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Introduction to METS

Wednesday, 24 Jun 2009 | by Mark Tucker

The Challenges

One of the challenges that need to be solved for online source citation is the ability to give structure to digital assets. Think of an online book that consists of a hundred images each representing a page. There are other images for the cover, title page, etc. There might even be text documents, audio files, or video associated with it. How do we keep track of all those individual files and relate them as a single digital entity? That is part of the problem that METS is trying to solve. In online citations, we also have the issue of source provenance. Where did the digital image file for the census come from? It came from a microfilm copy of the original census. Is it possible that METS can help keep track of this provenance? What about complex sources that are part of a collection in a series part of a record group at an archive? Can MET be used to keep track of this hierarchal information?

Let’s explore the basics of METS to see if we can find some answers.

Metadata Encoding & Transmission Standard – METS

Basically a METS document consists of 7 major sections:

1. METS Header
2. Descriptive Metadata
3. Administrative Metadata
4. File Section
5. Structual Map
6. Structual Links
7. Behavior

METS is usually used to manage digital assets where there is at least one digital file, but it doesn’t have to. The sections that are interesting for our discussion are Descriptive Metadata, Administrative Metadata, and Structual Map.

The Descriptive Metadata section either wraps or references metadata about the source and can be in various formats including MODS, DC.

“The descriptive metadata section of a METS document consists of one or more (Descriptive Metadata Section) elements. Each
element may contain a pointer to external metadata (an element), internally embedded metadata (within an element), or both.”

(see http://www.loc.gov/standards/mets/METSOverview.v2.html#descMD)

A dmdSec element has an ID attribute that can be referenced elsewhere in the document.

Among other things, the Administrative Metadata section can contain a sourceMD section that describes (also using MODS, DC, etc.) the source
that the digital representation came from. For example a digitized Census image could be described as a dmdSec while the microfilm that it was digitized from could appear in the sourceMD.

The Structual Map section can describe the physical or logical (or both or other) structure of the source.

“The organization may be specified to any level of granularity (intellectual and or physical) that is desired. Since the element is repeatable, more than one organization can be applied to the digital content represented by the METS document. The organization provided by the may be purely intellectual or logical (such as a book divided into chapters), purely physical (a book divided into sequences of pages), or a mixture of logical and physical (a book sub-divided into chapters and subsequently divided into a sequence of pages).”

(see http://www.loc.gov/standards/mets/METS%20Documentation%20final%20070930%20msw.pdf page 36)

If there are digitized files described in the File Section, these can be referenced using file pointers (fptr elements) in the structual map. The structure map can also contain child METS pointers to another METS document (mptr elements).

Here is an example of a structMap hierarchy for a book:


<mets:structMap TYPE="physical">
<mets:div TYPE="book" LABEL="Martial Epigrams II" DMDID="DMD1">
<mets:div TYPE="page" LABEL="Blank page"/>
<mets:div TYPE="page" LABEL="Page i: Series title page"/>
<mets:div TYPE="page" LABEL="Page ii: Blank page"/>
<mets:div TYPE="page" LABEL="Page iii: Title page"/>
<mets:div TYPE="page" LABEL="Page iv: Publication info"/>
<mets:div TYPE="page" LABEL="Page v: Table of contents"/>
<mets:div TYPE="page" LABEL="Page vi: Blank page"/>
<mets:div TYPE="page" LABEL="Page 1: Half title page"/>
<mets:div TYPE="page" LABEL="Page 2 (Latin)"/>
<mets:div TYPE="page" LABEL="Page 3 (English)"/>
<mets:div TYPE="page" LABEL="Page 4 (Latin)">
<mets:div TYPE="page" LABEL="Page 5 (English)"/>
<mets:div TYPE="page" LABEL="Page 6 (Latin)"/>
<mets:div TYPE="page" LABEL="Page 7 (English)"/>
</mets:div>
</mets:structMap>

Notice that the values of the TYPE attributes of (structMap, div) are up to us to define.

Here is another example using a multivolume book:


<mets:structMap TYPE="physical">
<mets:div TYPE="multivolume book" LABEL="Martial Epigrams I & II" DMDID="DMD1">
<mets:div TYPE="volume" LABEL="Volume I">
<mets:mptr LOCTYPE="URL" xlink:href="http://www.loc.gov/standards/mets/documentation MatrialEpigrams.xml"/>
</mets:div>
<mets:div TYPE="volume" LABEL="Volume II">
<mets:mptr LOCTYPE="URL" xlink:href="http://www.loc.gov/standards/mets/documentation/MatialEpigramsII.xml"/>
</mets:div>
</mets:div>
</mets:structMap>

There is not a limitation on the number of levels represented by div elements.

Profiles

METS Profiles can be created for different types of documents or for different purposes.

“By making use of all the components, an institution not only declares how it builds a METS document of a certain digital object type, or for a specific application or purpose, but can also provide an implicit description of the data model used for internal METS document creators, METS tool developers, and external recipients of their METS documents. This information can be an invaluable means to convey succinctly the critical information necessary to disaggregate a METS document for disposal within another institutional repository, for instance, or for the use of searching, navigating, displaying, and rendering applications or tools.”

(see http://www.loc.gov/standards/mets/METS%20Documentation%20final%20070930%20msw.pdf page 74)

Here are some interesting METS Profiles:

Library of Congress METS Profile for Bibliographic Records
http://www.loc.gov/standards/mets/profiles/00000020.html

UCSD Complex Object Profile
http://www.loc.gov/standards/mets/profiles/00000017.html

Here is an article that describes METS from the perspective of a Metadata Librarian implementing METS at UCSD:
http://gort.ucsd.edu/dlpwg/dletters/issue8.pdf
It seems possible that METS could help us solve some of the problems of citing online sources. Still more research to done.

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