# Multi-Purpose Ontology-Based Visualization of Spatio-Temporal Data: A Case Study on Silk Heritage
## 2 Related Work
### Visualization of Spatio-Temporal Data
It is usually possible to see a broad time level and zoom-down interactively to a concrete time period [27] or even to a precise month or day [13]. The use of moving elements and animations is also a good way to depict the flow of time. Several solutions have been proposed by the scientific community, such as spiral diagrams [28; 29; 30], river-flow diagrams [31; 32], spacetime cubes [11; 13; 33] and some otherless intuitive approaches [34]. For these purposes, it is important to identify the date at different time intervals. For instance, in a spiral or helix diagram, each revolution around the vertical axis represents a time period. Similarly, a layer in a spacetime cube represents a given time period. However, as the complexity of the visualization and the interaction increases, this becomes harder and harder. For instance, the work presented in [35] shows an immersive spacetime cube. The authors analyzed the usability and performance of this visualization in a series of tasks with young people and they concluded that the system receives positive opinions but does not improve conventional desktop-based space-time cube representation in terms of performance. As technology advances and gets more complex, so do data. Indeed, with increasingly complex information models, data tend to have many more attributes. For this reason, new ways of classifying and visualizing spatio-temporal data arise, such as the one proposed in [36]. In this work, [PERSON] added the who dimension to the [PERSON]'s proposal. This new dimension refers to objects, and the what dimension refers to the attributes of these objects. These four dimensions can be queried simultaneously or in combinations, performing queries with four different levels of complexity. The more dimensions are combined, the more filtered data are, and the easier they will be to visualize. On the contrary, the fewer dimensions are used, the more information will be shown, reducing the understandability of the analysis. Although this way of structuring spatio-temporal data for visualization purposes is useful, reality is often much more complex. For instance, the line between objects and attributes is not always clear, since sometimes objects can be at the same time objects and attributes. This could occur when an object is related to other objects and these latter objects become attributes of the former. In the case of CH, this happens frequently with manufactured objects, which can be crafted by using existing objects. If we need to show the spatio-temporal evolution of these objects, the problem gets increasingly complex. This is where ontologies and semantic information could provide useful insights. In addition, in CH, it is not uncommon to have objects that are poorly or loosely classified in terms of spatio-temporal properties. Some geographic and temporal data could be missing, fuzzy, incomplete, duplicated or even wrong, since it is not always possible to be certain about the origin and age of CH-related items. Small- and medium-sized museums do not always have the required resources to properly document their objects. This poses an additional challenge for the visualization of spatio-temporal data. Indeed, showing items that may have duplicated or incomplete spatio-temporal properties requires special strategies to be able to effectively communicate these situations to the user and show the relationship between the duplicated or incomplete information. Although this problem has been previously studied [37; 38; 39], very little attention has been put in visualizing this type of CH-related information [40]. Although the visualization of the uncertainty in data may increase significantly the complexity of the visual representation, its omission can also create obvious problems. For this reason, a trade-off between visual complexity and visual accuracy should be found [41]. This trade-off depends on the objectives and the audience of the visualization tool.