Dublin Core (DC) is an interoperable metadata standard that is primarily used to describe networked resources. It offers a simple and flexible descriptive framework that can be used across different domains and for multiple purposes. The DC is widely adopted internationally as a means for describing digital objects and more recently, it is increasingly being used to describe physical resources and artefacts. This post aims to review the DC metadata standard by looking at its origins, objectives, governance, functionality and best-practice guidelines. Specific reference will be made to the Irish context with a brief discussion (and appraisal) of the use of DC in Ireland.
Introduction: origins, usage, governance
Dublin Core (DC) is shorthand for the Dublin Metadata Core Element Set, which is a list of metadata elements first agreed at the OCLC/NCSA Metadata Workshop in March 1995. Around this time, it was recognised that there was a growing need for a metadata standard that could describe networked electronic information, and the workshop brought together interested parties from a range of professional backgrounds and disciplines such a librarians, computer scientists and text encoders (“OCLC/NSCA”). The motivation behind the DC was to reach a consensus on a small set of metadata that could be used to describe electronic information. From the very beginning, the DC was not intended to supplant other resource descriptions, but rather to complement them (“OCLC/NSCA”). While offering a simple framework for resource description it also aimed to provide a basis for semantic interoperability between other, more complicated, formats.
As such, the DC was designed for use more generally across different domains. The intention was to give authors and information providers a means to describe the resource themselves, without extensive training or specialist knowledge of established standards. While the DC was originally developed for describing document-like objects (DLOs), the metadata can be applied to a range of resources, physical and electronic. For example, the Digital Library Catalog at Berkley uses the DC to describe books, essays, speeches, and other textual material in HTML, technical reports (in various formats), photographs, engravings and other visual materials, and video and sound clips. JED Modes Repository, a collection of S-Lang scripts, uses DC metadata terms for the characterisation of the submissions. The MusicBrainz Project uses a metadata standard that is an extension of the DC to create a metadata catalogue of all music recordings around the world.
From 2000 onward, the Dublin Core community, known as the Dublin Core Metadata Initiative (DCMI), focused on using the DC together with other specialised vocabularies to meet particular implementation requirements. In 2005, a DCMI Abstract Model that was compatible with the Resource Description Framework (RDF) was designed to identify the components used in the DC metadata that could enable better mappings and cross-syntax translations. This signalled the beginning of an evolution in the DCMI approach to metadata. The scope of the DC metadata was broadened from “electronic resources” to include any object that could be identified, whether electronic, physical, or abstract, and particularly including resource types that were named in the DCMI Type Vocabulary (“User Guide – Wiki”). From the outset, the DC designers were committed to creating a standard that was extensible, syntax-independent, optional, repeatable and modifiable so that it could be used differently across different domains (“OCLC/NSCA”). However, while the standard can be applied to non-DLOs, the suitability of the DC for these types of resources depends to some extent on how closely their metadata resembles typical document metadata and also what purpose the metadata is intended to serve.
The DCMI is an open organisation that aims to support innovation and best practice in metadata design. Activities include consensus-driven working groups, global conferences and workshops, standards liaison, and educational efforts to promote widespread acceptance of metadata standards and practices. The governance of DCMI is structured through the co-operation of a governing board, a technical board, and advisory board, an executive committee and the directorate. See Figure 1.
Since 2001, all changes made to terms of the Dublin Core Metadata Element Set are reviewed by a DCMI Usage Board in the context of a DCMI Namespace Policy (“DCMES”). The namespace policy describes how DCMI terms are assigned Uniform Resource Identifiers (URIs) and sets limits on the range of editorial changes that are allowed to the labels, definitions, and usage comments associated with existing DCMI terms. The Usage Board ensures that there is order to the development of the metadata terms. It evaluates proposals for new terms (or changes to existing terms) in light of grammatical principle, semantic clarity, usefulness, and overlap with existing terms (“DCMI Usage”). It assigns a specific status to proposals that are accepted and it also evaluates other structures that use DCMI terms, such as application profiles.
This all takes place as part of a formal approval process where proposed changes are considered in five defined categories, ‘changes to DCMI metadata terms’ being one of these five categories (“Procedure”). Proposals can come from the Directorate (e.g. as a result of an outsourced activity), Affiliates, Communities, Task Groups, other organizations external to DCMI, or any individual.
Functionality: structure, terms, elements
The Dublin Core Metadata Vocabularies distinguish four types of terms: Properties, Classes, Syntax Encoding Schemes and Vocabulary Encoding Schemes. See Figure 2. The “Properties” or elements are the “core” attributes of the resources. The “Classes” are groups of resources that have certain properties in common and are put together as one concept. “Syntax Encoding Schemes” are the rules that specify how a value must be structured and “Vocabulary Encoding Schemes” identify controlled vocabularies (such as taxonomies, thesauri etc.) whose terms may be used as values. The terms in DCMI vocabularies are intended to be used in combination with terms from other, compatible vocabularies (“DCMES”).
