Core, you can't translate them back into MARC. Once they're Simple Dublin Core, you've lost the MARC information about what types of titles they are, so when you map from Simple Dublin Core back to MARC, all the data in the "Title" element maps to the basic MARC 245 Title Statement field. This is why crosswalks are said to be "lateral" (one-way) mappings from one scheme to another. Separate crosswalks would be required to map from scheme A to scheme B and from scheme B to scheme A. Difficulties in mapping Other mapping problems arise when: One scheme has one element that needs to be split up with different parts of it placed in multiple other elements in the second scheme ("one-to-many" mapping) One scheme allows an element to be repeated more than once while another only allows that element to appear once with multiple terms in it Schemes have different data formats (e.g. John Doe or Doe, John) An element in one scheme is indexed, but the equivalent element in the other scheme is not Schemes may use different controlled vocabularies Schemes change their standards over time Some of these problems are not fixable. As Karen Coyle says in "Crosswalking Citation Metadata: The University of California's Experience," "The more metadata experience we have, the more it becomes clear that metadata perfection is not attainable, and anyone who attempts it will be sorely disappointed. When metadata is crosswalked between two or more unrelated sources, there will be data elements that cannot be reconciled in an ideal manner. The key to a successful metadata crosswalk is intelligent flexibility. It is essential to focus on the important goals and be willing to compromise to reach a practical conclusion to projects." See also Meta element Metadata Database References External links "Metadata Crosswalk Depository" (SchemaTrans)(OCLC) "Mapping Between Metadata Formats" (UKOLN) "Crosswalks the Path to Universal Access?" (Getty) "Metadata Interoperability and Standardization - A Study of Methodology Part I" (D-Lib) Data management Knowledge representation Library cataloging and classification Metadata Technical communication