# The Big Picture: An Improved Method for Mapping Shipping Activities
## 5 Experimental Results and Evaluation
### Vessel Detections Based on EO Images
#### 5.2.2 Results  
First, following the pipeline described in the methodology chapter, we extracted detections of vessels for the European waters for the full month of October. This process was conducted for both the Sentinel-1 and Sentinel-2 datasets. Moreover, a density map was generated for each data source, based on the total number of detections located in each grid cell. For the purposes of this work, we used a grid with cells of lengths of 10 km (see Figure 13).  
Furthermore, we use the respective AIS data for that period and area to determine which of the detections cannot be matched to the existing trajectories and thus correspond to dark vessels. In an initial review of our findings, the correlation between highly congested areas and dark vessels was noted [14]. This phenomenon can be attributed to a possible high number of packet-collisions in the area [72; 73], where, due to high traffic, some AIS messages are not being received. In order to counter that effect and to differentiate between low AIS coverage due to packet collision and intentional switch-off of AIS transponders, we performed an analysis described in this section, which is based on identifying areas  
Figure 12: Positive detections from the proposed CNN-based framework, for both SAR images from Sentinel-1 (top three) and optical images from Sentinel-2 (bottom three), from October 2021.  
(e.g., grid cells) where vessels are in close distance, thus increasing the risk of losing AIS packets due to congestion.  
To achieve this, we applied the following formula to different snapshots of AIS data (we used the same snapshots of the respective Sentinel-1 images used to identify dark \"cells\"). For each grid cell, we calculated the mean nearest neighbour distance, i.e., the mean distance between a vessel and its neighbours:  
\[\underline{NND}=\frac{\sum^{n}NND}{n}\]  
with \(NND\) being the Distance with the Nearest Neighbour vessel and \(n\) the total number of vessels. This metric indicates that the lower the mean distance between neighboring vessels is, the higher the risk of congestion. Thus, we define the \"detectability\" maps as the maps that showcase congested areas that might lead to detectability issues. We constructed detectability maps for the whole area of interest; for visualization purposes, we include the respective map for the Mediterranean Sea in Figure 14.  
Using such information, a corrected density is calculated for each grid cell with the resulting density map depicted in Figure 15.  
Figure 14: Detectability map for the Mediterranean Sea area for the month of October 2021.  
Figure 13: Density maps based on vessel detections from EO imagery, for both Sentinel-1 (**left**) and Sentinel-2 (**right**) data, for vessels in EU waters for the month of October 2021. The grid is comprised of cells with edges of 10 km length, with the density computed as the total number of detections for each cell (according to the color caption).  
Finally, after producing the dark vessel density maps, we use this information to complement the Sat-AIS based density maps. Here, first we generated density maps for October 2021 using only SAT-AIS data (number of positions/cell). Then, the respective density map of dark vessels for the same period was created. We consider only the dark vessels, and not all vessels detected by satellites for duplicate elimination (so that we do not count the same vessel position twice); using this combination of data sources (SAT-AIS and Sentinel-1), we produce the combined density map, as shown in Figure 16.  
Figure 16: Density maps for vessels in EU waters for the month of October 2021, counting number of vessels per cell of 10 km edge length, from SAT-AIS only (**top left**), detections from Sentinel-1 data (**bottom left**) and for the combined dataset (**right**).  
Figure 15: Density map of dark vessels detected using imagery from Sentinel-1 correlated with SAT-AIS data, for the month of October 2021 (according to the color caption).