Résumé
The more ontologies are produced, the more need to identify mappings between them becomes important. Several practices and tools have been developed to support ontology alignment, but still, it remains a challenge. In the OBO world, ontology developers use cross reference annotations (formally using the oboInOwl:hasDbXRef property) to link a term to an external entity, including another term in another ontology (i.e., a mapping). These XRefs contains information of high value, because they were manually declared/verified by domain experts when the ontology was built. However, extracting and exploiting them remains a challenge for users due to their heterogeneous and chaotic descriptions. In this paper , we report on analysis of approximately 1 million XRefs in 30 ontologies from the OBO world. Our results show that 10.7% of these XRefs are ontology map-pings, but confirm that semantically-ambiguous usage of the cross-reference property which make it impractical or even impossible to reuse. We describe and quantify several issues identified (e.g., different kind of XRefs, miscellaneous formatting , ambiguous targets), and discuss some way to mitigate them especially by using more relevant specific properties taken from standard semantic Web metadata vocabularies.