New metadata standards for digital resources: MODS and METS

Bulletin of the American Society for Information Science & Technology, Dec 2002/Jan 2003 by Guenther, Rebecca, McCallum, Sally

Metadata has taken on a new look with the advent of XML and digital resources. XML provides a new versatile structure for tagging and packaging metadata as the rapid proliferation of digital resources demands both rapidly produced descriptive data and the encoding of more types of metadata. Two emerging standards are attempting to harness these developments for library needs. The first is the Metadata Object and Description Schema (MODS), a MARC-compatible XML schema for encoding descriptive data. The second standard is the Metadata Encoding and Transmission Standard (METS), a highly flexible XML schema for packaging the descriptive metadata and various other important types of metadata needed to assure the use and preservation of digital resources.

MODS Development

The Library of Congress' Network Development and MARC Standards Office developed MODS (www.loc.gov/standards/mods/) in consultation with interested experts to satisfy the expressed need for an abbreviated XML version of MARC 21. XML is being increasingly deployed in computer applications, particularly on the Web, as a richer, more flexible alternative to HTML. Many have expressed the need to move to XML for metadata in libraries and other cultural institutions. It is appropriate for an XML version of MARC to be investigated since it is perhaps the oldest metadata standard designed for use in computers.

Over the years people have expressed concerns about the number of data elements in MARC and their complexity. Some have suggested use of the Dublin Core Metadata Element Set (http://dublin core.org), although that set is intended to satisfy a broader range of purposes and communities than MARC 21. In order to address these concerns about MARC and also allow for a rich description, the Library of Congress developed MODS, an XML schema with language-based tags that includes a subset of data elements derived from MARC 21. It is intended to carry selected data from existing MARC 21 records as well as to enable the creation of original resource description records.

Features

MODS is intended to complement other metadata formats and to provide an alternative between a simple metadata format with a minimum of fields and little or no substructure such as Dublin Core and a very detailed format with many data elements having various structural complexities such as MARC 21. MODS has a high level of compatibility with MARC records because it inherits the semantics of the equivalent data elements in the MARC 21 bibliographic format. Thus, it is richer than Dublin Core and more compatible with library data than ONIX (www.editeur.org/onix.html), which was developed for the book industry, but it is also simpler than the full MARC format (either as ISO 2709 or full MARCXML). It is more "friendly" because it uses language-based tags that can be easily understood by anyone dealing with the "raw" record, as opposed to the numeric tags traditional to MARC.

Most elements that have been defined in MODS have equivalents in the MARC 21 bibliographic format. In addition the Library of Congress has made available mappings between MARC and MODS and vice versa (www.loc.gov/ standards/mods/modsmapping.html; www.loc.gov/standards/mods//mods2marcmapping.html). Since MODS elements inherit the semantics of MARC elements, an element in MODS has the meaning detailed in the MARC 21 bibliographic format.

In MODS some elements in MARC have been repackaged, for example in cases where several data elements are brought into one. This repackaging occurs in the MODS element genre, which uses controlled values that are used in various MARC elements, particularly in fixed fields. The Library of Congress has made available a controlled list of genre values found in various places in the MARC 21 bibliographic format to be used with the MODS genre element (www.loc.gov/ marc/sourcecode/genre/genrelist.html).

MODS, like MARC 21, does not assume the use of any particular cataloging code. It can accommodate record content that is full AACR2 with authoritative name and subject headings, uncontrolled by cataloging rules, or anything in-between.

Since MODS is a subset of MARC, decisions were made about which elements to include, which to combine with others to form a single element and which to drop altogether. For instance, there are numerous types of relationships that are expressed in the MARC linking entry fields. These are carried in MODS under relatedItem with a type attribute to express the type of relationship. Not all relationships in MARC are given type values.

Certain MODS elements define concepts that recur in more than one element as sub-elements. XML facilitates using the same definition for multiple elements. For example, "name" can be the primary name associated with the resource or a name associated with a related item; in MODS, both use the same definition. This concept is certainly present in MARC 21 but not as consistently as in MODS.

Since MODS includes a subset of MARC 21 bibliographic fields, it allows for a conversion from MARC 21 fields to MODS, while other MARC 21 fields may be dropped or carried in a less specific manner. The MODS schema does not target "round-tripability" with MARC 21. A converted record may lose some of its tagging, for instance, when the tagging is simpler, or accommodate some data even when there is not an equivalent data element. When an XML schema is desired that does not result in any data loss, the MARC 21 XML schema may be used (www.loc.gov/standards/marcxml/), since it allows for the expression of a full MARC record in XML. For any conversion between MARC in ISO 2709 format and MODS, it is expected that the record would first go through a conversion to MARCXML before a transformation to the subset that is MODS. The Library of Congress is providing tools for the conversion from MARC 21 to MARCXML with a further transformation to MODS.