Wow!
Thanks Pete!
I understood a fair amount of that :) I'm not sure if that is a good
sign of my mental health.
However, do you have a link to a simple list of these terms... or better
yet, an "head" example that uses the currently recommended setup?
I noted that alot of the links like
http://purl.org/dc/elements/1.1/creator
were not easily human readable.
An example is worth 1000 words.
Thanks!
-Jeff
Pete Johnston wrote:
> Hi Jeff,
>
>> I've been asked by a professor to help organize student
>> website projects on our webserver. I immediately though of
>> using meta tags in the header.
>>
>> After poking around the web for a while I found the Dublin
>> core project.
>> I figured using a meta tag schema that is documented and
>> has wide support is much better than me making up my own.
>>
>> I also found out about, and am playing with mksearch, a
>> spidering tool for Dublin core tagged webpages.
>>
>> I am a bit confused about some of the meta tag specifics. On
>> http://dublincore.org/documents/2007/11/05/dc-html/ , it
>> talks about using tags like this:
>>
>> <link rel="schema.DCTERMS" href="http://purl.org/dc/terms/"
>> /> <meta name="DCTERMS.title" content="Services to Government" />
>>
>> How is DCTERMS.title different than DC.title?
>
> Since January this year, for each property of the Dublin Core Metadata
> Element Set, i.e. the 15 properties with URIs of the form
> http://purl.org/dc/elements/1.1/xyz , there is a now a corresponding
> property in the DC Terms vocabulary with a URI of the form
> http://purl.org/dc/terms/xyz (Aside: note that the reverse is not true:
> there are terms with URIs of the form http://purl.org/dc/terms/xyz for
> which there is no corresponding terms with the URI
> http://purl.org/dc/elements/1.1/xyz )
>
> The introduction of this "parallel" set of properties was primarily due
> to the decision to specify domains and ranges for DCMI properties i.e.
> to make formal assertions in the descriptions of its terms that DCMI
> publishes which enable an applocation to make inferences about the type
> of the described resource or the type of the value when it encounters a
> statement using a specified property. See [1] for more details.
>
> In order not to risk creating contradictions for applications using the
> fifteen properties of the DCMES, which are frequently used with literal
> values, domain and range assertions were not made for these properties,
> but instead a new set of 15 properties were created, for which domain
> and range assertions _were_ made.
>
> So while no range is specified for the property with the URI
> http://purl.org/dc/elements/1.1/creator and no inferences can be made
> about the class of a value of that property, there is a new property
> with the URI http://purl.org/dc/terms/creator for which a range is
> specified, and for which an application can conclude that the value is
> an instance of a class of "Agents" identified by the URI
> http://purl.org/dc/terms/Agent .
>
> Formally the new property http://purl.org/dc/terms/creator is a
> subproperty - refinement - of the original property
> http://purl.org/dc/elements/1.1/creator
>
> (As a very minor side effect of this, the creation of these 15
> additional properties allows for some minor simplification in their use
> of concrete syntaxes since, if they can make use of the 15 new
> properties (i.e. their usage of those properties is consistent with the
> new domain/range assertions) then they require only one "namespace
> declaration" for the names of all the DCMI-owned properties.)
>
> The case of DC.Title and DCTERMS.Title is arguably slightly anomalous,
> since - at this point in time - no range assertion is made for the new
> property with the URI http://purl.org/dc/terms/title and so essentially
> - at this point in time - there is no real difference between the
> property http://purl.org/dc/terms/title and the property
> http://purl.org/dc/elements/1.1/title . However it is worth taking heed
> of the note in [1] regarding the new title property:
>
>> In current practice, this term is used primarily with literal values;
> however, there are important uses with non-literal values as well. As of
> December 2007, the DCMI Usage Board is leaving this range unspecified
> pending an investigation of options.
>
> It is expected that the range for the property
> http://purl.org/dc/terms/title will be changed in the near future.
>
> For more details, see [1] and [2].
>
>> If I have the link to the schema, is it okay for me to mix in
>> my own tags if I don't use the DC or DCTERMS?
