
width of 2 m or greater is a river; anything less is a stream. And, indeed this is a
solution that Natural Merea arrives at when faced with a similar problem in deter-
mining the difference between lakes and ponds; here, it decides that a lake is any
inland body of water greater than 2 hectares and a pond anything up to 2 hectares.
However, there is a subtle difference between the two organizations' operational
responsibilities that make this approach less attractive to Merea Maps. Natural
Merea is using the definition for internal uses; it helps them by providing a distinc-
tion that can be used to apply different management techniques, and the choice is
not completely arbitrary but founded on differences in management policies based
on physical size. By contrast, Merea Maps will not use the descriptions for internal
operational purposes, but as terms to enable end users ranging from professionals
to the general public to query the topographic data that they are publishing. Simply
imposing a size limit is unlikely to satisfy large numbers of users and could even
create heated debates between individuals with very different views of what consti-
tutes a river or stream. So, it decides to take a different approach. This approach is
based on the principle that the main difference between the two is largely an impre-
cise linguistic distinction (or rather indistinction), and the two classes are physically
identical. It therefore implements the following solution: It introduces a superclass
Watercourse that represents the physical representation of a river or a stream and
then creates two subclasses, River and Stream, that are distinguished by the way
they are named. It then classifies instance or Rivers or Streams based on the name of
the watercourse. So, the River Adder is classed as a river and the Isis Beck a stream
(as becks are defined as subclasses of streams). Unnamed watercourses are deemed
to be streams. This is not a complete solution. Although people can now return all
rivers and streams by asking for all watercourses and can return all rivers or streams
by querying the corresponding subclass, the latter could produce odd results related
to the vagueness between small rivers and large streams; end users, particularly pro-
fessionals, may still need a way to query by size. This is of course always possible
