METS: An Overview & Tutorial
Maintaining a library of digital objects of necessity requires
maintaining metadata about those objects. The metadata necessary
for successful management and use of digital objects is both more
extensive than and different from the metadata used for managing
collections of printed works and other physical materials. While
a library may record descriptive metadata regarding a book in its
collection, the book will not dissolve into a series of unconnected
pages if the library fails to record structural metadata regarding
the book's organization, nor will scholars be unable to evaluate
the book's worth if the library fails to note that the book was
produced using a Ryobi offset press. The same cannot be said for
a digital version of the same book. Without structural metadata,
the page image or text files comprising the digital work are of
little use, and without technical metadata regarding the digitization
process, scholars may be unsure of how accurate a reflection of
the original the digital version provides. For internal management
purposes, a library must have access to appropriate technical metadata
in order to periodically refresh and migrate the data, ensuring
the durability of valuable resources.
The Making of America
II project (MOA2) attempted to address these issues in part
by providing an encoding format for descriptive, administrative,
and structural metadata for textual and image-based works. METS,
a Digital Library Federation initiative,
attempts to build upon the work of 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). 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
Archival Information System (OAIS) Reference Model.
A METS document consists of seven major sections:
METS Header - The METS Header
contains metadata describing the METS document itself, including
such information as creator, editor, etc.
Descriptive Metadata - The descriptive
metadata section may point to descriptive metadata external
to the METS document (e.g., a MARC record in an OPAC or an
EAD finding aid maintained on a WWW server), or contain internally
embedded descriptive metadata, or both. Multiple instances
of both external and internal descriptive metadata may be included
in the descriptive metadata section.
Administrative Metadata - The
administrative metadata section provides information regarding
how the files were created and stored, intellectual property
rights, metadata regarding the original source object from
which the digital library object derives, and information regarding
the provenance of the files comprising the digital library
object (i.e., master/derivative file relationships, and migration/transformation
information). As with descriptive metadata, administrative
metadata may be either external to the METS document, or encoded
File Section - The file section
lists all files containing content which comprise the electronic
versions of the digital object. <file> elements may
be grouped within <fileGrp> elements, to provide for
subdividing the files by object version.
Structural Map - The structural
map is the heart of a METS document. It outlines a hierarchical
structure for the digital library object, and links the elements
of that structure to content files and metadata that pertain
to each element.
Structural Links - The Structural
Links section of METS allows METS creators to record the existence
of hyperlinks between nodes in the hierarchy outlined in the
Structural Map. This is of particular value in using METS to
Behavior - A behavior section
can be used to associate executable behaviors with content
in the METS object. Each behavior within a behavior section
has an interface definition element that represents an abstract
definition of the set of behaviors represented by a particular
behavior section. Each behavior also has a mechanism element
which identifies a module of executable code that implements
and runs the behaviors defined abstractly by the interface
A more detailed explanation of each section and their inter-relations
The METS Header element allows you to record minimal descriptive
metadata about the METS object itself within the METS document. This
metadata includes the date of creation for the METS document, the
date of its last modification, and a status for the METS document.
You may also record the names of one or more agents who have played
some role with respect to the METS document, specify the role they
have played, and add a small note regarding their activity. Finally,
you may record a variety of alternative identifiers for the METS
document to supplement the primary identifier for the METS document
recorded in the OBJID attribute on the METS root element. A small
example of a METS Header might look like the following:
<metsHdr CREATEDATE="2003-07-04T15:00:00" RECORDSTATUS="Complete">
<agent ROLE="CREATOR" TYPE="INDIVIDUAL">
<agent ROLE="ARCHIVIST" TYPE="INDIVIDUAL">
This example contains two attributes on the <metsHdr> element,
CREATEDATE and RECORDSTATUS, which are used to indicate the date
and time the METS record was created, and indicate the status of
the record's processing. Two individual agents are listed who
have worked on this METS record, the person responsible for creating
the record and an archivist responsible for the original material. Both
the ROLE and TYPE attributes on the <agent> element employ
controlled vocabularies. Allowable values for ROLE include "ARCHIVIST," "CREATOR," "CUSTODIAN," "DISSEMINATOR," "EDITOR," "IPOWNER" and "OTHER." Allowable
values for the TYPE attribute are "INDIVIDUAL," "ORGANIZATION" or "OTHER."
