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Last modified: March 19, 2005
Metadata Encoding and Transmission Standard (METS)

"METS, a Digital Library Federation initiative, attempts to build upon the work of the Making of America II project (MOA2) and provide 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). The MOA2 project attempted to address [the need for structural metadata] in part by providing an encoding format for descriptive, administrative, and structural metadata for textual and image-based works. A METS document consists of seven major sections, including a METS Header, Descriptive Metadata, Administrative Metadata, File Section, Structural Map, Structural Links, and Behavior. Depending on its use, a METS document could be used in the role of Submission Information Package (SIP), Archival Information Package (AIP), or Dissemination Information Package (DIP) within the Open Archival Information System (OAIS) Reference Model." [adapted from the project home page]

The METS schema provides a flexible mechanism for encoding descriptive, administrative, and structural metadata for a digital library object, and for expressing the complex links between these various forms of metadata. It can therefore provide a useful standard for the exchange of digital library objects between repositories. In addition, METS provides the ability to associate a digital object with behaviours or services. The above discussion highlights the major features of the schema, but a thorough examination of the schema and its included documentation is necessary to understand the full range of its capabilities..." [from the Overview]

A number of METS Profiles have been created: "METS Profiles are intended to allow an individual or organization to specify limitations and restrictions on the creation of METS instance documents in order to define a document class. METS instance documents which abide by those limitations and restrictions are said to conform to the profile. METS profiles may serve a variety of purposes, but their primary purpose is to promote interoperability and exchange of METS documents. By allowing any given institution to specify and publicize its practices with regards to creation and use of METS documents, it can assist others both in creating METS instances to be submitted to that institution and in processing METS documents created by that institution."

METS Rights Declaration Schema: A draft Rights Schema for METS has been prepared to support declaration and documentation of some basic facts about the digital collections being created and/or included in institutional digital repositories. This Rights Declaration schema has three main elements: (1) A simple declaration of type of rights [e.g., copyrighted, licensed, public domain, contractual, other] and the public statement of that Rights Declaration; (2) The naming of the Rights Holder(s) with appropriate contact information; (3) The Context(s) for the rights declaration based on type of users who have a set of permissions for a digital object or part of a digital object. If there are any constraints to the permissions, those are also expressed within the context by listing the constraints and explaining them in a constraint description element... This rights declaration schema focuses upon digital resources owned or controlled by the digital repository rather than e-resources accessed remotely, formally licensed and subscribed to by an organization..." See examples below. [adapted from the announcement]

[October 2001] Early description: Research Libraries Group (RLG) Backs METS. "The [METS] standard is currently maintained in the Network Development and MARC Standards Office of the Library of Congress. Having played a key role in moving this initiative forward and serving as the work coordinator, the DLF has helped to bring the METS work to the forefront. Now RLG will take over as the new coordinator. This is a natural step for a number of reasons. The METS standard will be applicable to RLG's member community of libraries, museums, archives, and historical societies. METS fits in nicely with much of RLG's ongoing work in digital preservation. The joint OCLC and RLG work on digital preservation metadata has identified the OAIS reference model as a potential starting point for developing the preservation metadata framework. RLG has actively supported the NISO standards work, such as Technical Metadata for Digital Still Images, which RLG cosponsors. The work of this group will fit neatly into the administrative section of METS. Finally, RLG has always advocated community standards such as EAD and Z39.50, and METS is viewed as an emerging standard. For the next six to eight months, RLG will continue the process of education, information dissemination, and gathering of feedback on METS. Then RLG, like the DLF, will review METS based on use." [from the October 2001 RLG Focus report]

METS document structure: "METS has a very simple structure with just four major components: descriptive metadata, administrative metadata, file inventory, and structural map. Only the file inventory and structural map are required by the schema.

  • The descriptive metadata, used for discovery and identification, is optional. A METS object can contain a Metadata Reference or a Metadata Wrapper. A Metadata Reference is a link to external descriptive metadata. A Metadata Wrapper packages descriptive metadata associated with the object, as either Base64 encoded binary data or XML. METS does not require a particular scheme for description, so the implementer can choose the most appropriate descriptive scheme.
  • The administrative metadata, also optional, has four optional subcomponents for technical metadata, rights metadata, source metadata, and preservation metadata. Each of these subsections acts like the descriptive section in that the metadata can be encoded ('wrapped') within the METS document or pointed to in an external location ('referenced').
  • The file inventory allows for listing all the files associated with a digital object. Files can be grouped; some groupings might include master files, thumbnails, etc. The files may be pointed to or can be contained internally as Base64 encoded binary data.
  • The structural map forms a simple or complex tree structure that describes the digital object. The map outlines a hierarchical structure linking the content files and metadata to the digital object. [from Merrilee Proffitt's report in RLG Focus, October 2001]

