[Tdwg-tag] RDF versus GML; old but relevant (?)

Blum, Stan sblum at calacademy.org
Sun Feb 12 03:57:10 CET 2006

I (we) desperately need to solve the puzzle of how to compose (re-use)
conceptual specifications.  Can we create a flexible system of base classes
that can be snapped together to make useful data exchange applications?  I
think this is one of, if not THE most import tasks for the incipient TDWG
Architecture group.
So I'm trying to educate myself about RDF versus XML (and their respective
schema tools).  I came across this comment on RDF versus GML,
http://www.mapbureau.com/gml/, which contained this:

[...] GML is not directly composable with other XML languages. Entities that
are described by other languages cannot legally play the role of geographic
features in GML. This because all types of geographic features are required
to derive from the GML abstract class gml:AbstractFeatureType. Even if it
were not for this formal requirement, the lack of conventions about how to
represent even simple semantic notions in XML languages would prevent
effective integration of GML with XML languages developed independently. 

The non-composability of GML requires that it absorb as application schemas
the multitude of other domains to which geographical information is relevant.
Failing this, non-standard mechanisms of some kind must be used to relate GML
content with external data.

Indeed, GML positions itself as a universal, rather than geography-specific,
semantic standard by including its own general formalisms for collections,
assertion of properties (in a style very much like RDF), time and processes,
and reference between content in separate files (via Xlink). GML can be
viewed as an alternative not just to geography in RDF, but to RDF itself. 

This seems like a problem for us because some aspects of our biodiversity
information are decidedly not spatial.  Is this a problem with XML Schema
generally or just the way it was used to create GML?  Several TDWGers are
getting enthusiastic about RDF, despite the cautions of McCool (referenced by
Bob Morris earlier on the TDWG-GUID list).  Should we go ahead and cast
DarwinCore as a GML application while we gear up for a coordinated switch to


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