The descriptive metadata section of a METS document consists of
one or more <dmdSec> (Descriptive Metadata Section) elements.
Each <dmdSec> element may contain a pointer to external metadata
(an <mdRef> element), internally embedded metadata (within
an <mdWrap> element), or both.
External Descriptive Metadata (mdRef): an mdRef element
provides a URI which may be used in retrieving the external metadata.
For example, the following metadata reference points to the finding
aid for a particular digital library object:
<mdRef LOCTYPE="URN" MIMETYPE="application/xml" MDTYPE="EAD"
LABEL="Berol Collection Finding Aid">urn:x-nyu:fales1735</mdRef>
The <mdRef> element of this <dmdSec> contains four
attributes. The LOCTYPE attribute specifies the type of locator
contained in body of the element; valid values for LOCTYPE include
'URN,' 'URL,' 'PURL,' 'HANDLE,' 'DOI,' and 'OTHER.' The MIMETYPE
attribute allows you to specify the MIME type for the external
descriptive metadata, and the MDTYPE allows you to indicate what
form of metadata is being referenced. Valid values for the MDTYPE
element include MARC, MODS, EAD, VRA (VRA Core), DC (Dublin Core),
NISOIMG (NISO Technical Metadata for Digital Still Images), LC-AV
(Library of Congress Audiovisual Metadata) , TEIHDR (TEI Header),
DDI (Data Documentation Initiative), FGDC (Federal Geographic Data
Committee Metadata Standard [FGDC-STD-001-1998] ), and OTHER. LABEL
provides a mechanism for describing this metadata to those viewing
a METS document, in a 'Table of Contents' display of the METS document,
Internal Descriptive Metadata (mdWrap): An mdWrap element
provides a wrapper around metadata embedded within a METS document.
Such metadata can be in one of two forms: 1. XML-encoded metadata,
with the XML-encoding identifying itself as belonging to a namespace
other than the METS document namespace, or 2. any arbitrary binary
or textual form, PROVIDED that the metadata is Base64 encoded and
wrapped in a <binData> element within the mdWrap element.
The following examples demonstrate the use of the mdWrap element:
<mdWrap MIMETYPE="text/xml" MDTYPE="DC" LABEL="Dublin Core Metadata">
<dc:title>Alice's Adventures in Wonderland</dc:title>
<dc:date>between 1872 and 1890</dc:date>
<mdWrap MIMETYPE="application/marc" MDTYPE="MARC" LABEL="OPAC Record">
Note that all <dmdSec> elements must possess an ID attribute.
This attribute provides a unique, internal name for each <dmdSec> element
which can be used in the structural map to link a particular division
of the document hierarchy to a particular <dmdSec> element.
This allows specific sections of descriptive metadata to be linked
to specific parts of the digital object.
<amdSec> elements contain the administrative metadata pertaining
to the files comprising a digital library object, as well as that
pertaining to the original source material used to create the object.
There are four main forms of administrative metadata provided for
in a METS document: 1. Technical Metadata (information regarding
files' creation, format, and use characteristics), 2. Intellectual
Property Rights Metadata (copyright and license information), 3.
Source Metadata (descriptive and administrative metadata regarding
the analog source from which a digital library object derives),
and 4. Digital Provenance Metadata (information regarding source/destination
relationships between files, including master/derivative relationships
between files and information regarding migrations/transformations
employed on files between original digitization of an artifact
and its current incarnation as a digital library object). Each
of these four different types of administrative metadata has a
unique subelement within the <amdSec> portion of a METS document
in which that form of metadata can be embedded: <techMD>, <rightsMD>, <sourceMD>,
and <digiprovMD>. Each of these four elements may occur
more than once in any METS document.
