Andrew said: > It's not clear to me why you would want to invent a new > namespace for the definition of dctag when it was related to > but not purporting to be dc:subject. This seems similar to > what is done with encoding schemes, e.g., dct:DDC, dct:LCC, > dct:LCSH, dct:MESH, dct:NLM, dct:TGN and dct:UDC. Taken from > "Expressing Qualified Dublin Core in RDF /XML" document: > > <dc:subject> > <dcterms:MESH> > <rdf:value>D08.586.682.075.400</rdf:value> > <rdfs:label>Formate Dehydrogenase</rdfs:label> > </dcterms:MESH> > </dc:subject> > > It seems like you could just define a new encoding scheme, > e.g., dcterm:TAG, to handle the semantics of social tagging. > However, that might not be enough. Organizations such as > Flickr, YouTube, etc. may desire slightly different semantics > for their social tagging. DCMI probably doesn't want to keep > defining new encoding schemes on a regular basis. Leaving aside the "which namespace do we use" issue for a second, I think we need to be careful not to confuse two very different types of thing, two different types of term used in DC metadata: properties and vocabulary encoding schemes. A property is a specific type of relationship. The dc:subject property is one specific type of relationship, defined and named by DCMI (using a DCMI-owned URI) and described by DCMI in human-readable terms as "The topic of the content of the resource." A vocabulary encoding scheme, on the other hand, is something quite different. According to the DCMI Abstract Model, it is a class of which the value is an instance. N.B. This is one of the areas where a change is susggested in the proposed revisions to the DCAM - the suggestion is that we change the concept of VES to something like "an enumerated set of which the value is a member", and that is _not_ represented as an instance/class relationship, but for the purposes of this discussion I don't think that matters too much. The point is that a VES is a different thing from a property and "plays a different role" in DC metadata. So, when I use the dc:subject property in an RDF triple or a DC statement, I'm making an assertion that resource:A has-as-topic resource:B Or maybe more colloquially resource:A is-about resource:B I could specify that resource:B is an instance/member of dcterms:LCSH or an instance/member of dcterms:DDC (i.e. I could specify a vocabulary encoding scheme for the value). That provides some additional information about the value - it's an instance/member of some specified class/set - but that doesn't change the nature of the relationship that I'm asserting between resource:A and resource:B. The property referred to in my triple/statement is still the same: the dc:subject property. I'm still asserting a "has-topic"/"is-about" relationship. If resource:B is a tag, and I use it as the object in an RDF triple or a DC statement with the dc:subject property, then I'm making an assertion that resource:A has-as-topic tag:T Or resource:A is-about tag:T I could specify that tag:T is an instance/member of petej:TagSet (i.e. I could specify a vocabulary encoding scheme for that value), but - as in the example above of LCSH and DDC - adding the vocabulary encoding scheme provides additional information about the value, but it does not change the assertion I am making about the nature of the relationship between resource:A and tag:T. It's the property which specifies the nature of the relationship, and as long as I'm using the dc:subject property, I'm asserting a "has-topic"/"is-about" relationship. And in my previous message, I was arguing that when people "tag" resources, yes, they are asserting a relationship between the tagged resource and a tag (but see also note below), but it is _not_ true that the relationship they are asserting is always a "has-topic"/"is-about" relationship. On the contrary, people use tagging to represent all sorts of relationships - ownership, status, "rating", related-location. A resource tagged "to-read" on del.icio.us isn't "about" the concept of not having been read yet. Well, yes, I accept that somewhere out there someone has written a weblog post describing the pile of paperbacks on their bedside table and a del.icio.us user has indeed tagged it as "to-read" with that notion in mind, but in the vast majority of cases that isn't the case! ;-) So representing all tagged-resource/tag relationships as statements using the dc:subject property not only fails to capture the particular relationship that someone had in mind when they tagged a resource, but asserts a relationship which - in many cases - the tagger did not intend. (Actually, I should have highlighted yesterday that del.icio.us doesn't only represent tags using dc:subject, it also uses the property http://purl.org/rss/1.0/modules/taxonomy/topics from the RSS taxonomy module. But I'd suggest that the same issue arises. Tagging is used with intent other than to indicate a has-topic relationship.) So, in the general case, a property other than dc:subject (or taxo:topic/taxo:topics) would be required. You could argue that the dc:relation property does the job - there is some unspecified type of relationship between the resource and the tag - or you could argue for a more specific "is-associated-with-tag" or "is-tagged-with" property. I'd argue against putting "subject" in the name/URI because I think we want to avoid suggesting (even to a human reader) any relationship with the dc:subject property. And indeed the ontology I referred to yesterday provides such a property http://www.holygoat.co.uk/owl/redwood/0.1/tags/taggedWithTag Described as "Indicates that the subject has been tagged with the object tag. This does not assert by who, when, or why the tagging occurred. For that information, use a reified Tagging resource." So we can say resource:A tags:taggedWithTag tag:T The final part of that description is what I was referring to in my "but see also note below" above. Depending on what information it is desirable/necessary/useful to capture about the "tagging", then you may wish to adopt the approach of describing that "event" in more detail. If I understand it correctly, the ontology supports both the simple resource:A tags:taggedWithTag tag:T approach, and it also supports a richer, more complex approach which seeks to represent more of the "context", particularly the agent who performed it and the point in time they did so, by representing a "tagging event" as a resource ("reifying the tagging"). See http://www.holygoat.co.uk/projects/tags/ for more discussion, examples. > However, DCMI would not need to define new encoding schemes > on a regular basis since the above qualified Dublin Core > really boils down to: > > <dc:subject> > <rdf:Description> > <rdf:type rdf:resource="http://purl.org/dc/terms/MESH"/> > <rdf:value>D08.586.682.075.400</rdf:value> > <rdfs:label>Formate Dehydrogenase</rdfs:label> > </rdf:Description> > </dc:subject> > > Which generates the same exact RDF triples. This implies > that anybody can create new encoding schemes and semantics, > albeit not in the dcterms: namespace since it is controlled > by DCMI, and still be compatible with the DCMI model. So if > Flickr wanted to define their definition of tags they could just do: > > <dc:subject> > <rdf:Description> > <rdf:type rdf:resource="http://www.flickr.com/photos/tags/"/> > <rdf:value>D08.586.682.075.400</rdf:value> > <rdfs:label>Formate Dehydrogenase</rdfs:label> > </rdf:Description> > </dc:subject> > > Which would provide interoperability with Dublin Core without > DCMI lifting a finger. Internally at OCLC, for research > projects, we have been using this interoperability practice > for defining new encoding schemes to controlled vocabularies > that DCMI has not defined. > For example, GSAFD, NGL, RVM, etc. Oh, yes, in terms of the name/URI for the term, I quite agree that there's no requirement that a DCMI-owned URI is assigned. I don't mind whether it's a DCMI-owned URI or a URI owned by another agency (as long as it's an agency I trust to (a) manage their URIs sensibly so as to ensure a "reasonable" degree of persistence and (b) provide consistent representations of the identified resources in a way which makes those representations accessible to my tools using simple widely-deployed mechanisms in accordance with W3C guidelines.) But (IMHO) the requirement would not be satisfied by coining a new vocabulary encoding scheme, whether that scheme was identified by a DCMI-owned URI or a URI owned by another agency. Pete