Various ontologies are available defining the semantics of digital identity information. Due to the rise in use of lowercase semantics, such ontologies are now used to add metadata to digital identity information within web pages. However concepts exist in these ontologies which are related and must be mapped together in order to enhance machinereadability of identity information on the web. This paper presents the Social identity Schema Mapping (SISM) vocabulary which contains a set of mappings between related concepts in distinct digital identity ontologies using OWL and SKOS mapping constructs.
The semantic web provides a web of machine-readable data. Ontologies form a vital component of the semantic web by providing conceptualisations of domains of knowledge which can then be used to provide a common understanding of some domain. A basic ontology contains a vocabulary of concepts and definitions of the relationships between those concepts. An agent reading a concept from an ontology can look up the concept and discover its properties and characteristics, therefore interpreting how it fits into that particular domain. Due to the great number of ontologies it is common for related concepts to be defined in separate ontologies, these concepts must be identified and mapped together.
Web technologies such as Microformats, eRDF and RDFa have allowed web developers to encode lowercase semantics within XHTML pages. It is commonplace for different vocabularies to be used within such pages, particularly when describing identity information. It must be declaratively specified to the agent how an unknown vocabulary is related to the vocabulary set that it knows and understands. The need to specify mappings between digital identity ontologies is currently being investigated by the W3C Social Web Incubator Group1. Our contribution in this paper is the presentation of a mapping vocabulary between ontologies used to define digital identity information called the Social Identity
Schema Mapping (SISM). Each mapping provides a relation between two concepts in disparate ontologies where the relation defines the semantics of the mapping which can cover equivalence, associative and hierarchical relations.
This paper is structured as follows: Section 2 defines the problems that have motivated the creation of mappings between digital identity ontologies. Section 3 presents the Social Identity Schema Mapping (SISM), detailing the semantics of the mappings, explaining the design choices and providing example mappings. Section 4 discusses how SISM is currently being used in applications. Section 5 presents related work and section 6 discusses the conclusions we have drawn from SISM and plans for future work. 2 2.1
Metadata models can now be embedded within web pages using lowercase
semantics: The Resource Description Framework in Attributes (RDFa) [
The majority of web users now have profiles on several distributed Social Web platforms. Such platforms open up their data through an API allowing it to be reused and combined with data from other platforms. In most cases the response returned by the API is XML according to an XML schema used by the platform and due to each platform using a distinct schema, reusing data automatically becomes limited. If we consider that each Social Web platform that a given web user is a member of contains a distinct facet of his/her digital identity, then compiling these facets together would create a complete profile of the web user. This profile could then be reused by the web user when accessing additional Social Web platforms, allowing personal information access to be controlled. 3
SISM solves the problem of information heterogeneity by providing mappings between different identity ontologies. An agent is able to consult the mappings and derive a relation between a parsed concept and a known concept where the relation describes the semantics of the mapping in a machine-readable format. SISM is available on the web2 as RDF containing mappings defined as triples containing a source concept as the subject and the target concept as the object, the predicate contains the semantics of the mapping. 3.1
SISM contains mappings between five ontologies which contain concepts used to define identity information: The Friend of a Friend ontology 3, the ontology for VCards 4, the XFN ontology 5, and the Nepomuk ontologies; Personal Information Model Ontology 6, and the Nepomuk Contact Ontology 7.
Mapping constructs from the Web Ontology Language (OWL) [
Figure 1 shows mappings between the foaf:Person and vcard:VCard classes where
each class is defined as an instance of skos:Concept. According to the SKOS
primer [
#sism rdf:type skos:ConceptScheme skos:inScheme skos:inScheme
skos:inScheme skos:inScheme vcard:VCard skos:related
pimo:Person skos:related owl:equivalentClass
skos:related foaf:Person skos:related
nco:PersonContact skos:member skos:member skos:member
skos:member #person rdf:type skos:Collection
Figure 1 shows mappings between classes in different identity ontologies. We define the foaf:Person and vcard:VCard concepts as being related in some way but not equivalent: vcard:VCard defines an information resource containing contact information whereas foaf:Person defines an instance of a person. We therefore regard each concept to be related in some way, and express this using the skos:related relation. The relation between foaf:Person and pimo:Person is more straightforward as each ontology defines a resource as a person. Therefore we used the strict expressivity of OWL to relate the two concepts as being equivalent using the owl:equivalentClass relation.
Hierarchical relations between concepts in SISM are dealt with through the use of the SKOS relationship definitions skos:broader and skos:narrower. SISM contains a collection of mappings between URL concepts, the foaf:homepage property is defined as having a broader concept defined by the vcard:url property. Therefore using skos:narrower, the inverse of skos:broader, vcard:url can also be defined as having a narrower concept defined by the foaf:homepage property. We chose SKOS constructs to express generalisation and specialisation due to their relaxed semantics, OWL constructs also allow the definition of hierarchical relations but impose strict restrictions on subsumption. Equivalent property concepts in distinct schemas are defined as equivalent using the owl:equivalentProperty relation. 4 4.1
To date the main application of SISM has been the transformation of metadata
models. As we explained in section 2, one of the main problems that motivated
the creation of SISM was the increase of lowercase semantics within XHTML
markup and the disparate ontologies used. We have successfully applied SISM
to normalise metadata models to our desired identity ontologies by generating
inference rules which when applied to a given RDF model transforms the model.
Transformed models can then be integrated and the internal data compared more
easily given the same ontological concepts. This approach has been successfully
deployed in an application that automatically monitors personal information on
the web [
The second application of SISM has been to map XML schemas used by Social
Web sites to concepts within SISM. Through this additional mapping metadata
models, defined as XML, returned from querying Social Web platforms are lifted
to RDF. By lifting these responses to RDF representations we have been able to
integrate information from such platforms [
The UMBEL project8 has investigated and produced a lightweight ontology to provide a mapping layer between RDF models published on the web. This works by linking similar concepts in distinct ontologies to an upper concept, therefore providing mappings in an upper level. SISM differs by using collections for meditation rather then concepts.
Work by [
9 http://www.sioc-project.org described using SIOC and when the web user exports his content from one site to another it is machine-readable and therefore reusable. This strategy has been deployed as a fully functional WordPress plugin10. 6
In this paper we have presented the Social Identity Schema Mapping (SISM), a currently deployed and very much in-use vocabulary of mappings between disparate digital identity ontologies. At the time of writing this paper SISM was the only mapping vocabulary for digital identity ontologies available on the web. The need for SISM is highlighted in the W3C Social Web Incubator Group’s current work investigating the portability of social data and the overlap in conceptual elements available to define the semantics of such data. SISM is based on both SKOS and OWL constructs which capture the possible semantics of relations between concepts and is currently supporting two areas of work: transforming metadata models and interlinking identity fragments distributed across the Social Web.
Future work will include the addition of more ontologies into SISM to cover the maximum range of digital identity ontologies. We hope that SISM becomes the basis for interpreting relations between digital identity ontologies, therefore we are currently implementing a RESTful service which returns an RDF model from a web page according to required ontologies.