Thursday, 28 May 2015

Semantic Web for the Working Ontologist: chapter 8

Chapter 8 is titled RDFS-Plus. It introduces a selection of OWL – Web Ontology Language – keywords and demonstrates their use.
Clearly, OWL's originators aren't Winnie the Pooh fans, else they'd have stuck with the acronym WOL. Unless they were avoiding confusion with "Wake-on-LANan Ethernet standard that allows computers to be turned on by a network message" and a bunch of other stuff (see Wikipedia).
It felt much easier to understand most of this than I expected - but I suspect that was partly down to the assistance I had this week from Paul Rissen, Data Architect for BBC News, Paul kindly gave up a lunchtime to explain stuff to me, show me how and where the BBC are using linked data, and how he taught himself to develop ontologies. He also introduced me to two useful resources – BBC Ontologies and BBC Things which are hugely helpful for putting all this into context.

Anyway, the new functions covered in chapter 8 are…

It's not unusual when federating data from different sources, to discover different URIs referring to the same thing, or class of things. Specifying that two classes are equivalent indicates that they will always have the same members. One important point the authors make here is that "when two classes are equivalent, it only means the two classes have the same members. Other properties of the classes are not shared".

This, unsurprisingly functions similarly to equivalentClass, but for properties of things, rather than classes of things. So, for example, in the authors example of library books, "borrows" is an owl:equivalentProperty of "checkedOut".

Where equivalentClass and equivalentProperty are the ways of expressing and defining identity relationships between classes and properties of things, sameAs is for individuals. The examples given here include Shakespeare: the Shakespeare that Anne Hathaway married and who had three children is the sameAs the Shakespeare who wrote plays and sonnets. I

Used where one property is the inverse of another: the examples given include hasParent and hasChild and, for checking out of library books, :signedTo owl:inverseOf :signedOut. So, it specifies a mutual relationship between two properties.

In OWL, a transitive property is one that applies throughout a chain of relationships. So, as in the instance given here, ancestors are transitive, parents are not (although parents are a subset of ancestors). "owl:TransitiveProperty is a class of properties, so a model can assert that a property is a member of the class

:P rdf:type owl:TransitiveProperty."

"is an aspect of a single property… expressed in OWL as a Class." It applies in situations where, for instance, you need to specify that two people are married to each other, or are siblings. It applies in any case where a :isWhateverOf b, means that b :isWhateverOf a.

"RDFS-Plus borrows the name functional to describe a property that, like a mathematical function, can only take one value for any particular individual."So, for an individual, age and National Insurance Number could each be a FunctionalProperty.

But FunctionalProperty isn't as useful as InverseFunctionalProperty, which is considered by some pwople "the most important modeling construct in RDFS-Plus, especially in situations in which a model is being used to manage data from multiple sources." It applies in a situation where "a single value of the property cannot be shared by two entities". National Insurance Number can be an InverseFunctionalProperty, but age can't. In RDFS-Plus, where two entities share the same InverseFunctionalProperty, the inference will be made not that there has been an error, but that the two entities are, in fact, the same.

Some properties – like National Insurance Number – can be both functional and inverse functional properties.

ObjectProperty and DatatypeProperty 
I struggled to understand the distinction between these two from the details provided in the book, so resorted to Googling, which led me to a forum – where someone asked the
difference between the two, and the answer was "Datatype properties relate individuals to literal data (e.g., strings, numbers, datetypes, etc.) whereas object properties relate individuals to other individuals".  Which makes a lot of sense!

Next week's chapter – "Using RDFS-Plus in the wild" – looks at real world applications. So, I'm hoping that should be comprehensible!

Until then…

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