No matter what metadata standards your digital library exports, services you need to interact with will occasionally require a different standard.
Metadata crosswalks are a common way of dealing with this issue as they are mappings from one metadata format to another.
Many standards bodies offer crosswalks. For instance the Library of Congress provides standard crosswalks from MARC to Dublin Core and Dublin Core to MARC.
To give a concrete example, the LOC’s MARC-Dublin Core crosswalk maps the MARC fields 245 and 246 to dc:title.
However, crosswalking is dangerous. Most organizations follow the best practice guidelines to ensure that they catalog to a particular metadata format correctly. And even then it is difficult to ensure consistency in the use of a metadata field in a single project.
If you add in that a remote project’s metadata may not have originated in the same format, but in a field ‘like’ the one you use, you have potential chaos. For instance, although your dc:title field might have certain best practices, the MARC 245 field it was mapped from may have had completely different best practice standards.
Because of this, crosswalks are frowned upon in some fields, and it is best to use direct crosswalks between formats. It’s also important to understand the best practices of both formats if you plan on creating your own.
Despite the danger of garbled fields, crosswalks are often necessary as projects from various fields try to interoperate.
Next time I’ll talk about how digital libraries exchange metadata records using OAI.
The D-Light of Digital Collections 
Hey Lyle,
Interesting, crosswalks seem very necessary with no one metadata standard being used acoss the board. I understand the concept of what the crosswalk does, but how does it work? Is it some form of software that allows the two to communicate or something entirely different? Any good links you know of that explain (OAI too)?
Hi,
Crosswalks are generally not implemented, they are just textual mappings. It is generally up to the person wanting to crosswalk something to figure out how to actually do it.
(I say that, but for some things you can find xslt stylesheets that can be used to transform some xml metadata formats to others.)
However, a more common form is this mapping that shows how DC can be mapped to MODS: http://www.loc.gov/standards/mods/dcsimple-mods.html
Regarding OAI, I’ll be getting to OAI pretty soon, but I recommend the OAI-PMH tutorial available from http://www.oaforum.org/tutorial/