Encoding Dublin Core Metadata in HTMLCenter for Knowledge Management, University of California530 Parnassus Ave, Box 0840San FranciscoCA94143-0840USA+1 415-476-4653jak@ckm.ucsf.edu
The Dublin Core is a small set of metadata elements for
describing information resources. This document explains how these
elements are expressed using the META and LINK tags of HTML
. A sequence of metadata elements embedded in an HTML file
is taken to be a description of that file. Examples illustrate
conventions allowing interoperation with current software that
indexes, displays, and manipulates metadata, such as ,
, , , , etc., and the Perl
scripts in the appendix.
(see above)
The Dublin Core (DC) metadata initiative has produced a
small set of resource description categories , or elements of
metadata (literally, data about data). Metadata elements are
typically small relative to the resource they describe and may, if
the resource format permits, be embedded in it. Two such formats are
the Hypertext Markup Language (HTML) and the Extensible Markup
Language (XML); HTML is currently in wide use, but once standardized,
XML in conjunction with the Resource Description Framework
promise a significantly more expressive means of encoding
metadata. The specification actually describes a way to use
RDF within an HTML document by adhering to an abbreviated syntax.
This document explains how to encode metadata using HTML 4.0
. It is not concerned with element semantics, which are
defined elsewhere. For illustrative purposes, some element semantics
are alluded to, but in no way should semantics appearing here be
considered definitive.
The HTML encoding allows elements of DC metadata to be interspersed
with non-DC elements (provided such mixing is consistent with rules
governing use of those non-DC elements). A DC element is indicated
by the prefix "DC", and a non-DC element by another prefix; for
example, the prefix "AC" is used with elements from the A-Core .
The META tag of HTML is designed to encode a named metadata element.
Each element describes a given aspect of a document or other
information resource. For example, this tagged metadata element,
says that Homer Simpson is the Creator, where the element named
Creator is defined in the DC element set. In the more general form,
the capitalized words are meant to be replaced in actual
descriptions; thus in the example,
Within a META tag the first letter of a Dublin Core element name is
capitalized. DC places no restriction on alphabetic case in an
element value and any number of META tagged elements may appear
together, in any order. More than one DC element with the same name
may appear, and each DC element is optional. The next example is a
book description with two authors, two titles, and no other metadata.
The prefix "DC" precedes each Dublin Core element encoded with META,
and it is separated by a period (.) from the element name following
it. Each non-DC element should be encoded with a prefix that can be
used to trace its origin and definition; the linkage between prefix
and element definition is made with the LINK tag, as explained in the
next section. Non-DC elements, such as Email from the A-Core ,
may appear together with DC elements, as in
This example also shows how some special characters may be encoded.
The author name in the first element contains a diacritic encoded as
an HTML character entity reference -- in this case an accented letter
E. Similarly, the last line contains two double-quote characters
encoded so as to avoid being interpreted as element content
delimiters.
The LINK tag of HTML may be used to associate an element name prefix
with the reference definition of the element set that it identifies.
A sequence of META tags describing a resource is incomplete without
one such LINK tag for each different prefix appearing in the
sequence. The previous example could be considered complete with the
addition of these two LINK tags:
In general, the association takes the form
where, in actual descriptions, PREFIX is to be replaced by the prefix
and LOCATION_OF_DEFINITION by the URL or URN of the defining
document. When embedded in the HEAD part of an HTML file, a sequence
of LINK and META tags describes the information in the surrounding
HTML file itself. Here is a complete HTML file with its own embedded
description.
HTML allows more flexibility in principle and in practice than is
recommended here for encoding metadata. Limited flexibility
encourages easy development of software for extracting and processing
metadata. At this early evolutionary stage of internet metadata,
easy prototyping and experimentation hastens the development of
useful standards.
Adherence is therefore recommended to the tagging style exemplified
in this document as regards prefix and element name capitalization,
double-quoting (") of attribute values, and not starting more than
one META tag on a line. There is much room for flexibility, but
choosing a style and sticking with it will likely make metadata
manipulation and editing easier. The following META tags adhere to
the recommendations and carry identical metadata in three different
styles:
Use of these recommendations is known to result in metadata that may
be harvested, indexed, and manipulated by popular, freely available
software packages such as , , ,
, and , among others. These conventions also work
with the metadata processing scripts appearing in the appendix, as
well as with most of the applications referenced from
the site. Software support for the LINK tag and qualifier
conventions (see the next section) is not currently widespread.
Ordering of metadata elements is not preserved in general. Writers
of software for metadata indexing and display should try to preserve
relative ordering among META tagged elements having the same name
(e.g., among multiple authors), however, metadata providers and
searchers have no guarantee that ordering will be preserved in
metadata that passes through unknown systems.
In actual resource description it is often necessary to qualify
Dublin Core elements to add nuances of meaning. While neither the
general principles nor the specific semantics of DC qualifiers are
within scope of this document, everyday uses of the qualifier syntax
are illustrated to lend realism to later examples. Without further
explanation, the three ways in which the optional qualifier syntax is
currently (subject to change) used to supplement the META tag may be
summarized as follows:
Accordingly, a posthumous work in Spanish might be described with
Note that the qualifier syntax and label suffixes (which follow an
element name and a period) used in examples in this document merely
reflect current trends in the HTML encoding of qualifiers. Use of
this syntax and these suffixes is neither a standard nor a
recommendation.
