On Wed, Aug 17, 2016 at 9:48 AM, Markus Kroetzsch <markus.kroetzsch@tu-dresden.de> wrote:

 As Gerard said, "exact" correspondence might be difficult in most cases, but something slightly weaker should be ok. Something that one should note is that, even in cases where two things are about the same "idea", their usage in RDF is usually different if you compare Wikidata RDF to external RDF. For example, a class in an external RDF document might have instances assigned via rdf:type whereas a class-like item in Wikidata has instances assigned via a chain of properties

This is exactly why the property P2888 is based on skos:exactMatch When I proposed the "exact match" property this issue surfaced as well [1].  skos:exactMatch provides more freedom in expressing similarity between concepts then the related owl:sameAs 
 

possibly with further quantifiers assigned to the middle element. So you cannot get a simple, direct correspondence on a structural level anyway.


Actually with skos:exactMatch you can. 
 
There are also properties "equivalent class" (P1709), "external superproperty" (P2235), and "external subproperty" (P2236). See

https://tools.wmflabs.org/sqid/#/browse?type=properties&datatypes=5:Url

for the list of all 34 URL-type properties. Maybe you can discover others that are relevant for you.

The reason I proposed the exact match property was with federated queries in mind. Other resources can have a higher level of granularity on a given topic. Being able to reach out to these resource can have benefits. In the past we were able to federate over for example Wikdiata and Uniprot where the URI was composed by generating  the linking URI on the fly: e.g. BIND(IRI(CONCAT("http://purl.uniprot.org/uniprot/", ?wduniprot)) as ?uniprot) 

At first we experimented with the equivalent class property. But it was pointed out by ontologists that using the property for class equality is problematic as is partly expressed in its W3C definition: "NOTE: The use of owl:equivalentClass does not imply class equality". [1]   

Being able to store external URIs of wikidata URIs, would enable us to really make WIkidata central in de linked data cloud, allowing representing more granularity then currently captured in Wikidata. 

Cheers, 

Andra

[1] https://www.wikidata.org/wiki/Wikidata:Property_proposal/exact_match