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Re: [pedantic-web] The OWL Ontology URI

Richard Cyganiak May 6, 2010 2:51 PM
Posted in group: Pedantic Web Group
Niklas, please go to and look at item  
#5. I removed semantic-web from the cc list.

On 5 May 2010, at 19:54, Niklas Lindström wrote:
> how come the OWL Ontology itself is defined as
> <>, and not
> <>? The latter (hash URI, ending in "#")
> is linked to, via rdfs:isDefinedBy, for all classes and properties,
> but the former is the resource described as the owl:Ontology.
> Is this really intentional? It seems to conflate the document and the
> ontology. And ontologies aren't considered to be information
> resources, are they?

I think that ontologies are information resources. An ontology is a  
document, written down in OWL or another ontology language.

> Neither classes and properties are,

That may or may not be true, but it's besides the point. An ontology  
defines (and thereby describes) classes and properties. Information  
resources can describe any kind of thing, including classes and  

> nor is the
> thing linked to via isDefinedBy (itself not described further in the
> document).

I agree that this is a problem; the target of the isDefinedBy links  
should either be further described in the document, or it should be a  
document itself. (rdfs:isDefinedBy is a subproperty of rdfs:seeAlso,  
so it certainly makes sense that the target would be a document.)

> Also notice that neither RDF nor RDFS are described like this -- they
> both use the hash URI as identifier for the Ontology (also linked to
> with rdfs:isDefinedBy from their classes/properties).

This is an interesting question, so I did a little study using the  
collection of all namespace URIs from . The goal was  
to find out how owl:Ontology and rdfs:isDefinedBy are used in the wild.

346 total namespace URIs
238 are hash URIs
192 could be dereferenced and parsed as RDF (using any23)
105 contain rdfs:isDefinedBy or owl:Ontology

Of the 102 unique resources typed as owl:Ontology, 80.4% are hash-less  
URIs, the other 19.6% end in a hash.

Of the 97 unique resources that are targets of rdfs:isDefinedBy  
51.5% are hash URIs *with* fragment (most of them pointing to anchors  
within HTML documents),
26.8% are hash-less URIs,
21.6% are URIs ending in a hash.

Finally I looked at those vocabularies where the namespace URI,  
owl:Ontology resource, and rdfs:isDefinedBy are either identical or  
differ only in the presence/absence of the trailing hash. There were  
40 of those documents. Results:

47.5% - Ontology and isDefinedBy target do not end in hash
27.5% - Ontology does not end in hash, isDefinedBy ends in hash
22.5% - Neither ontology nor isDefinedBy target end in hash
  2.5% - Ontology ends in hash, isDefinedBy target does not

The detailed lists for each of these groups are attached at the end of  
this message.

I conclude that pedants should not use trailing hashes -- neither for  
the owl:Ontology typed resource, nor for the target of  
rdfs:isDefinedBy triples. This is the popular thing anyway, it means  
that only 19.6% of owl:Ontology URIs and 21.6% of rdfs:isDefinedBy  
targets needs fixing, and it embraces the consistent view that classes  
and properties are defined by ontologies, which are documents.

A number of high-profile vocabularies do not use this approach: OWL,  
RDF, RDFS, DOAP, SIOC. But all of these predate the W3C TAG's  
httpRange-14 decision, so they were designed at a time when the  
interactions between RDF and URIs and HTTP were not yet settled. So  
I'd say let's not copy their archaic style, but let's get them to fix  

Any disagreement? Bring it on ;-)


Ontology and isDefinedBy have no hash

Ontology has no hash, isDefinedBy has hash

Ontology and isDefinedBy have hash

Ontology has hash, isDefinedBy has no hash

> Best regards,
> Niklas