Metadata principle 2: Good metadata supports interoperability.

Teaching, learning, and research today take place in a distributed networked environment. It can be challenging to find resources that are distributed across the world's libraries, archives, museums, and historical societies. To alleviate this problem, cultural heritage institutions must design their metadata systems to support the interoperability of these distributed systems.

The goal of interoperability is to help users find and access information objects that are distributed across domains and institutions. Use of standard metadata schemes facilitates interoperability by allowing metadata records to be exchanged and shared by systems that support the chosen scheme. Another way to achieve interoperability is to map elements from one scheme to those of another scheme. These mappings, or crosswalks, help users of one scheme to understand another, can be used in automatic translation of searches, and allow records created according to one scheme to be converted by program to another. If a locally created metadata scheme is used in preference to a standard scheme, a crosswalk to some standard scheme should be developed in anticipation of future interoperability need.

• The Getty Standards Program maintains crosswalks relevant to art, architecture, and cultural heritage information on their Metadata Standards Crosswalks page. http://www.getty.edu/research/conducting_research/standards/intrometadata/3_crosswalks/index.html
• The Library of Congress maintains crosswalks to and from MARC21. http://lcweb.loc.gov/marc/marcdocz.html
• The NSDL Standards Working Group has a metadata resources page largely devoted to crosswalks. http://metamanagement.comm.nsdlib.org/IntroPage.html
• A Spectrum of Interoperability: The Site for Science Prototype for the NSDL. D-Lib Magazine, Vol. 8, no. 2 (Jan. 2002). The paper describes how NSDL achieves interoperability. http://www.dlib.org/dlib/january02/arms/01arms.html
• University of Washington Digital Collections has mapped various metadata standards to Dublin Core. http://www.lib.washington.edu/msd/mig/datadicts/default.html

Another way to increase interoperability is to support the metadata format and harvesting protocol of the Open Archives Initiative (OAI). Systems that support the OAI Protocol for Metadata Harvesting can expose their metadata to harvesters, allowing their metadata to be included in large databases and used by external search services.

• The Open Archives Initiative home page links to the Protocol for Metadata Harvesting and guidelines for implementers. http://www.openarchives.org/
• The University of Michigan's OAIster search service contains more than three million records for digitized cultural heritage materials harvested from nearly 300 collections. http://oaister.umdl.umich.edu/o/oaister/

Another way to increase interoperability is to support protocols for cross-system searching, also called "metasearch." Under this model, the metadata remains in the source repository, but the local search system accepts queries from remote search systems. The best known protocol for cross-system search is the international standard Z39.50. http://lcweb.loc.gov/z3950/agency/

Many systems have implemented Z39.50 for metasearch. Two selected examples are:

• Searchlight. http://searchlight.cdlib.org/cgi-bin/searchlight
• PHAROS. http://pharos.calstate.edu/webpac/pharosstart.html

Z39.50 International Next Generation (ZING) is an initiative to modernize Z39.50 for the Web environment. Two emerging standards, SRW and SRU, allow Z39.50-like queries and responses to be exchanged using common Internet protocols. http://www.loc.gov/z3950/agency/zing/

Source: adapted from 'A Framework of Guidance for Building Good Digital Collections', published by the National Information Standards Office, 2nd edition, 2004.