1. UCD++: XML-based UCDs Motivation –following on discussion of atomizing UCDs: PCD, PAD, … –2 approaches to tagging metadata 1.XML-based: … good for registries that describe complex, heterogeneous entities 2.UCD-based Both are useful and necessary –5 WGs dealing with metadata Data ModelsUCDRegistryVOQLData Access –Dont want 2-3 different metadata systems! –Various atomized proposals not that far from XML Goal –establish a unified framework for metadata that can be used transparently across all contexts

2. XML-based UCDs Claim Proposed PCDs are a pointer into a data model. XML and Data Models –XML Schema is a definition of a Data Model –XPath is a standard for pointing to locations in model PHOT/FLUX Note that an XML instance document need not exist for the path to carry meaning. Can be parameterized: PHOT[FREQ=1.4 GHz]/FLUX Proposal: migrate to XPath-based UCDs

3. XML-based UCDs Advantages –existing standard; allows for leveraging OTS software –does everything we need to do with UCDs –provides natural, direct relationship with Data Models (dmwg); UCDs fall right out. –namespaces for free –provides framework for introducing/standardizing new UCDs XML Schema is formal definition; UCDs fall out schemas are extensible; namespaces automated data dictionaries –translation between entities in VOTable and XML for registries –current work on modeling, assignment still applicable

4. XML-based UCDs Transition –consider style rules for XML Schema definition –support backward-compatability with current list schema should note where element relates to UCD in current list –coordinate with DMWG –expect gradual transition