Figure 1: A density map of vessel traffic in front of Rotterdam during a single day. The density map is a combination of four density fields each covering a quarter of the day. The following manually defined color map is used: night is dark blue, morning is bright yellow, afternoon is dark yellow, and evening is bright blue. Furthermore, the saturation of the color represents the density field contribution and the hue is given by the period with the highest density. To discriminate daylight patterns from nighttime, the night and evening use half the kernel radius of the other periods. This figure shows that the main routes are the most used during daylight, while in the night deviations from these routes occur.
ABSTRACTWe present a method to interactively explore multiple attributes in trajectory data using density maps, i.e., images that show an aggregate overview of massive amounts of data. So far, density maps have mainly been used to visualize single attributes. Density maps are created in a two-way procedure; first smoothed trajectories are aggregated in a density field, and then the density field is visualized. In our approach, the user can explore attributes along trajectories by calculating a density field for multiple subsets of the data. These density fields are then either combined into a new density field or first visualized and then combined. Using a widget, called a distribution map, the user can interactively define subsets in an effective and intuitive way, and, supported by high-end graphics hardware the user gets fast feedback for these computationally expensive density field calculations. We show the versatility of our method with use cases in the maritime domain: to distinguish between periods in the temporal aggregation, to find anomalously behaving vessels, to solve ambiguities in density maps via drill down in the data, and for risk assessments. Given the generic framework and the lack of domain-specific assumptions, we expect our concept to be applicable for trajectories in other domains as well.