Metadata Resources Metadata Encoding and Transmission Standard (METS)

The Metadata Encoding and Transmission Standard (METS) schema is a standard for encoding descriptive, administrative, and structural metadata about digital library objects and the complex links between these types of metadata within a repository. It does so by providing an XML document format for encoding metadata necessary for both management of digital library objects within a repository and exchange of such objects between repositories (or between repositories and their users). METS is being developed as an initiative of the Digital Library Federation (DLF) and is being maintained in the Network Development and MARC Standards Office of the Library of Congress.

Work on METS began in May 2001 and is a continuation of work undertaken as part of the Making of America II (MOA2) testbed project (a DLF project led by the University of California at Berkeley). MOA2 was itself was a follow-up to the Making of America (MOA) digital library initiative (a collaboration between the the University of Michigan and Cornell University), and proposed to create a digital library object standard by providing an encoding format for descriptive, administrative and structural metadata for both text and image based items. This standard was in response to the need for more appropriate metadata about digital objects taking into account that structural metadata is vital in understanding how a digital item is organized and technical metadata is vital in understanding how accurate a reflection of the original the digital surrogate is. The MOA2 project attempted to address these issues by offering a Document Type Definition (DTD) that defined the digital object’s elements and encoding, in doing so promoting interoperability, scalability and digital preservation. The project introduced the 4 part structure now seen in METS, however it did have several limitations including only allowing encoding of a limited range of digital objects which did not include audio and video formats. It also lacked external linking facilities. The METS project has increased the audience of the schema by generalizing the elements and converting the DTD to XML Schema Definition language (XSD).

The initiative’s Web pages claim that METS has a similar role to that of information packages, as defined in the Reference Model for an Open Archival Information System (OAIS).

A METS document is made up of 4 sections: Descriptive Metadata, Administrative Metadata, File Groups and a Structural map. Metadata can either be included in the METS hub document or referenced via an identifier or locator. The following descriptions are based on those published in the METS overview & tutorial provided by the Network Development and MARC Standards Office of the Library of Congress.

• Descriptive Metadata – This section points to descriptive metadata that is either external to the document (e.g., a Dublin Core (DC) or MARC record maintained on a separate server) or embedded internally, or both, using <dmdSec> (Descriptive Metadata Section) elements. External metadata is linked to by a <mdRef> element and internal by <mdWrap>.
• Administrative Metadata – This section provides information on how the files were created and stored (technical metadata), intellectual property rights metadata, metadata about the source object from where the digital object derives and digital provenance metadata. It also includes information on source/destination and master/derivative relationships between files and migration/transformation information. As mentioned previously this metadata can be held externally or internally.
• File Groups – This section lists all the files that make up the digital object e.g thumbnails, master archival images, pdfs. Each file is given a unique ID attribute which allows it to be referenced by other parts of the document. The <GROUPID> attribute allows two versions of the digital object (such as a WAV file and an MP3), that contain the same information to be linked.
• Structural map – The structural map outlines the hierarchical structure for the digital library object allowing users to navigate clearly. The map links to content files and metadata for each element. The <StructMap> element encodes this hierarchy as a nested set of <div> elements. Each div element may contain pointers and file pointers. METS pointers specify other documents as containing the relevant information, which is useful in keeping the size down when encoding large collections of material.

METS Resources:

Official METS Web site:

http://www.loc.gov/standards/mets/

Example METS XML Document:

Courtesy of Rick Beaubien, U.C. Berkeley Library Systems Office

http://www.loc.gov/standards/mets/sfquad.xml

METS Documentation:

http://www.loc.gov/standards/mets/mets_xsd/mets.html

METS Overview & Tutorial:

http://www.loc.gov/standards/mets/METSOverview.html

Making of America II testbed project:

http://sunsite.berkeley.edu/MOA2/

Report on the Making of America II Testbed Project:

Hurley, B.J., Price-Wilkin, J., Proffitt, M. & Besser, H., The Making of America II Testbed Project: a digital library service model. Washington, D.C.: Council on Library and Information Resources, 1999.

http://www.clir.org/pubs/abstract/pub87abst.html

Maintained by: Marieke Napier of UKOLN.
Last updated: 26-Oct-2001.

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