# Multi-Purpose Ontology-Based Visualization of Spatio-Temporal Data: A Case Study on Silk Heritage
## 3 STMaps Visualization Tool
### Multipurpose Ontology-Based Spatio-Temporal Visualization
For instance, if several items are placed in the same cluster, an icon depicting the number of items is shown, instead of showing all the items in a very small area, which is confusing. If the user wants to see each individual item, they need to zoom in in order to split these clusters into items or into further sub-clusters. This process is performed up to a configurable depth (defined in the stylesheet file in _fromLevel_ and _toLevel_ properties). This clusterization is essential to keep the readability of the map without losing information. Figure 2 shows an example of clusterization. The map can be viewed both in 2D and 3D. In both cases, icons are placed to represent the objects (or groups of objects within a cluster) placed at each particular location. These icons are defined in the stylesheet file, in the <_URL_File>_ field. The 3D view is rather convenient for showing the spatial relationship between the objects, as distant objects (icons) appear smaller and feel distant. This does not occur in the 2D representation. Finally, time evolution is shown in a particular fashion. Although the 'time' property of the objects can be shown with any ontology-defined visualization mechanism, time can also be shown dynamically with two specific visualization tools implemented in STMaps. The first one is an interactive timeline tool that allows seeing the evolution of the distribution of objects on the map through time. By moving the timeline, the user can see how the icons on the map appear and disappear in the local area chosen by the user moving through the map with the pan and zoom tools. The second time-specific visualization is a layered visualization mechanism by which STMaps shows simultaneously several maps (of the same geographical area) corresponding to different periods of time. This way, the user can quickly see how the objects are distributed through time. This stacked-map visualization is interactive, and the user can choose the number of layers (one layer per period) and navigate through each of them by clicking a particular period of time. When clicked, the selected period will appear in foreground, and the adjacent periods will still be visible, but in the background. Figures 3 and 4 show these two time-related visualization mechanisms. \begin{table}
\begin{tabular}{c c c} \hline \hline
**New Class** & **Class Extended** & **Properties Used** \\ \hline _Graphic Representation 3D_ & _Graphic Representation_ & \\ _Graphic Object 3D_ & _Graphic Object_ & _File, Primitive, Color_ \\ _Interactive Graphic Representation 3D_ & _Interactive Graphic Representation_ & _Tilt\_allowed_ \\ _Map 3D_ & _Map 3D, Interactive Graphic Representation 3D_ & _Clustering\_allowed, Data\_allowed_ \\ _Cluster_ & _Graphic Object_ & _Clustering (Spatial, Shape), Domain, Ring_ \\ _Cluster 3D_ & _Cluster, Graphic Object 3D_ & _Clustering (Spatial, Shape), Domain, Ring_ \\ _Time Map_ & _Dynamic Map_ & _Levels_ \\ _Point_ & _Time Map, Dynamic Map 3D_ & _Point_ \\ _Point_ & _Data Structure_ & _X, Y_ \\ _Spatial Data_ & _Point_ & \\ _Time Data_ & _Point_ & \\ _Relation Ring_ & _N-Ary Graphic O2O Relation_ & _Color, Legend_ \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Extensions to the VISO ontology implemented by STMaps. Figure 2: WGS84-based map and some examples of clusterization (with 72 and 97 items collapsed).