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Commonly defined as "data about data," metadata constitutes the
documentation of all aspects of digital files essential to their
persistence and usefulness and should be inextricably linked to each
digital image. Metadata is captured in the form of a prescribed list
of elements, or fields, known as a metadata schema. Each schema is
tailored to a specific domain or purpose (see Selecting a Metadata
Schema). Metadata schemas should be distinguished from metadata
format: information in any given metadata schema can be formatted
in a number of different ways—as a physical card-catalogue entry,
a set of fields for a database or collection management system
record, or an XML (Extensible Markup Language) document—and
still convey the same meaning. The advantage of the increasingly
popular XML format is that it provides a mechanism for encoding semantic
data, as defined by a metadata schema, that facilitates data sharing
and automated data processing. (For a more detailed discussion of
the XML format, see Metadata Format.) However, the quality
and consistency of metadata are more important than the particular
format in which it is expressed or the software used to generate
it: bad metadata in a sophisticated native-XML database will
be less valuable than good metadata in a simple desktop spreadsheet,
which can likely be migrated to new formats if need be. "Good" metadata
was defined in 2001 by the Digital Library Forum as fulfilling the
following criteria: it is appropriate to the materials digitized
and their current and likely use; it supports interoperability; it
uses standard controlled vocabularies to populate elements
where appropriate; it includes a clear statement on the terms of
use of the digital object; it supports the long-term management of
digital objects; and it is persistent, authoritative, and verifiable. 1
The depth and complexity of metadata captured will vary from one
project to another depending on local policies and user needs, but
images without appropriate metadata will quickly become useless—impossible
to find, view, or migrate to new technology as this inevitably becomes
necessary. It is metadata that allows collection managers to track,
preserve, and make accessible digital images and enables end users
to find and distinguish between various images. Metadata also allows
digital image collections to be reused, built upon, and become part
of larger cultural heritage offerings within and across institutions.
Metadata can be divided into three broad types, which may be simply
defined as follows: descriptive, which describes content; administrative,
which describes context and form and gives data-management information;
and structural, which describes the relationships between parts and
between digital files or objects.2 The first is most akin
to traditional cataloguing and would describe what a digital image
depicts or its essence. This is essential for end-user access and
to allow efficient search and retrieval. Administrative metadata
records information such as how and why a digital object was created,
and is used in the management of digital objects. Structural metadata
documents information such as the fact that one image is a detail
of the upper-right corner of another image, that a particular image
depicts page two of a book of thirty-four pages, or that an image
is one item in a given series.
Metadata may also be divided into more specific categories: for
instance, rights metadata is used in rights management and
documents with whom the intellectual property (IP) rights
of a particular image or collection reside and describes access and
usage restrictions. It may specify at what quality an image may be
reproduced or the credit line that is required to accompany its display.
Technical metadata documents aspects of files or collections that
are distinct from their intellectual content or essence, such as
production, format, and processing information. Preservation metadata
documents the information necessary to ensure the longevity of digital
objects. There is obvious crossover among these categories; preservation
metadata is primarily a combination of administrative and structural
metadata elements, or alternatively is primarily a subset of technical
metadata.
It is important in this context to mention CBIR, or content-based
information retrieval. CBIR technology is able to retrieve images
on the basis of machine-recognizable visual criteria. Such indexing
is able to recognize and retrieve images by criteria such as color,
iconic shape, or by the position of elements within the image frame.
Stock-photo houses that cater to the advertising industry have had
some success in using this automatic indexing to answer such queries
as "find images with shades of blue in the top part of the frame
and shades of green in the bottom part" (i.e., landscapes). As this
technology becomes more sophisticated, it is likely that automatic
and manual indexing will be used together to describe and retrieve
images.
Metadata Crosswalks and Controlled Vocabularies
To make different metadata schemas work together and permit broad
cross-domain resource discovery, it is necessary to be able to map
equivalent or nearly equivalent elements from different schemas to
each other, something that is achieved by metadata crosswalks.
The Getty Research Institute and the Library of Congress offer crosswalks
between various metadata schemas, and UKOLN (UK Office for Library
and Information Networking) maintains a Web page linking to a variety
of crosswalk and metadata mapping resources. When integrated into
search software, such crosswalks allow the retrieval of diverse records
contained in different repositories and aid the migration of data
to new systems. (See Metadata Format for a discussion of the use
of RDF—Resource Description Framework—in Web crosswalks.)
Crosswalks are only a part of a coherent data structuring. Controlled
vocabularies, thesauri, authorities, and indices provide
accurate and consistent content with which to populate metadata elements.
Their use improves searching precision and recall and enables automated
interoperability. For example, a streamlined arrangement of the totality
of data describing an image file might include a distinction between
intrinsic and extrinsic information, the latter being ancillary information
about persons, places, and concepts. Such information might be important
for the description and retrieval of a particular work but is more
efficiently recorded in separate "authority" records than in records
about the work itself. In this type of system, such information is
captured once (authoritatively) and may be linked to all appropriate
work records as needed, thus avoiding redundancy and the possible
introduction of error.
Examples of controlled vocabularies include the Library of Congress
Subject Headings (LCSH), Name Authority File (NAF),
and Thesaurus for Graphic Materials I and II (TGM-I and TGM-II), all maintained by the Library of Congress;
the Art & Architecture Thesaurus (AAT), the Getty
Thesaurus of Geographic Names (TGN), and the Union
List of Artist Names (ULAN), all maintained by the Getty
Research Institute; and ICONCLASS, a subject-specific international
classification system for iconographic research and the documentation
of images. These and other vocabularies and classification systems—many
disciplines and professions have developed their own thesauri,
tailored to their particular concerns—provide a wide range of controlled
terminology to describe the people, places, things, events, and
themes depicted in images, as well as the original objects themselves.
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