SKOS and classifications

“Oh, and we’d really like SKOS as well”.


We have names and references.
Names appear in references at “instances” of a name.
Some name instances are taxonomic concepts.

We have a classification made up of tree nodes.
A tree node has many subnodes.
A tree node may be used by many supernodes as a subtree.
A tree node (for classification trees) refers to a taxonomic concept instance.

Now here’s the thing.

The basic job of SKOS in our data is to say “this concept is narrower-than that concept”. X ∈ A → X ∈ B. Naively, we’d put this over the node ids and say job done.

But our tree nodes are most certainly not taxonomic concepts. They are placements of concepts in some classification. It’s the concepts that are concepts. So the skos output must declare that it is the taxon concepts that are narrower-than other taxon concepts.

The problem becomes that the data becomes a mishmash. If a concept is moved from one place to another, then our classification data (which holds the entire history of a tree) will declare in one node that A⊂B and in another that that A⊂C, where B and C are different taxa at the same level.

Not good.

The problem becomes clearer when you consider having two entirely separate and incompatible classifications. It makes sense to say A⊂B according to the people at NSW University, but A⊂C according to the Norwegian National Classification of Herring. These classifications are sources of assertions that you can choose to trust or to not trust.

The nature of our classification nodes becomes clear. Each node is a document – a source of triples – that asserts certain facts and that trusts the assertion of facts in the nodes below it.

All I need to do is to find a suitable ontology for expressing this. The W3C provenance ontology looks promising. It even has – Oh My Lord! I has these predicates

prov:wasRevisionOf prov:invalidatedAtTime prove:wasInvalidatedBy

Which match exactly certain columns in my data.

(edit: why is this important? It indicates that the provenance ontology and my data may very well be talking about the same kinds of thing. If your data has a slot for “number of wheels” and “tonnage” and so does mine, there’s a good chance we are both discussing vehicles.)

I’m pretty comfortable, at this point, deciding that each tree node in a classification is a prov:Entity object. In fact, it seems to be a prov:Bundle in the sense of its subnodes, and a prov:Collection in the sense of the reified subnode axioms, which are skos terms.

I’m probably over-engineering things again. But there you go.

PS: not all subnode relationships are skos broader/narrower term relationships. In particular, hybrids. A hybrid cucumber/pumkin is not simply a kind of cucumber in the same way that a broadbean is a type of legume. I cannot simply assert skos inclusion over a tree – the actual link types and node types matter.


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