What is METS? XML for Digital Library Objects

The Metadata Encoding and Transmission Standard (METS) is an XML-based schema developed by the digital library community to manage, exchange, and preserve complex digital objects. Maintained by the Library of Congress, METS provides a standardized framework that bundles all metadata types—descriptive, administrative, and structural—along with references to the digital content files themselves. This article explains the purpose of METS, breaks down its core structural components, and highlights its critical role in digital archiving and repository interoperability.

The Purpose of METS

Digital objects such as digitized books, audio recordings, or archival collections often consist of multiple files, including master preservation scans, access derivatives, OCR text transcripts, and descriptive records. Managing these disparate assets as a single, coherent object poses significant technical challenges.

METS solves this problem by acting as a digital wrapper. It does not replace existing metadata standards like Dublin Core, MODS, or PREMIS; instead, it provides an XML container that references or embeds these standards, establishing the relationships between the metadata and the physical or digital files.

Core Structure of a METS Document

A METS document is structured into seven distinct sections, each serving a specific role in defining the digital object:

  1. METS Header (<metsHdr>): Contains metadata about the METS document itself, such as the creation date, modification history, and the agent or software responsible for creating the record.
  2. Descriptive Metadata (<dmdSec>): Stores or points to external descriptive metadata describing the intellectual content (e.g., bibliographic data, creator names, titles).
  3. Administrative Metadata (<amdSec>): Contains information regarding how files were created and stored, intellectual property rights, source object details, and digital provenance (preservation history).
  4. File Section (<fileSec>): Lists all digital files comprising the object. Files are typically grouped into file groups (such as thumbnails, master images, or text representations).
  5. Structural Map (<structMap>): The essential core of every METS document. It outlines the hierarchical structure of the digital object (such as chapters, pages, or audio tracks) and links each structural node directly to its corresponding files and descriptive metadata.
  6. Structural Links (<structLink>): Records hyperlinks between different elements within the structural map, which is useful for non-linear structures or hypermedia objects.
  7. Behavior Section (<behaviorSec>): Associates executable behaviors or code modules with the digital object, defining how specific files should be rendered or processed by software.

Key Benefits in Digital Libraries and Preservation