[Tdwg-tag] A very simple question stated again.

[Gregor Hagedorn]

Roger wrote:
> ... It also means that 
> we hard link structure to meaning. It is very difficult for some one to 
> come along and extend this schema saying "I have an element and my 
> element represents a collector that isn't a person" because the notion 
> of collector is hard coded to the structure for representing a person. 
> They can't abstractly say "This machine is a type of collector".

Partly you can. You can create a revised schema in which the complex type 
PersonType is replaced by AgentType and PersonType as well as SoftwareAgent, 
InstitutionalAgent etc. are derived from AgentType, 

The original document would remain valid both syntactically (what xml-schema 
does) and semantically (additional interpretation of schema typing).

What you loose is that in earlier documents data in AgentType were not 
undecided about its subtypes, but fixed to the specific subtype PersonType. 
However, since the content of PersonType is simple string, it is very likely 
that many documents under the first version in fact did *not* have a person 
there, despite the name of the type, so relatively little is lost. 

You can do evolution with schema. You can not validate semantics with schema. 
However, turning your example to rdf, I think it is likely that there is no way 
to validate that a string or URI refers indeed to a person. You can validate 
internal semantic consistency, but not actual usage.
 
> My central question is how we map between existing and future schemas. 
> If we can't say where the meaning is encoded in our current schemas then 
> we can't even start the process.

Meaning is not encoded, but documented. You do not have to make use of element 
nesting (= OO composition) but is often help mapping xml-data to software 
design and it usually help intuition.

The technical answer to questions about mapping is xslt. But devising these 
mappings is a lot of work, where semantic processors clearly would help. 

I understand what we can gain by using semantic tools. I still have not clearly 
understood what we gain by RDF.  What is wrong with simply tagging the schema 
elements with id attributes (or even rdf attributes - schema-schema is defined 
to support attributes from any other namespace) and then have an external 
ontology based on this?

Gregor----------------------------------------------------------
Gregor Hagedorn (G.Hagedorn at bba.de)
Institute for Plant Virology, Microbiology, and Biosafety
Federal Research Center for Agriculture and Forestry (BBA)
Königin-Luise-Str. 19           Tel: +49-30-8304-2220
14195 Berlin, Germany           Fax: +49-30-8304-2203