# A System to Improve Port Navigation Safety and Its Use in Italian Harbours
## 2 Materials and Methods  
### Bathymetry Updating and \"Virtual Traffic Lights\"  
Here, we describe the software tool developed to update automatically the bathymetry of a basin (based on sea level measured or forecasted) and to implement the idea of virtual  
Figure 1: The test sites in Livorno and Bari harbours, in Italy (image from Google Earth). traffic lights (by means of comparison with two threshold levels); this represents a helpful graphical interface to support authorities in avoiding or managing critical situations by detecting hazardous areas [31; 32; 33; 34; 35; 36; 37; 38; 39]. The initial condition (referred to time t\({}_{0}\)) of the processing procedure is the georeferenced bathymetric map of a port as it usually results from multibeam surveys; this represents the static bathymetry because it changes only after a new survey following variations of the basin topography, for instance, due to dredging operations or natural phenomena such as coastal erosion, sediment deposit, sea bottom subsidence. Figure 1(a) represents the bathymetric map of the port of Livorno, that extends over an area of approximately 18 [km\({}^{2}\)]; the grid spacing is equal to 5 [m]; the coordinates are represented in UTM (Universal Transverse of Mercator coordinates, Zone 32T) and bottom depth values are referred to IGM's 0 sea level (Italian Military Geographic Institute) with a resolution of \(10^{-2}\) [m]. On the contrary, Figure 1(b) shows the map of Bari harbour (area of about 4.8 [km\({}^{2}\)]; spatial resolution 1 [m]; UTM Zone 33T); bathymetric data have been kindly provided by Port Authorities of Livorno and Bari. It is possible to set two thresholds, a lower and an upper level, variable from ship to ship depending on the draught of the vessel under monitoring, to divide the bathymetry into different areas represented by three colours: seafloor depth values smaller than the lower threshold (usually set equal to the vessel draught) identify zones that are forbidden for that vessel and are represented by red colour, while depth values greater than the upper threshold identify deep or allowed zones (green: bottom depth more than the upper threshold, in its turn greater than the ship draught: there is a wide under-keen clearance (the space between the lowest point of the vessel keel and the sea bottom under it)); finally, depth values between lower and upper values identify shallow or warning zones (yellow: a gradual transition from the green area to the red one, with bottom depth values between the lower threshold and the upper one: there is a little space between the vessel keel and the seafloor). The partition of the harbour area into three different colours shows prohibited, allowed or warning zones for that vessel and so it implements the idea of virtual traffic lights customized for each ship: in a hypothetical operational scenario, the choice of thresholds would be a task of end users (port communities). Figure 2: _Cont._  
For example, regarding the port of Livorno, for a ship having a draught of 10 m thresholds could be set at 10 and 11 m, respectively; it follows the representation shown in Figure 3a: therefore, for that ship, we consider as forbidden (red) the areas with bottom depth less than 10 m, warning areas (yellow) those in which the depth is between 10 and 11 m, and allowed areas (green) those where the depth is greater than 11 m. This is the so-called static analysis (the map does not change over time). In Figure 3b the same map is shown but after different thresholds have been chosen: for a hypothetical ship with a greater draught than the previous case, e.g., 11.5 m, choosing threshold levels set at 11.5 and 12.5 m, respectively, leads to expand the red (prohibited) zone and to reduce the green (allowed) area: we consider as forbidden the zones with a seafloor depth of less than 11.5 m, warning areas those with a depth between 11.5 and 12.5 m, and allowed areas those with a depth greater than 12.5 m; obviously, for a ship with a deeper draught, a larger (red) area will be forbidden.