## 3 Results
### Crop Classification
#### 3.2.4 Optimal Combination of SAR Images and Polarimetric Observables  
The corresponding accuracy assessments using single-date image (i.e., first step) with optimal combination of polarimetric observables are listed in Table 7. SAR images in June and September still produced more accurate results. The selected optimal combination in the first step was still using the image on 3 September. Figure 13 shows the RF OA for the optimal combination produced by the forward image selection procedure using optimal combination of polarimetric observables. Similar to the previous tests, the most important acquisitions corresponded to summer and autumn (from June to September). The best OA was achieved using six images (in May, June, July, August, and September), with an OA of 94.04%, which was the highest value among all classification tests in this study. The corresponding classification map is given in Figure 14, which corresponds to the best crop classification performance over the study area in this research. It suggests that the combined use of optimal combination of polarimetric observables and acquisition dates may produce the best crop classification performance.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Acquisition Date** & **OA (\%)** & **Kappa** \\ \hline
12 April 2015 & 46.11 & 0.32 \\
6 May 2015 & 53.48 & 0.42 \\
23 June 2015 & 72.56 & 0.65 \\
17 July 2015 & 60.72 & 0.50 \\
10 August 2015 & 56.23 & 0.45 \\
3 September 2015 & 73.71 & 0.66 \\
27 September 2015 & 72.44 & 0.65 \\ \hline \hline \end{tabular}
\end{table}
Table 6: Accuracy assessment of RF classifications using single-date SAR image with all polarimetric observables.  
Figure 12: The RF overall accuracies for the optimal combination SAR image produced by a forward image selection procedure using all polarimetric observables. Note that numbers in the figure denote the best accuracy in each step (at the bottom), and combinations of SAR images (at the top), for example “9(1), 6, 8, 9(2)” represents the combination of images dated 3 September, 23 June, 10 August and 27 September (see Table 1).  
Figure 14: Classification map with the best accuracy simultaneously using optimal combination of polarimetric observables and images (OA = 94.04%).  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Acquisition Date** & **OA (\%)** & **Kappa** \\ \hline
12 April 2015 & 46.72 & 0.32 \\
6 May 2015 & 55.66 & 0.45 \\
23 June 2015 & 72.68 & 0.65 \\
17 July 2015 & 62.14 & 0.51 \\
10 August 2015 & 56.57 & 0.45 \\
3 September 2015 & 73.51 & 0.66 \\
27 September 2015 & 69.29 & 0.61 \\ \hline \hline \end{tabular}
\end{table}
Table 7: Accuracy assessment of RF classifications with optimal combination of polarimetric observables using single SAR image on each date.  
Figure 13: The RF overall accuracies for the optimal combination SAR image produced by a forward image selection procedure using optimal combination of polarimetric observables. Note that numbers in the figure denote the best accuracy in each step (at the bottom), and combinations of SAR images (at the top), for example “9(1), 6, 8, 5” represents the combination of images dated 3 September, 23 June, 10 August and 6 May (see Table 1).