Database Reference
In-Depth Information
simply not been done for the great majority of braided rivers. Merea Maps therefore
has three options. It can use some vague and undefined property “has multiple”:
Every Braided Stretch has
multiple Channels.
Class:BraidedStretch
SubClassOf: hasMultiple some Channel
It can set some lower limit that it believes most people will agree with:
Every Braided Stretch has at
least 5 Channels.
Class:BraidedStretch
SubClassOf: hasChannel min 5 Channel
Or, it can say nothing about the number of channels at all. Saying nothing is not quite
doing nothing; Merea Maps can still subclass the Braided Rivers and classify rivers
as braided, and users can ask questions about them. This is an important example
of using the open world assumption to advantage—a precise definition cannot be
produced, so do not try; rather, define a class and let experts on the ground apply the
class. If they record the number of channels, then over time it might even be pos-
sible to deduce a minimum. There will be many occasions when you can spend a
lot of time trying to find precise definitions that simply cannot be nailed down. The
important thing is to recognize early on the nature of the beast and to quickly come
to terms with an incomplete description.
10.4.7 D
efineD
c
lasses
We have been careful when talking about the specification of a class to say that we
are
describing
rather than
defining
it. This is because there is a way of specifying a
class that specifically results in what are termed
defined classes
. A defined class dif-
fers from a described class in that it enables a reasoner to automatically classify any
individual as a member of the class providing that individual meets the criteria of
the defined class. To understand how a defined class works, you need to understand
necessary and sufficient conditions.
10.4.7.1 Necessary and Sufficient Conditions
To this point, we have been dealing with what are called primitive classes, classes
that are simply subsets of other classes. In DL terms, if A is a subset of B we say
that B is a necessary condition for A. If we look at geographic examples, then for
a river to be a river, it is necessary for it to be a type of watercourse, so being a
watercourse is a
necessary
condition
for being a river; for a schoolhouse to be a
schoolhouse, it is
necessary
for it to be a building and for it to have the purpose of
providing education, so being a building and providing education are
necessary
conditions
for being a schoolhouse.
If we look at the problem from the other direction, then we can see that if we
know something is a river, then it must also be a watercourse. In this case, we say
that a river is a
sufficient
condition
for something to be a watercourse: If something
is a river, it must also be a watercourse. But equally, being a river is not a necessary
condition for being a watercourse since a canal or drain is also a watercourse but
are not a river.