>
> The short answer is "yes" :-)
>
> To provide a longer answer and a bit of explanation, I probably need to
> take a step back and emphasise that - as that doc you refer to above [3]
> emphasises - the convention used in HTML meta and link elements is a
> convention for "encoding" a data structure known as a DC description
> set, the form of which is described by the DCMI Abstract Model [4]. So
> the "Expressing DC metadata using X/HTML meta & link elements" doc
> describes a mapping bewteen the components of that data structure and
> the components of X/HTML meta and link elements.
>
> Now, in the DC description set, all metadata terms - properties,
> classes, vocabulary encoding schemes and syntax encoding schemes are
> referred to using URIs. Those can be URIs owned by anyone, not just URIs
> owned by DCMI - and it is perfectly possible for a DC description set to
> contain no references at all to terms owned by DCMI! :-)
>
> And in the meta data profile described by the "Expressing DC metadata
> using X/HTML meta & link elements" doc, the "prefixed names" (like
> "DC.Title") used as the values of X/HTML name, scheme and rel attributes
> are abbreviations/shorthands for thoes term URIs i.e. the prefixed name
> "DC.Title" is (typically - it depends on the association between prefix
> and "namespace URI" provided by a link element) mapped to the URI
> http://purl.org/dc/elements/1.1/title (using the convention described in
> section 3.1.2 of [3]).
>
> So if you wish to refer to terms other than those owned by DCMI, you
> need a URI for each of those terms, provided by the owner of those
> terms, either a third party (like the Library of Congress for the case
> of the MARC Relator Codes) or, as in this case, yourself. So you need to
> coin a URI in some URI-space where you can create URIs, and ideally if
> you expect your metadata instances to be used over the medium/lon term
> then that that URI would be chosen so that it was reasonably persistent.
>
> And then to represent that URI as a name/rel/scheme attribute value, you
> need to choose a suitable prefix for abbreviating the term URIs and add
> a corresponding "namespace declaration" using the link/@rel="schema.XYZ"
> convention described in section 3.1.2 of [3] e.g.
>
> <link rel="schema.JEFF" href="http://jeff.example.org/terms/" />
> <link rel="JEFF.isRelatedInSomeWeirdAndWonderfulWayTo" href="doc123" />
>
> where the "prefixed name" "JEFF.isRelatedInSomeWeirdAndWonderfulWayTo"
> is an abbreviation/shorthand for the URI
> http://jeff.example.org/terms/isRelatedInSomeWeirdAndWonderfulWayTo
>
> Incidentally, although DCMI hasn't yet addressed this in any of its own
> specifications (but I think it probably will at some point!), you may
> also be interested in the recently published working draft from the W3C
> for RDFa [5], which specifies how to use a set of attributes to embed
> RDF metadata in XHTML documents (and other XML documents, I think, but
> the focus in the current draft is mainly on XHTML). Any RDF graph can be
> represented using RDFa, so it's very powerful and flexible, and once you
> become familiar with the basic RDFa "patterns", it is quite easy to use,
> I think.
>
> Finally, it's probably worth adding that _if_ your _only_ requirement is
> to provide data to a local search engine and you have control over that
> document set, you could just go ahead and use meta and link elements
> without worrying too much about the conventions recommended by DCMI.
> i.e. you could just use
>
> <meta name="abcdef" content="blahblahblah" />
>
> without worrying about the mapping of the @name attribute value to a URI
> via the link/@rel="schema.XX" mechanism.
>
> Cheers
>
> Pete
>
> [1] http://dublincore.org/documents/2008/01/14/domain-range/
> [2] http://dublincore.org/usage/decisions/2008/dcterms-changes/
> [3] http://dublincore.org/documents/2007/11/05/dc-html/
> [4] http://dublincore.org/documents/2007/06/04/abstract-model/
> [5] http://www.w3.org/TR/2008/WD-rdfa-syntax-20080221/
>
> ---
> Pete Johnston
> Technical Researcher, Eduserv Foundation
> Web: http://www.eduserv.org.uk/foundation/people/petejohnston/
> Weblog: http://efoundations.typepad.com/efoundations/
> Email: [log in to unmask]
> Tel: +44 (0)1225 474323
>
--
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Jeff Albro - Information Technology Manager
Boston University School of Education
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