## 5 Discussion
### Window Size Selection  
When using the MCC method to invert the SSC field, the selection of window size will affect the SSC inversion. The selection of the window should not only satisfy the requirement that \"template window\" should include enough spatial structure information of feature quantity (TSM), and that \"search window\" must cover the maximum moving distance of feature quantity, but also ensure that the obtained SSC field possesses better spatial consistency [(20; 48)]. In this study, we used the area of 122.5\({}^{\circ}\)-125.5\({}^{\circ}\)E, 34.5\({}^{\circ}\)-37.5\({}^{\circ}\)N at 10:30-11:30 am on 5 August 2012 (less cloud cover, better inversion effect). For the case study, different window sizes were selected to compare and analyze the inversion results of the respective current fields. As shown in Table 3, W1-W7 are the different parameter settings of the seven window sizes used in the MCC method. For example, W1 represents the template window size of \(10\times 10\) pixels and the search window size of \(24\times 24\) pixels. Figure 14 shows the SSC vector diagram obtained under each parameter. Parameters W1, W2 and W4 have better inversion effects, but the current speed obtained by W2 is too large, and the current field obtained by W3 has obvious inversion error. Table 4 shows the SSC characteristics and vector coverage of the current field obtained under each parameter, where the vector coverage is the ratio of the actually obtained vector number to the theoretically obtained vector number. We can see that the average speeds obtained by W1, W4 and W7 are close to the measured average speed of 0.45 m/s, but the coverage rate of W7 is lower. Although the results obtained by W2 have a large coverage rate, the average current speed is relatively large, the maximum current speed reaches 2.95 m/s, and the error is relatively large. For the spatial consistency test of the obtained vectors, we use the method of neighborhood requirements [(27)]; that is, to compare the current speed difference and angle deviation between the target vector and its adjacent vector (up to eight, each direction includes an adjacent vector of the diagonal), and use the method mentioned in Section 3.4 for evaluation. Here, we stipulate that the number of adjacent vectors around the target vector should not be smaller than 5. As shown in Figure 14h), I-III are the positions of the three target vectors selected from south to north in order. Table 5 shows the AME and AAE values of the three target vectors and their adjacent vectors under each parameter. The AME and AAE values of the target vectors obtained by W1, W2, W4 and W5 are smaller than those of the adjacent vectors and have better spatial consistency. In summary, the current field obtained by selecting the window with the parameter size of W1 or W4 is better. Taking all the factors into account, we decided that W4 should be used as the standard setup for application of the MCC on GOCI-derived image pairs with T = 1 h. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & **W1** & **W2** & **W3** & **W4** & **W5** & **W6** & **W7** \\ \hline Max-speed (m/s) & 1.11 & 2.95 & 1.57 & 1.39 & 1.18 & 2.95 & 1.46 \\ \hline Min-speed (m/s) & 0.23 & 0.36 & 0.79 & 0.22 & 0.01 & 0.15 & 0.05 \\ \hline Ave-speed (m/s) & 0.60 & 1.09 & 1.00 & 0.68 & 0.28 & 0.74 & 0.51 \\ \hline PCV (\%) & 86.54 & 97.65 & 56.20 & 96.58 & 70.09 & 87.61 & 77.56 \\ \hline \hline \end{tabular} Note: Ave-speed: average speed; PCV: percentage coverage values.  
\end{table} 
Table 4: SSC characteristics and vector coverage of SSC field under different window sizes. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**MCC** & **W (=H)** & **W1** & **W2** & **W3** & **W4** & **W5** & **W6** & **W7** \\ \hline \(T_{sub}\) & pixels & 10 & 10 & 20 & 20 & 28 & 20 & 28 \\ \(S_{sub}\) & pixels & 24 & 36 & 24 & 36 & 36 & 48 & 48 \\ \hline \hline \end{tabular} Note: W(=H): width(=height); \(T_{sub}\): template window; \(S_{sub}\): search window.  
\end{table}