This has come up independently in a couple of messages I've had
off-list, so I thought I'd post something here, as it raises a general
point about the DC CD AP and DCAPs in general, which I think we may need
to extend the DC CD AP to cover.

According to the DCMI Abstract Model [1], a DC metadata description is a
set of statements about a single resource, where each statement asserts
a relationship between that "subject" resource and a second resource,
and the DCAM calls that second resource the "value".

A statement consists of:

- exactly one reference to a property ("property URI");
- a reference to a value ("value URI"), or a representation of the value
("value string"), or both;
- optionally a reference to a class that indicates the type of the value
("vocabulary encoding scheme URI")

(A representation can be something other than a value string, and value
strings can be associated with syntax encoding schemes, but let's stick
to the simple case.)

So, e.g., to represent the fact that a book is authored by a person. In
terms of the DCAM, the resource is the book, and the value is the
person. I could construct a description using a value string

Description
  Resource URI : http://example.org/book/1234
  Statement
    Property URI : http://purl.org/dc/elements/1.1/creator
    Value String : "Fred Smith"

This just says the creator is some resource represented by the string
"Fred Smith"; it doesn't tell me which "Fred Smith" it is.

Or I could use a value URI (I know there is the question that people
typically don't have URIs assigned to them, but let's suppose they did):

Description
  Resource URI : http://example.org/book/1234
  Statement
    Property URI : http://purl.org/dc/elements/1.1/creator
    Value URI : http://example.org/person/Fred

Now I know that the relationship is between two uniquely identified
resources, but I don't have any human-readable information about that
value.

Or I could use both a value string and a value URI:

Description
  Resource URI : http://example.org/book/1234
  Statement
    Property URI : http://purl.org/dc/elements/1.1/creator
    Value URI : http://example.org/person/Fred
    Value String : "Fred Smith"

And I could add a vocabulary encoding scheme URI too to indicate the
type of the value:

Description
  Resource URI : http://example.org/book/1234
  Statement
    Property URI : http://purl.org/dc/elements/1.1/creator
    Value URI : http://example.org/person/Fred
    Value String : "Fred Smith"
    Vocabulary Encoding Scheme URI : http://some.org/type/Person


Now, what a DCAP is doing (well, this is my take on it - unfortunately
DCMI doesn't currently have a formal description of what a DCAP actually
is, so it's difficult to say this with any authority!) is: for some
class of descriptions (actually, description sets, but let's stick with
the simple case for the purposes of this discussion) e.g. those used
within a particular application, or those that describe some class of
resources, it specifies

- what properties I should use in statements in that class of
descriptions
- additional information about how those properties are deployed in that
class of descriptions, which includes
- information about the types of value that those properties should take
(i.e. the vocabulary encoding schemes to be used) in that class of
descriptions

So I sometimes think a DCAP might be better named a "description (set)
profile": it describes how to construct a description (set).

OK, looking back to the example above. I emphasised that I could include
in my statement either a value string "Fred Smith" or a value URI
http://example.org/person/Fred or both.

And further, the choice of vocabulary encoding scheme is independent of
whether I use a value URI or a value string. I do not use a vocabulary
encoding scheme to signal the use of a value URI.

So the question that has come up regarding the DC CD AP is:

(For some property specified by the DC CD AP), should I use a (value)
string or a (value) URI?

The answer (as the DC CD AP stands at the moment) is that _any_ of the
properties specified (except maybe dc:identifier) may be used with
either a value string or a value URI or both.  e.g. to specify that the
owner of my collection is Fred Smith, I can use any of the forms of
description above with a property URI of
http://www.loc.gov/marc.relators/own

i.e. the DC CD AP is silent on whether a value string or value URI is
used in the statement.

So the question is whether this is what we want, or whether we should
include in the DC CD AP an indication that (for some specified
properties e.g. the relationships between collections?) a value URI
should be deployed. That would not rule out the use of a value string as
well as a value URI.

This is not done by specifying that the "vocabulary encoding scheme" is
dcterms:URI - that would state that the value, the resource, _is_ a URI,
but in the example above the value is a person. And I don't think it is
done by saying that the "syntax encoding scheme" is dcterms:URI because
value URIs are distinct from value strings in the DCAM.

Indeed none of the specifications for guidance for creating DCAPs (that
I've seen) address this point. So I think it would require a separate
statement in the "property usage" - the description of how the property
is used in the DC CD AP - to state that the use of a value URI is
required.

i.e. an additional row in the tables like

http://www.ukoln.ac.uk/metadata/dcmi/collection-application-profile/#dct
ermshaspart

to say something like:

Requires Value URI = Yes

Similarly - I'm less sure about this - there may be cases where we want
to indicate that, for specified properties, the use of a value string is
required. That would not exclude the use of a value URI too. Again, I
think that would require a separate statement in the property usage.

Pete
-------
Pete Johnston
Research Officer (Interoperability)
UKOLN, University of Bath, Bath BA2 7AY, UK
tel: +44 (0)1225 383619    fax: +44 (0)1225 386838
mailto:[log in to unmask]
http://www.ukoln.ac.uk/ukoln/staff/p.johnston/