The core attributes or “Properties” are used for the structured description of resources. They originally included two levels: “Simple”, which comprised fifteen elements and “Qualified”, which included three additional elements (Audience, Provenance and RightsHolder) as well as a group of element refinements (called qualifiers) that could add more detail to the basic element set. Since 2012, “Simple” and “Qualified” have been incorporated into the “DCMI Metadata Terms” as a single set of terms that are reliant on the RDF.
The original element set exists now as the “Dublin Core Metadata Element”, for which a separate namespace is maintained. A detailed description of all the elements is available on the DCMI website but the names of the elements are, for the most part, self-explanatory; Contributor, Coverage, Creator, Date, Description, Format, Identifier, Language, Publisher, Relation, Rights, Source, Subject, Title, Type. The “Relation” element could be considered “structural” metadata, while the “Rights” and “Identifier” elements could be considered “administrative” metadata. The remaining twelve elements could be considered “descriptive” metadata.
There is no prescribed order for presenting or using the elements and each element is optional and may be repeated. Implementers may choose to use these fifteen properties either in the original set or as part of the “DCMI Metadata Terms” set of elements (“DCES”). Each term in the “DCMI Metadata Terms” has a unique URI in the namespace. The list is as follows (the original fifteen elements are highlighted):
abstract, accessRights, accrualMethod, accrualPeriodicity, accrualPolicy, alternative, audience, available, bibliographicCitation, conformsTo, contributor, coverage, created, creator, date, dateAccepted, dateCopyrighted, dateSubmitted, description, educationLevel, extent, format, hasFormat, hasPart, hasVersion, identifier, instructionalMethod, isFormatOf, isPartOf, isReferencedBy, isReplacedBy, isRequiredBy, issued, isVersionOf, language, license, mediator, medium, modified, provenance, publisher, references, relation, replaces, requires, rights, rightsHolder, source, spatial, subject, tableOfContents, temporal, title, type, valid
Describing each element is beyond the scope of this blog post but two examples are:
Access Rights: Information about who can access the resource or an indication of its security status, may include information regarding access or restrictions based on privacy, security, or other policies.
Has Format: A related resource that is substantially the same as the pre-existing described resource, but in another format.
The extension of the DC standard to include the “qualifiers” or “DCMI Metadata Terms” has enabled the DC to move beyond the “core” fifteen element set that had been repeatedly criticised as simplistic or generalist (Beall). As Harper notes, today the DC standard provides a framework and a model, as well as a set of principles for designing metadata, and the real value and success of the DC is its commitment to interoperability and its applicability to any metadata scheme, element set or implementation syntax (22).
Best practice application: guidelines, qualification, controlled vocabularies
The original fifteen metadata terms are endorsed in IETF RF 5013, ISO Standard 15836-2009 and NISO Standard Z39.85. Detailed explanations and best-practice guidelines are available for each metadata type on the DCMI website. The use of encoding and vocabulary schemes is encouraged and very specific ways to refine elements are recommended. Established vocabulary encoding schemes and authority files are encouraged such as DDC, LCC, LCSH, TGN (e.g. TGN is recommended for the dc:coverage element). A DCMI Type Vocabulary has also been developed, which provides a general list of approved terms that may be used for the Resource Type element to identify (e.g. dcmitype: collection, dcmitype: dataset). While the DC standard does not require any particular syntax, established syntax encoding schemes such as Box, ISO3166, W3CDTF are encouraged (e.g. it is recommended to use W3CDTF profile or ISO 8601 in the dc:date element). The DC standard is generally implemented in XML, RDF/XML, HTML or XHTML.
A User Guide is available, which provide an entry point for users of DC. A 2005 version of the guide introduces users to the principles of DC while a revised version of this guide is currently being developed in wiki style (“Using”, “User Guide-Wiki”). Other metadata training resources such a webinars, tutorials, glossaries and FAQs are available through the DCMI community, and are updated regularly. A formal registry, the “Dublin Core Metadata Registry”, which is designed to promote the discovery and reuse of properties, classes, and other metadata terms is also available (“Open”). It provides an up-to-date source of authoritative information about DCMI metadata terms and related vocabularies. The DCMI Abstract model provides a reference model against which particular DC encoding guidelines can be compared, independent of any encoding syntax (“DCMI Abstract”). This reference model allows users to gain a better understanding of the kinds of descriptions they are trying to encode and facilitates the development of better mappings and translations between different syntaxes.
The shift in focus from a metadata format to a metadata vocabulary means that the DC standard has evolved to encompass a much broader scope that the original fifteen element set would have allowed. Ongoing work of the DCMI seeks to align the DC standard with the RDF and the Linked Data movement. This outward-looking approach is demonstrated by the “Guidelines for Dublin Core Application Profiles”, which aim to help package together application-specific metadata records for describing a particular resource (“Guidelines”). For example, in 2013, Bair et al. reviewed the Dublin Core Pre-modern Manuscripts Application Profile (PMAP). They found that although the Application Profile was still in development, it demonstrated potential as a simple and straightforward profile that could assist metadata creation for pre-modern manuscripts among non-specialists (11).
