## 4 Results
### Assessment Results
#### 4.3.3 Comparison with VNG  
The VNG maps generated for the analysed VIIRS data are plotted in Figure 5, where pixels with an increasing percentage of floodwater fractions are depicted from cyan to red. In these maps it is possible to identify different and quite persistent flooded area, mostly located along the Basento and Lato rivers as well as at the border of the Monte Cotugno and San Giuliano dams. Finally, the north-eastern sector of Figure 5a,b,d is characterized by some possible false positives, probably related to cloud shadows.  
All these features are also evident in Figure 13, where flooded pixels, detected by means the ALICE\({}_{\text{NDSI}}\) implementation (red pixels), have been superimposed on those identified by VNG (yellow pixels), regardless their confidence level. Pixels commonly identified as flooded by both the algorithms have been depicted in green.  
The two products generally show a good agreement, considering that both the algorithms are able to detect the main spatial features along the Basento and Lato rivers as well as the areas bordering the Monte Cotugno and San Giuliano dams. Even taking into account a different sensitivity in the identification of probably flooded pixels, residual biases are due to effects related to the different land-sea mask used as well as to the already mentioned cloud shadows issues. Table 4 summarizes the  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{2}{c}{**All scene**} & \multicolumn{2}{c}{**Black Box**} \\ \cline{2-5}  & **Pixels Number** & **\%** & **Pixels Number** & **\%** \\ \hline
**included in PAI area** & 617 & 68\% & 537 & 76\% \\
**not included in PAI area** & 295 & 32\% & 172 & 24\% \\ \hline \hline \end{tabular}
\end{table}
Table 3: Number of anomalous ALICE\({}_{\text{NDSI}}\) pixels detected in the 4-8 December 2013 period, included or not within the PAI area all over the scene and only for the white box (Figure 12b).  
number of probably flooded pixels detected by RST-FLOOD and VNG considering both the whole scene as well as the black box for the five analysed days, exploring also common detections.  
Figure 13: Comparison between flooded areas detected by RST-FLOOD using ALICE\({}_{\text{NDSI}}\geq 3\) (red pixels) and VNG (yellow pixels) on VIIRS data acquired on: (**a**) 04/12/2013; (**b**) 05/12/2013; (**c**) 06/12/2013; (**d**) 07/12/2013; (**e**) 08/12/2013. Common detections are depicted in green.  
VNG seems to detect fewer anomalous pixels than RST-FLOOD, especially along the Basento River, as is clearly observable in Figure 13. This effect is more evident considering the detection statistics related to the black box, where the area most affected by the event is included. The comparison between VNG results and those achieved by the change detection scheme on Landsat data, as already done for RST in Section 4.3.1, returns a 26% of common detections (i.e., 49 pixels) on the whole scene and a 47% in the black box. Starting from the 6 December 2013 (Figure 13c) map, any flooded area around the Basento River has been detected by VNG, whose identifications are mainly focused on Lato River. It is worth noting that, to reduce the false detection [11], the minor flood detection within VNG is only done around the determined floodwater pixels or existing normal water pixels (rivers, lakes or reservoirs) and therefore a water mask not perfectly updated could be at the basis of these discrepancies.