The <techMD>, <rightsMD>, <sourceMD> and <digiprovMD> elements
employ the same content model as <dmdSec>: they may contain
an <mdRef> element to point to external administrative metadata,
an <mdWrap> element to use when embedding administrative
metadata within a METS document, or both. Multiple instances of
these elements may occur within a METS document, and all of them
must carry an ID attribute so that other elements within the METS
document (such as divisions within the structural map or <file> elements)
may be linked to the <amdSec> subelements which describe
them. One might, for example, have an <techMD> element which
includes technical metadata regarding a file's preparation:
<mdWrap MIMETYPE="text/xml" MDTYPE="NISOIMG" LABEL="NISO Img. Data">
A <file> element within a <fileGrp> might then identify
this administrative metadata as pertaining to the file it identifies
by using an ADMID attribute to point to this <techMD> element:
<file ID="FILE001" ADMID="AMD001">
The file section (<fileSec>) contains one or more <fileGrp> elements
used to group together related files. A <fileGrp> lists all
of the files which comprise a single electronic version of the
digital library object. For example, there might be separate <fileGrp> elements
for the thumbnails, the master archival images, the pdf versions,
the TEI encoded text versions, etc.
Consider the following example of a file section from a digital
library object for an oral history which has three different versions:
a TEI-encoded transcript, a master audio file in WAV format, and
a derivative audio file in MP3 format:
<file ID="FILE001" MIMETYPE="application/xml" SIZE="257537" CREATED="2001-06-10">
<file ID="FILE002" MIMETYPE="audio/wav" SIZE="64232836"
<fileGrp ID="VERS3" VERSDATE="2001-05-18">
<file ID="FILE003" MIMETYPE="audio/mpeg" SIZE="8238866"
In this case, the <fileSec> contains three subsidiary <fileGrp> elements,
one for each different version of the object. The first is an XML-encoded
transcription file, the second is a master audio file in WAV format,
and the third is a derivative audio file in MP3 format. While such
a basic example does not really seem to need the <fileGrp> elements
to distinguish the different versions of the object, <fileGrp> becomes
much more useful for objects consisting of large numbers of scanned
page images, or indeed any case where a single version of the object
consists of a large number of files. In those cases, being able
to separate <file> elements into <fileGrp>s makes identifying
the files belonging to a particular version of the document a simple
You may note the presence of the GROUPID attributes with identical
values on the two audio <file> elements; these indicate that
the two files, while belonging to different versions of the object,
contain the same basic information (you can use the GROUPID for
the same purpose to indicate equivalent page image files in digital
library objects containing many scanned page images).
You should also note that all of the <file> elements have
a unique ID attribute. This attribute provides a unique, internal
name for this file which can be referenced by other portions of
the document. You’ll see this type of referencing in action
when we look at the Structural Map Section.
It should be mentioned that <file> elements may possess
an <FContent> element rather than an <FLocat> element. <FContent> elements
are used to embed the actual contents of the file within the METS
document; if this is done, the file contents must either be in
XML format or be Base64-encoded. While embedding files is not something
one would typically do when preparing a METS document for use in
displaying a digital library objects to users, it can be a valuable
feature for exchanging digital library objects between repositories,
or for archiving versions of digital library objects for off-site
The structural map section of a METS document defines a hierarchical
structure which can be presented to users of the digital library
object to allow them to navigate through it. The <structMap> element
encodes this hierarchy as a nested series of <div> elements.
Each <div> carries attribute information specifying what
kind of division it is, and also may contain multiple METS pointer
(<mptr>) and file pointer (<fptr>) elements to identify
content corresponding with that <div>. METS pointers specify
separate METS documents as containing the relevant file information
for the <div> containing them. This can be useful when encoding
large collections of material (e.g., an entire journal run) to
keep the size of each METS file in the set relatively small. File
pointers specify files (or in some cases either groups of files
or specific locations within a file) within the current METS document's <fileSec> section
that correspond to the portion in the hierarchy represented by
the current <div>.