METS document summary from the Version 1.0 epsilon XML Schema documentation: "METS is intended to provide a standardized XML format for transmission of complex digital library objects between systems. As such, it can be seen as filling a role similar to that defined for the Submission Information Package (SIP), Archival Information Package (AIP) and Dissemination Information Package (DIP) in the Reference Model for an Open Archival Information System. A METS document consists of five possible subsidiary sections: metsHdr (METS document header), dmdSec (descriptive metadata section), amdSec (administrative metadata section), fileGrp (file inventory group), and structMap (structural map). It also has five possible attributes: [1] ID: (an XML ID); [2] OBJID: a primary identifier assigned to the original source document; [3] LABEL: a title/text string identifying the document for users; [4] TYPE: a type for the object, e.g., book, journal, stereograph, etc.; [5] PROFILE: the registered profile to which this METS document conforms."

From "METS: Metadata Encoding for Digital Library Objects":

METS is a generalized metadata framework, developed to encode the structural metadata for objects within a digital library and related descriptive and administrative metadata. Launched by the Digital Library Federation, the METS initiative seeks to provide a single, standard mechanism for encoding all forms of metadata for digital library objects. METS will simplify both the exchange of objects between repositories and the development of software tools for search and display of those objects. Additionally, METS encoding will provide a coherent means for archiving digital objects and their metadata. The METS initiative has two major components, a technical one aimed at developing a standard encoding format for metadata, and an organizational one aimed at developing mechanisms for maintenance and further development of the format.

The technical component of the METS initiative has completed a draft XML schema for the encoding format and made it publicly available for review. The METS schema provides mechanisms for encoding descriptive, administrative, and structural metadata regarding objects within a digital library, and is expressed using the XML schema language of the World Wide Web Consortium. The METS schema tries to support the dual and sometimes competing requirements of ensuring interoperability and exchange of documents between different institutions while also allowing for significant flexibility in local practice with regards to descriptive and administrative metadata standards. The organizational component of the METS initiative is also being pursued. The Network Development and MARC Standards Office of the Library of Congress has agreed to help maintain the standard and make it publicly available through their web site. Active development of the METS schema is currently being pursued by the Digital Library Federation; when the schema has been adequately reviewed by the membership of DLF, the Research Libraries Group will partner with the Library of Congress in maintaining the standard, with RLG taking on the role of work coordinator (continuing development of the schema and related technical work required to support METS' use).

The US Library of Congress Digital Audio-Visual Preservation Prototyping Project is proposing extension schemas for the Metadata Encoding and Transmission Standard. The prototyping project "plans [2001-12] to use the generic descriptive metadata extension schema gdm.xsd developed at the University of California at Berkeley. Some of the specific audio-visual extension schemas are placeholders awaiting development from other organizations. For example, the extension schema for image technical information is intended to embrace the elements and attributes expressed by the data dictionary for the NISO image metadata set."

[July 05, 2001] A posting from Jerome McDonough (Digital Library Development Team Leader, Elmer Holmes Bobst Library, New York University) announces the release of a beta version of the Metadata Encoding and Transmission Standard (METS) XML schema. METS "provides an XML-based framework for encoding descriptive, administrative, and structural metadata for a digital library object. It can be used both as a standardized mechanism for exchanging digital library objects between repositories, and as an encoding mechanism for local storage, retrieval and display of digital library objects. METS has been developed as an initiatve of the Digital Library Federation and is being maintained in the Network Development and MARC Standards Office of the US Library of Congress." The development team solicits feedback from the library community regarding the design of the METS XML schema. Comments on the XML schema should be sent to the Listserver by August 31, 2001 in anticipation of a review meeting in September. Interested parties may subscribe to the METS Forum, an unmoderated computer forum open to members of the METS development community.

METS history: "METS had its beginnings in 1996 as the brainchild of the Digital Library Federation (DLF) Architecture Group, which identified metadata and architecture problems as an area of critical need for digital libraries. Thus was born the Making of America 2 (MOA2) project, sponsored by the DLF in the early stages and funded by the National Endowment for the Humanities. New York Public Library and the libraries of Cornell, Penn State, and Stanford collaborated under the leadership of the University of California, Berkeley Library, contributing images and data towards an investigation of structural and administrative metadata for digital objects. As more and more institutions created digital images in the course of projects, there was growing concern about sensible storage for the digital objects, defined as digital files plus associated metadata. It was the beginning of a serious discussion, tying together many important aspects of digital library research. The MOA2 Document Type Definition (DTD), which was the direct predecessor of METS, was developed for the MOA2 project to encapsulate what were then seen as the required metadata elements..." [from the October 2001 RLG Focus report]

Principal References

Earlier references [maintenance needed]:

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