same and synonym Re: IAU thesaurus in RDF (an update)

[Bernard Vatant]

Ed
> Is infrared_radiation synonymous with infrared_emission?  I don't see 
> a way for SKOS to say SameAs.  How does one say iau:stars sameas 
> cds:stars?
This is a complex issue - and to say the truth my favourite one - with 
different aspects. So I have to bite here :-)

1. Synonymy is dealt with in SKOS as in standard thesaurus practice. 
Defining a controlled vocabulary is all about defining a sort of closed 
linguistic world where the "unique name assumption" holds, which means 
distinct concepts bear distinct labels, and every concept has a single 
(preferred) label (in a given language). So if you have synonymous like 
"infrared emission" and "infrared radiation" (supposing they are 
actually synonymous, which IMHO seems questionable, but this is not the 
point), the controlled vocabulary authority, based on what is considered 
as the best naming practice. Suppose you recommend "infrared radiation" 
over "infrared emission", you will have in classical thesaurus :

/Infrared Emission
    /USE   Infrared Radiation/
/
Infrared Radiation
    UF /Infrared Emission
/
In SKOS, the same semantics is achieved using skos:prefLabel and 
skos:altLabel.

iau:_123456  skos:prefLabel   'infrared radiation' @en
iau:_123456  skos:altLabel   'infrared emission' @en
iau:_123456  skos:prefLabel   'rayonnement infrarouge' @fr
iau:_123456  skos:altLabel   'émission infrarouge' @fr

Note that I deliberately used some opaque string for the concept URI, to 
make clear that this is a question concerning labels and not concepts 
themselves. There is only one concept here, with four labels.

2. A different issue arises when two different authorities have issued 
vocabularies and you want to declare a posteriori equivalence of such 
concepts. SKOS does not prevent you to use owl:sameAs to express such an 
equivalence, since skos:Concepts are individuals. If you look e.g. at 
http://www.geonames.org/ontology/ontology_v2.0_Lite.rdf, you will see 
that I have defined here a few owl:sameAs equivalences between 
geographical feature codes (a subclass of skos:Concept) and concepts 
defined by the environmental GEMET thesaurus (also in SKOS).

So something like the following would be perfectly consistent with both 
OWL and SKOS semantics

iau:stars   rdf:type   skos:Concept
cds:stars   rdf:type   skos:Concept
iau:stars   owl:sameAs   cds:stars

Of course, you might want things more subtle than that, because for some 
reason you want to declare a mapping without merging the two concepts. 
In that case you can use SKOS mapping properties, but note that this 
draft has no formal status whatsoever so far on W3C track. See 
http://www.w3.org/2004/02/skos/mapping/spec/

Hopes that helps

Bernard
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*Bernard Vatant
*Knowledge Engineering
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