The following provides an example of an extremely simple structural
<div ID="div1" LABEL="Oral History: Mayor Abraham Beame"
<div ID="div1.1" LABEL="Interviewer Introduction"
<area FILEID="FILE001" BEGIN="INTVWBG" END="INTVWND"
<area FILEID="FILE002" BEGIN="00:00:00" END="00:01:47"
<area FILEID="FILE003" BEGIN="00:00:00" END="00:01:47"
<div ID="div1.2" LABEL="Family History" ORDER="2">
<area FILEID="FILE001" BEGIN="FHBG" END="FHND"
<area FILEID="FILE002" BEGIN="00:01:48"END="00:06:17"
<area FILEID="FILE003" BEGIN="00:01:48" END="00:06:17"
<div ID="div1.3" LABEL="Introduction to Teachers' Union"
<area FILEID="FILE001" BEGIN="TUBG" END="TUND"
<area FILEID="FILE002" BEGIN="00:06:18" END="00:10:03"
<area FILEID="FILE003" BEGIN="00:06:18" END="00:10:03"
This structural map shows that we have an oral history (with Mayor
Abraham Beame of New York City) that includes three subsections:
an opening introduction by the interviewer, some family history
from Mayor Beame, and a discussion of how he came to be involved
with the teachers' union in New York. Each of these subsections/divisions
is linked to three files (taken from our earlier example of file
groups): an XML transcription, and a master and derivative audio
file. A subsidiary <area> element is used in each <fptr> to
indicate that this division corresponds with only a portion of
the linked file, and to identify the exact portion of each linked
file. For example, the first division (the interviewer introduction)
is linked to a portion of the XML transcription file (FILE001)
which is found between the two tags in the transcription file with
ID attribute values of "INTVWBG" and "INTVWND." It is also linked
to the two different audio files; in these cases, rather than specifying
ID attribute values within the linked files, the begin and end
points of the linked material within the files is indicated by
a simple time code value of the form HH:MM:SS. So, the interviewer
introduction can be found in both audio files in the segment beginning
at time 00:00:00 in the file and extending through time 00:01:47.
The structural links section of the METS format is the simplest
in form of any of the major METS sections, containing only a single
element, <smLink> (although that element may be repeated). The
structural links section of METS is intended to allow you to record
the existence of hyperlinks between items within the structural
map, usually <div> elements. This is a useful facility if
you wish to use METS to archive web sites, and wish to maintain
a record of the hypertext structure of the sites separately from
the HTML files of the site itself.
As an example, consider the case of a METS document for a web
page containing an image which is hyperlinked to another page. The <structMap> element
would probably contain <divs> like the following for the
<div ID="P1" TYPE="page" LABEL="Page 1">
<div ID="IMG1" TYPE="image" LABEL="Image Hyperlink to
<div ID="P2" TYPE="page" LABEL="Page 2">
If you wished to indicate that the image file in the <div> contained
with the first page <div> is hyperlinked to the HTML file
in the second page <div>, you would have a <smLink> element
within the <structLink> section of the METS document as follows:
<smLink from="IMG1" to="P2" xlink:title="Hyperlink from
JPEG Image on Page 1 to Page 2" xlink:show="new"
The <smLink> link element above uses a slightly modified
form of the XLink syntax; all of the XLink attributes are used,
but the "to" and "from" attributes are declared to be of type IDREF
rather than NMTOKEN as in the original XLink specification. This
allows you to indicate the existence of links between any two nodes
in the structural map, and also use XML processing tools to confirm
that the linked nodes actually exist.
A behavior section can be used to associate executable behaviors
with content in the METS object. A behavior section contains one
or more <behavior> elements, each of which has an interface
definition element that represents an abstract definition of the
set of behaviors represented by a particular behavior section.
A <behavior> also has a <mechanism> element which is
used to point to a module of executable code that implements and
runs the behavior defined abstractly by the interface definition.
Digital object behaviors can be implemented as linkages to distributed
web services as in the following example from the Mellon Fedora project.
<METS:behavior ID="DISS1.1" STRUCTID="S1.1" BTYPE="uva-bdef:stdImage"
CREATED="2002-05-25T08:32:00" LABEL="UVA Std Image Disseminator"
<METS:interfaceDef LABEL="UVA Standard Image Behavior Definition"
<METS:mechanism LABEL="A NEW AND IMPROVED Image Mechanism"
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.