# Interactive 3D Exploration of RDF Graphs through Semantic Planes
## 3 Use Cases
### Use Case #2: Exploring DBpedia
The knowledge base adopted in this use case is a very simple one showing relationships among people and projects. Organizations (yet a concept borrowed from FOAF) confirms the relationships

Figure 12: Use Case #2, Question 2: A semantic plane hosting influencers is created on top of the ground plane (**a**). A second semantic plane containing the living artists stands above the semantic plane of the influencers (**b**). No links between these two planes exists (**c**). At the bottom, the rest of the knowledge base. Figure 13: Use Case #2, Question 3: A semantic plane hosting artists is created on top of the ground plane (**a**). Unnecessary edges are hidden and the remaining green spheres refer to the dbo:deathDate property (**b**). Having more than one of these spheres means that the related artist has more than one death date (**c**). by also adding a start date and the envisioned end date (if any). The confirmation of a relationship between a person and a project, in this sample knowledge base, is expressed through the reification pattern. In this specific case, the standard reification [12] (involving then the rdf:Statement class) will be take as a reference point. A summary of the knowledge base is proposed in Table 2.

Before presenting the example in detail, a brief introduction to standard reification is proposed. Through RDF, information can be easily represented as a set of triples (subject, predicate and object). Here is an example:

foaf:Person1 foaf:currentProject foaf:ProjA

Sometimes, this it is not enough and there is the need to state something more about a given triple. This can be achieved through the reification pattern, where a triple \(t=(s,p,o)\) is broke down into four triples. The first is used to declare a statement, and the others to express its components. Thus, the previous triple can be decomposed as follows:

ns:St1 rdf:type rdf:Statement ns:St1 rdf:subject foaf:Person1 ns:St1 rdf:predicate foaf:currentProject ns:St1 rdf:object foaf:ProjA

(where ns is a custom namespace). In this way, information related to the previous triple can be expressed by simply referring to the statement as follows:

ns:St1 ns:ackBy foaf:Organization1 ns:St1 ns:startDate ''...'' ns:St1 ns:endDate ''...''

The unfiltered content of the knowledge base is presented in Figure 14, while Figure 15 shows one of the present statements on a dedicated plane. This visualization allows viewing the subject, predicate and object composing the triple (again, a click on these items allows seeing all the related information). In the described scenario, one instance of the class rdf:Statement is used to link a person to its current project. Organizations may acknowledge the triple and append information to each statement as the start and end date of the collaboration. Link outgoing from the statement (i.e., the pink sphere, since in our case is a blank node) represent the information appended to it by the Organization: in this case, only a data property is bound to the statement, so no end date is envisioned for the collaboration of \begin{table}
\begin{tabular}{l c} \hline \hline
**OWL Ontology T-Box Content** & \\ \hline Classes & 20 \\ (Person, Project, Organization \(\in\) foaf, Statement \(\in\) rdf) & 5 \\ Object Properties (currentProject \(\in\) foaf, ackBy, startDate, endDate \(\in\) ns, subject, predicate, object \(\in\) rdf) & 20 \\ Datatype Properties (name, surname, status \(\in\) foaf, object \(\in\) rdf) & 20 \\ \hline
**OWL Ontology A-Box Content** & 20 \\ \hline Persons & 20 \\ Projects & 5 \\ Organizations & 2 \\ Links persons-projects (i.e., foaf:currentProject) & 20 \\ Links organizations-persons (i.e., foaf:member) & 20 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Use Case #3: Summary of the Knowledge Base. the person with that project. The presence of the underlying semantic plane (i.e., the ground) name allows maintaining a view on the rest of the knowledge base, even when looking at a single statement.