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DC-ARCHITECTURE  October 2012

DC-ARCHITECTURE October 2012

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Subject:

Prefix for http://purl.org/dc/elements/1.1/ in RDFa Initial Context?

From:

Thomas Baker <[log in to unmask]>

Reply-To:

DCMI Architecture Forum <[log in to unmask]>

Date:

Wed, 31 Oct 2012 18:47:22 -0400

Content-Type:

text/plain

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Parts/Attachments

text/plain (70 lines)

Dear all,

Currently, the prefixes "dc:" and "dcterms:" are both mapped to the namespace URI
http://purl.org/dc/terms/ (here: "/terms/") in the RDFa Initial Context [1].  

The Initial Context provides default mappings of namespace URIs to prefixes to
be used as fallbacks for cases in which authors have neglected to explicitly
declare prefixes in their markup.  The decision by the W3C RDFa Working Group
to map "dc:" and "dcterms:" to /terms/ was based, as I understood it at the
time, on empirical data from crawlers which appeared to show that virtually all
of the RDFa data sources crawled were mapping the prefix "dc:" to
http://purl.org/dc/terms/ [2].

Since then, it has been pointed out that a considerable amount of RDFa data has
used the prefix "dc:" with the namespace URI http://purl.org/dc/elements/1.1/
(here: "/1.1/").  For example, see [4] and the very large project Europeana.
It should be noted that some of the empirical data used to establish the
Initial Context showed strong use of /1.1/ though not, as I understood it,
specifically for RDFa data [3].

The history of, and rationale for, the existence of DCMI properties using the
separate /terms/ and /1.1/ namespace URIs is explained in detail in a FAQ
answer [5].  In a word, most of the /terms/ properties have "ranges" which
require that they be used only with literal values (such as the name "Madonna",
a string) or only with non-literal values (such as a URI denoting Madonna),
while the /1.1/ properties can be used with either a literal or non-literal
value.

The Initial Context is intended as a convenience method or fallback option, not
as a replacement for the explicit declaration of prefixes.  However, it arguably
also serves to draw attention to frequently used vocabularies and to reinforce
conventional prefixes for their namespace URIs.

In light of the above, I would like to propose to the RDFa Working Group that a
prefix be added to the Initial Context for the /1.1/ namespace URI.  This would
not solve the problem of people who have used the prefix "dc:" with /1.1/ in
the past and hoped to rely on the Initial Context to automate their conversion
from RDFa 1.0 to 1.1.  However, for those who continue to use /1.1/ properties
-- in some cases, precisely _because_ they are unspecified as to range --
inclusion in the Initial Context would emphasize that /1.1/ has not been
forgotten or deprecated, reinforce the message that /1.1/ properties should not
be "upgraded" to /terms/ properties in a mechanical way, and provide a
conventional prefix to use for the properties.

This issue has been discussed in more detail in a very interesting thread in
Google+ [6], and a draft explanation of the issues, with text from Dan
Brickley, has been put into a wiki page for further editing [7].

My questions to this list:

1. Do you agree that DCMI should request a prefix for /1.1/?

2. If so, what should we propose as the prefix?  I would propose "dc11:",
   as in [8], but if anyone here knows of implementations that use "dce:"
   I would suggest we go with whatever prefix is used more frequently.

Tom

[1] http://www.w3.org/2011/rdfa-context/rdfa-1.1
[2] http://www.w3.org/2010/02/rdfa/profile/data/
[3] http://www.w3.org/2010/02/rdfa/profile/Sindice-crawl.html
[4] http://lists.w3.org/Archives/Public/public-rdfa/2012Aug/0002.html
[5] http://wiki.dublincore.org/index.php/FAQ/DC_and_DCTERMS_Namespaces
[6] https://plus.google.com/108363728773973627360/posts/ffuC1uTJCT6
[7] http://wiki.dublincore.org/index.php/Dublin_Core_Prefixes
[8] http://rdf.greggkellogg.net/distiller?format=turtle&in_fmt=rdfa&uri=http://dublincore.org/2012/06/14/dcmi-terms/index.shtml

-- 
Tom Baker <[log in to unmask]>

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