## 3 Results
### Crop Classification
#### 3.2.3 Optimal Combination of SAR Images  
The corresponding accuracy assessment using single images (i.e., first step) with all polarimetric observables are listed in Table 6. SAR images acquired in June and September produced more accurate results than at other dates. The reasons may be attributed to the large morphological and structural differences among crops on these dates due to the crop calendar. As can be seen from ground truth photos in Figure 4, big differences existed in June and September. In June, winter wheat was tall and dense while corn and soybean were sparse with large proportions of bare soil shown. In September, winter wheat had already been harvested, while corn and soybean were fully mature and started to senesce. In addition, there was a decrease in OA in the month of July and August. The reason might be corn and soybean have dense canopy structures and large canopy moisture, which increases difficulties for radar to distinguish them. Since the value of OA on September 3 was the largest in the first step, optimal combination in the next steps always included the image on this date. Figure 12 shows the RF overall accuracies for the optimal combination \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Crop Class**} & \multicolumn{2}{c}{**LP**} & \multicolumn{2}{c}{**Pauli**} & \multicolumn{2}{c}{**Span**} & \multicolumn{2}{c}{**Ra**} & \multicolumn{2}{c}{**Ro**} \\ \cline{2-11}  & **PA** & **UA** & **PA** & **UA** & **PA** & **UA** & **PA** & **UA** & **PA** & **UA** \\ \hline Corn & 94.77 & 84.00 & 93.45 & 84.95 & 82.17 & 92.4 & 91.22 & 80.68 & 91.99 & 85.52 \\ Forest & 98.33 & 94.28 & 95.79 & 97.39 & 90.30 & 94.36 & 98.86 & 94.21 & 98.94 & 97.72 \\ Grass & 90.98 & 67.70 & 87.41 & 69.30 & 66.17 & 87.39 & 52.62 & 42.35 & 45.39 & 52.15 \\ Soil & 89.51 & 100.00 & 89.89 & 100.00 & 94.80 & 76.76 & 86.43 & 100.00 & 90.7 & 90.87 \\ Soybean & 92.64 & 92.36 & 95.54 & 90.13 & 82.14 & 90.7 & 81.72 & 90.89 & 86.23 & 89.26 \\ Tobacco & 62.13 & 99.47 & 65.12 & 97.03 & 50.26 & 63.12 & 25.17 & 63.56 & 28.24 & 95.51 \\ Watermelon & 68.28 & 100.00 & 80.91 & 98.04 & 53.22 & 58.9 & 21.17 & 100.00 & 43.37 & 89.93 \\ Wheat & 76.44 & 97.40 & 77.71 & 96.80 & 95.17 & 65.76 & 88.8 & 97.66 & 93.22 & 94.49 \\ OA & 89.19 & & 89.45 & & 84.23 & & 86.34 & & 88.66 \\ Kappa & 0.86 & & 0.86 & & 0.80 & & 0.82 & & 0.85 \\ \hline \multirow{2}{*}{**Crop Class**} & \multicolumn{2}{c}{**Pha**} & \multicolumn{2}{c}{**FD**} & \multicolumn{2}{c}{**CP**} & \multicolumn{2}{c}{**ND**} & \multicolumn{2}{c}{**RVI**} \\ \cline{2-11}  & **PA** & **UA** & **PA** & **UA** & **PA** & **UA** & **PA** & **UA** & **UA** \\ \hline Corn & 90.02 & 86.60 & 95.83 & 87.58 & 90.52 & 82.02 & 93.95 & 89.73 & 66.37 & 57.06 \\ Forest & 80.66 & 76.62 & 99.07 & 97.65 & 99.74 & 97.77 & 99.71 & 97.94 & 99.3 & 94.80 \\ Grass & 48.47 & 26.42 & 88.38 & 56.62 & 52.62 & 48.48 & 77.12 & 64.93 & 39.52 & 33.94 \\ Soil & 70.41 & 76.89 & 91.08 & 100.00 & 82.54 & 99.77 & 83.79 & 99.18 & 78.89 & 100.00 \\ Soybean & 79.33 & 76.35 & 94.43 & 93.31 & 83.2 & 87.47 & 88.92 & 91.25 & 61.47 & 63.94 \\ Tobacco & 2.33 & 10.45 & 55.81 & 91.30 & 28.57 & 100.00 & 32.23 & 100.00 & 16.85 & 45.19 \\ Watermelon & 1.29 & 40.00 & 71.84 & 98.67 & 52.1 & 81.73 & 55.66 & 96.09 & 30.74 & 62.09 \\ Wheat & 66.21 & 86.21 & 75.38 & 96.57 & 90.49 & 96.88 & 93.18 & 97.42 & 60.69 & 72.77 \\ OA & 76.69 & & 89.64 & & 87.67 & & 91.57 & & 65.64 \\ Kappa & 0.70 & 0.87 & & 0.83 & & 0.89 & & 0.55 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Accuracy assessment of RF classifications using different combinations of polarimetric observables. Note that abbreviations are explained in Table 3.  
produced by the forward image selection procedure using all polarimetric observables. It is evident that some combinations produced better results than using all images, and the best OA was achieved using five images (in May, June, August, and September), with an OA of 92.80%.