This section consists of very simple Dublin Core encoding examples,
arranged by element.
The syntax rules for encoding Dublin Core metadata in HTML that are
documented here pose no direct risk to computers and networks.
People can use these rules to encode metadata that is inaccurate or
even deliberately misleading (creating mischief in the form of "index
spam"), however, this reflects a general pattern of HTML META tag
abuse that is not limited to the encoding of metadata from the Dublin
Core set. Even traditional metadata encoding schemes (e.g., )
are not immune to inaccuracy, although they are generally followed in
environments where production quality greatly exceeds that of the
average Web site.
Systems that process metadata encoded with META tags need to consider
issues related to its accuracy and validity as part of their design
and implementation, and users of such systems need to consider the
design and implementation assumptions. Various approaches may be
relevant for certain applications, such as adding statements of
metadata provenance, signing of metadata with digital signatures, and
automating certain aspects of metadata creation; but these are far
outside the scope of this document and the underlying META tag syntax
that it describes.
This section contains two simple programs that work with versions 4
and 5 of the Perl scripting language interpreter. They may be
taken and freely adapted for local organizational needs, research
proposals, venture capital bids, etc. A variety of applications are
within easy reach of implementors that choose to build on these
scripts.
Here is a simple Perl script that correctly recognizes every example
of metadata encoding in this document. It shows how a modest
scripting effort can produce a utility that converts metadata from
one format to another. Minor changes are sufficient to support a
number of output formats.
When the conversion script is run on the metadata file example from
the LINK tag section (section 4), it produces the following output.
The creation and maintenance of high-quality metadata can be
extremely expensive without automation to assist in processes such as
supplying pre-set or computed defaults, validating syntax, verifying
value ranges, spell checking, etc. Considerable relief could be had
from a script that reduced an individual provider's metadata burden
to just the title of each document. Below is such a script. It lets
the provider of an HTML document abbreviate an entire embedded
resource description using a single HTML comment statement that looks
like
Our script processes this statement specially as a kind of "metadata
block" declaration with attached title. The general form is
This statement works much like a "Web server-side include" in that
the script replaces it with a fully-specified block of metadata and
triggers other replacements. Once installed, the script can output
HTML files suitable for integration into one's production Web server
procedures.
The individual provider keeps a separate "template" file of
infrequently changing pre-set values for metadata elements. If the
provider's needs are simple enough, the only element values besides
the title that differ from one document to the next may be generated
automatically. Using the script, values may be referenced as
variables from within the template or within the document. Our
variable references have the form "(--mbVARNAME)", and here is what
they look like inside a template:
The above template represents the metadata block that will describe
the document once the variable references are replaced with real
values. By the conventions of our script, the following variables
will be replaced in both the template and in the document:
Here's an example HTML file to run the script on.
Note that because replacement occurs throughout the document, the
provider need only enter the title once instead of twice (normally
the title must be entered once in the HTML head and again in the HTML
body). After running the script, the above file is transformed into
this:
Here is the script that accomplishes this transformation.
Art and Architecture ThesaurusGetty Information InstituteThe A-Core: Metadata about Content Metadata(in progress)
Dublin Core Metadata for Resource DiscoveryOCLC Online Computer Library Center, Inc., Office of Research6565 Frantz Rd.DublinOH43017USA+1 614 764 6081+1 614 764 2344weibel@oclc.orgUniversity of California, San Francisco, Center for Knowledge Management530 Parnassus Ave, Box 0840San FranciscoCA94143-0840USA+1 510 525 8575+1 415 476 4653jak@ckm.ucsf.eduCornell University, University Library and Department of Computer ScienceIthacaNY14853USA+1 607 255 6046+1 607 255 4428lagoze@cs.cornell.eduReuters Limited85 Fleet StreetLondonEC4P 4AJUK+44 171 542 6722+44 171 542 8314misha.wolf@reuters.com
Applications
discoveryDublin CoremetadataDublin Core Initiative Home PageProjects Using Dublin Core MetadataDublin Core Type List 1DC Type Working GroupThe enhanced freeWAIS distributionGlimpse Home PageHarvest Web IndexingHTML 4.0 SpecificationIsearch Resources PageCode for the representation of names
of languages.International Organization for Standardizationhttp://www.iso.chISO 8601, Data elements and interchange formats -- Information
interchange -- Representation of dates and timesInternational Organization for Standardizationhttp://www.iso.chUSMARC Format for Bibliographic DataUS Library of CongressProgramming Perl, Second EditionResource Description Framework (RDF) Model and Syntax SpecificationTags for the Identification of LanguagesSimple Web Indexing System for Humans - EnhancedThesaurus of Geographic NamesGetty Information InstituteDate and Time FormatsExtensible Markup Language (XML) 1.0World Wide Web ConsortiumMIT Laboratory for Computer Science545 Technology SquareCambridgeMA02139US+ 1 617 253 2613+ 1 617 258 5999http://www.w3c.orgtbray@textuality.comjeanpa@microsoft.comcmsmcq@uic.edu