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Re: [foaf-dev] FOAF, geonames, and more

From: Bernard Vatant <bernard.vatant@mondeca.com>
Date: Mon, 29 Jan 2007 14:52:47 +0100
Message-ID: <45BDFC2F.6090009@mondeca.com>
To: Benjamin Nowack <bnowack@appmosphere.com>
Cc: Alexandre Passant <alex@passant.org>, public-xg-geo@w3.org

Hi Benjamin

Thanks for those clarifications on RDFS semantics. Actually many people 
are not clearly aware of the two aspects (necessary and sufficient) of 
rdfs:range and rdfs:domain semantics.

I'm pretty sure most applications relying on those semantics, as well as 
their builders and users, consider only the "necessary" aspect.
(1) "If a resource R is (of class X) in the domain of P, then you can 
attach a P-value to R".

wheras the "sufficient" aspect
(2) "If a resource R has a P-value, then R belongs to (some class in) 
the domain of P"

will be used in those applications in its contrapositive form
(3) "If a resource R is (of class X) not in the domain of property P, 
then you can't attach a P-value to R"

(1) and (3) are used to control edition interfaces, whereas (2) will 
need a reasoner/classifier. The former are more frequent and familiar 
than the latter, hence (2) is sometime forgotten or misunderstood.

> On 28.01.2007 17:46:28, Alexandre Passant wrote:
>> Right, but what about using based_near for something that is not a
>> spatial thing ?
>> The only way to do is to assert this is a Spatial Thing, isn't it ?
> I can't think of a non-spatial thing that's based near something 
> (unless you use "near" in a temporal sense).
>> But since foaf:Organisation is not a subclass of geo84:SpatialThing
>> (but foaf:Person is), I have to create an union class with foaf:org +
>> geo84:ST to use based_near with this org ?
> No, "Descriptive, not prescriptive" means that you don't have to 
> pre-define rules before you can use certain RDF terms. The orgs
> you'd like to use based_near with are spatial things.
> From
>    :org foaf:based_near :x .
> you can infer that
>    :org a geo:SpatialThing .
>    :x a geo:SpatialThing .
> The description
>    foaf:based_near rdfs:domain geo:SpatialThing .
> does not say that only resoures explicitly typed as spatial things are
> "allowed" to use foaf:based_near. It's exactly the other way round:
> Resources which use foaf:based_near *are* spatial things. Independent
> of other types they may have. (In OWL you can construct axioms to
> identify/prevent inconsistencies, but not in RDF Schema. The latter 
> can only increase the total number of triples, but never reduces them.)
> Bottom line: You simply don't use foaf:based_near with resources
> [in the subset of foaf:Organization] that aren't spatial things. 
> cheers,
> Benjamin

*Bernard Vatant
*Knowledge Engineering
*3, citÚ Nollez 75018 Paris France
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Mail:     bernard.vatant@mondeca.com <mailto:bernard.vatant@mondeca.com>
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Received on Monday, 29 January 2007 13:52:56 UTC

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