# Assessing the Added Value of Sentinel-1 PolSAR Data for Crop Classification
## 5 Discussion
### Feature Importance of the Combined Sentinel-1/2 Feature Space  
As discussed in Section 4.1.4, our custom GA was employed for a feature importance estimation analysis, by running multiple GA experiments and recording how many times each individual feature appeared in the optimal subset. This exhaustive and computational expensive procedure was followed in order to extract a list of the most valuable predictors with satisfactory statistical significance. Figures 11 and 12 illustrate the first 25 most important features in descending order, as recorded for the original case and the case of artificially generating clouds, respectively (the lists are available in the Supplementary Material, Tables S8 and S9). The relative importance of each feature was calculated in the same way as the importance values in Figure 9. In both cases, PSRI and NDWI were the two features with the highest relative importance. The great contribution of these particular attributes is quite reasonable, since the different crop types of this study exhibited large differences in senescence phase onset and their water content across the observation period. When the original clear Sentinel-2 imagery was used, optical features dominate the list of the 25 most important features, occupying its three quarters. Apart from  
Figure 10: Evolution of the Sentinel-1 (red) and Sentinel-2 (blue) ratios inside the list of the most important features as a function of threshold value t for the original case (**a**) and the artificial clouds scenario (**b**). The size of each point corresponds to the number of features for each threshold considered. Black demarcation lines delineate the threshold range for the optimal performance of SVM classifier in each case. this disproportion, the optical features have generally higher relative importance values, ranging from 3.10% to 11.91%, contrary to the polarimetric features, ranging from 0.24 to 3.79%. All vegetation indices, as well as all Sentinel-2 spectral bands, are included in the most important features list, in accordance with our crop classification results in Section 4.1. This finding was expected, since optical data hold vital information about the biophysical properties of plants [22,138]. Despite the superiority of optical features, several polarimetric parameters related to Shannon entropy and to covariance matrix's eigenvalues appear significant contribution. Concerning the second scenario, the presence of artificial clouds in the Sentinel-2 imagery revealed the hidden value of PolSAR data. Shannon entropy, second eigenvalue, as well as the normalised Shannon entropy were placed among the top 5 features, followed by Shannon entropy intensity and the normalised value of Shannon entropy intensity in positions 7 and 8, respectively. Moreover, there is almost a balance between the number of two types of features. These results are verified by the findings of recent publications [49, 50, 51, 52, 63], which clearly demonstrate the great potential of Shannon entropy, entropy and mean alpha angle in crop monitoring, and crop classification. Apart from this analysis, we also performed an importance analysis for each month of the observation period both for the original case and the case of artificially cloudy images. The relative importance value of each month was calculated in the same fashion as in features' relative importance analysis. As illustrated in Figure 13, in both cases, July is the month with the greater contribution. July's high competence can be interpreted, considering that, during this month, summer and winder crops are going through a different stage of development (inflorescence and maturation, respectively) [137] and, as a result, present large differences in biophysical and structural properties. For the original case, June and April are the second and third most informative months, respectively. According to Figure 2, June is a transitional period in crops' growth cycle, since winter crops gradually approaching their full development and summer crops pass into their flowering stage.