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Metadata can be used to annotate entities of any kind and size, from documents to applications. They can serve different functions, including descriptive, structural, and

administrative. The purpose of metadata is to improve information seeking and retrieval, and also information understanding and use.

Semantic metadata have emerged from the semantic web initiative. Semantic metadata are ontology-based in that they are created using ontologies as their vocabularies (Corcho, 2006). As a knowledge representation tool, the ontology offers a conceptual foundation for formally defining the meaning of the metadata terms for semantic markup of the digital resources. The

18 _{RSS is a family of web feed formats used to publish frequently updated digital content. RSS was initially} developed as an RDF application before multiplying in numerous languages.

19 _{Protégé is an ontology editor and knowledge-base framework developed at Stanford University} http://protege.stanford.edu/.

notion of semantically enriched or deep annotation is at the foundation of the semantic web (Handschuh, Staab, & Volz, 2003). As Mizoguchi (2003) describes, “metadata used in semantic web is built on the basis of an ontology which constrains and partially defines the meaning of each tags and values [sic]. Interpretation and translation of the metadata can be done via ontologies. Ontologies thus play the role of glue which guarantees semantic interoperability among metadata” (p. 375).

In the area of digital libraries, ontology-driven annotations offer significant advantages to subject access to resources over traditional bibliographic descriptions. As Slavic (2003) notes, the importance of subject access to web resources has been often stressed (Koch & Day, 1997; Kwaśnik, 1999; Hodge, 2000), but the educational domain represents a specific area where subject access can be particularly important.

The limitations of subject access to online library catalogs have been analyzed in past studies (Sridhar, 2004). One of the major factors that reduce effectiveness in the use of subject search is the difficulties users find in formulating subject queries based on the Library of

Congress Subject Headings (LCSH) (Larson, 1999). Indeed, the terminology provided by subject headings can be very remote from the vocabulary of the user.

Many scholars have identified the need to adapt existing subject cataloging practices to a completely new information environment such as the web. Chan and Hodges (2000) argue that LCSH should be modified to achieve: “simplicity, interoperability and scalability” (p. 229). Specifically, the syntax needs to be simplified by separating the terminology from the syntax; LCSH should become more faceted and post-coordinate in order to be compatible with mapping to other vocabularies. The necessity to develop alternative subject access tools to overcome the limitations of the traditional subject searching as conceived and enabled by traditional subject cataloging practice has long been advocated (Lancaster, Connell, Bishop, & McCowan, 1991).

Fischer (2005) proposes an updated critical bibliography on LCSH that spans the decade 1990- 2001. A number of the drawbacks and cultural biases of LCSH are addressed. For example, intrinsic racial connotation of African American subject descriptors are described (Nuckolls, 1994, p. 243). Inconsistencies and irregularities are among the limitations of LCSH (Chan & Hodges, 2000). The lack of specificity of terminology in subject areas was another issue described by Fischer (2005) as a major obstacle for mapping LCSH to other controlled

vocabularies, such as the Art and Architecture Thesaurus (AAT). As libraries change and adapt to the new information environment, the question of whether or not LCHS will remain the primary tool for subject access is under debate. As Calhoun (2006) points out, the nature of the library catalog is shifting and needs to be integrated in a broader set of discovery tools. The risk for libraries to be marginalized for relying exclusively on the catalog for resource access has also been discussed (Coyle & Hillmann, 2007). A report from the University of California

Bibliographic Services Task Force (2005) seemed to agree: “For the past 10 years online

searching has become simpler and more effective everywhere, except in library catalogs” (p. 11). New models of content description have emerged in the online information environment to address subject access to information. Web-based descriptive systems such as Dublin Core, Encoded Archival Description (EAD), Text Encoding Initiative (TEI), and Visual Resources Association (VRA) Core have been developed over the last decade and are now part of a

sandbox of tools for digital libraries. However, there is still enormous need for effective subject access (Fischer, 2005). One increasingly significant example is social tagging. This “bottom-up” or user-centered approach to the subject classification of information subverts more traditional models used by libraries and other organizations.

Semantic annotation is another method digital libraries could explore for content

In fact, the semantic web is based on the notion of “common formats for integration and

combination of data drawn from diverse sources, whereon the original web mainly concentrated on the interchange of documents.”20

A major purpose of semantic annotations is to improve information access by supporting concept-based retrieval. While traditional metadata are recognized syntactically by machines and the retrieval is based on the matching of data string tags, semantic metadata, which are formally and explicitly specified by the ontology, enable information retrieval at the conceptual level. This implies that user queries are mapped onto the conceptual structure of the ontology and searches can be performed not only against attribute values, but also against relations.

Research has shown that having metadata based on ontologies can be beneficial. In a study addressing ontology-based metadata, Weinstein (1998) converted a bibliographic catalog encoded in MARC format into a knowledge base realized by rooting metadata in an ontology. The logical foundation of ontology-based metadata showed empowerment of the existing bibliographic relationships and support for more accurate queries. In a comparison between ontologies and bibliographic description systems, Weinstein and Alloway (1997) identified characteristics that show that ontologies are more expressive because each term is defined in relation to other terms in a complex web of meaning. Ontologies are also more precise due to the computational power of logic.