The ‘first-principles’ of the DCMI community – interoperability, cross-disciplinary, simplicity, flexibility – have guided the DC standard to evolve as a style of metadata (“UserGuide – Wiki”). This has served to extend and improve the metadata standard beyond the initial fifteen descriptive elements.
Use in Ireland
The DC standard has been widely adopted in Ireland. As early as 2002, the Irish Public Sector developed an ‘Irish Public Service Metadata Standard User Guide’, which was based on the original DC metadata element set (“IPSMES”). More recently, a survey of digital preservation and access practices, led by the Digital Repository of Ireland, found that the DC standard was the most commonly used metadata standard among the cultural institutions, libraries, higher- education institutions and other bodies surveyed. It reported that 61% of interviewees used the DC standard, with the next most popular metadata schema being ISAD(G), which was used by 39% of interviewees (O’Carroll 33). Based on this survey, the DRI has committed to supporting DC metadata (“Metadata”).
Various digital projects in the Arts & Humanities sector have adopted the DC standard including the OPSIT project, the National Irish Visual Arts Library Artists’ database and CAIN (Conflict Archive on the Internet). Other Irish projects tested the DC standard but decided against using it. The IVRLA initially experimented with the DC standard but found it less suitable than MODS, due to the variety of resources encompassed by the IVRLA project.
The DC standard is, however, used extensively in Institutional Repositories (IR) in Ireland such as TARA (TCD), ARAN (NUIG) and CORA (UCC). The IRs of the seven university libraries repositories can be searched via the RIAN portal, which uses DC for harvesting metadata records from compliant Irish institutional repositories. While other metadata standards such as MARC, s-GMS, TEI or METS could have been used for the IRs, the DC standard is most suitable for this project for a number of reasons. Firstly, the primary resource type of an IR will be a networked DLO and the DC is most suited to describing these types of resources. Secondly, because RIAN uses the internet protocol OAI-PMH to harvest the metadata records, the DC is the most suitable metadata standard to use in this instance. The OAI technical infrastructure (OAI-PMH) defines a mechanism for data providers to expose their metadata, which declares that these must be mapped to the DC standard.
Conclusion
While an IR is well-suited to the descriptive element set and the interoperability of the DC standard, other types of resources are not. As noted earlier, the suitability of the DC for non DLOs depends to some extent on how closely their metadata resembles typical document metadata and also what purpose the metadata is intended to serve. The experience at the IVRLA is telling; while the DCMI community are making efforts to develop particular application profiles suited to certain domains, the element set is still viewed as unsuitable for some. In 2012, a study investigating the implementation of the DC standard on the Internet showed that while metadata remains to be used and embedded in source code, the DC standard has not been as widely implemented as originally hoped, and that website creators tended to use only five of the available fifteen elements (Phelps 333). Ten years ago, and again this year, Beall heralded the death of the DC. In a 2004 article, (and again in 2014) he critiqued the DC standard, citing the simplistic and flexible approach and the organisational management of DCMI as significant weaknesses (Beale). More recently, the simplicity and flexibility, which focuses on interoperability, has been recognised as a significant strength (Harper 22). While the DC is not widely implemented by website creators of networked electronic information, it remains to be heavily used for describing document-like objects. The continuing application and use of the DC is evidenced by the high number of publications referencing the standard in a wide range of domains over the last ten months (as evidence by a Google Scholar search for “Dublin Core”, refined to 2014).
Works cited
Bair, Sheila A., and Susan M. B. Steuer. “Developing a Premodern Manuscript Application Profile Using Dublin Core.” Journal of Library Metadata 13.1 (2013): 1–16. Taylor and Francis+NEJM. Web. 12 Nov. 2014.
Beall, J. “Dublin Core: An Obituary.” Library Hi Tech News 21.8 (2004): 40–41. emeraldinsight.com (Atypon). Web. 12 Nov. 2014.
—. “Dublin Core Is Still Dead.” Library Hi Tech News 31.9 (2014): 11–13. emeraldinsight.com (Atypon). Web. 12 Nov. 2014.
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Harper, Corey. “Dublin Core Metadata Initiative: Beyond The Element Set.” Information Standards Quarterly 22.1 (2010): 20-28. Web.
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“OCLC/NCSA Metadata Workshop Report.” Dublin Core Metadata Initiative. N.p., n.d. Web. 12 Nov. 2014.
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“Procedure for Approval of Proposals by DCMI.” Dublin Core Metadata Initiative. N.p., n.d. Web. 12 Nov. 2014.
“The Open Metadata Registry.” N.p., n.d. Web. 12 Nov. 2014.
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“Using Dublin Core.” Dublin Core Metadata Initiative. N.p., n.d. Web. 12 Nov. 2014.