# Mapping Rice Fields in Urban Shanghai, Southeast China, Using Sentinel-1A and Landsat 8 Datasets
## 3 Results  
### Optimal Polarization Images  
In determining the optimal polarization based on the temporal increase in paddy rice backscatter, backscatter profiles of training polygons were critically examined at both VV and VH polarizations. It is observed at these two polarizations that at the start of the growing season, out of the 42 paddy rice training polygons, 14 showed lowest backscatter at the first available image (9 June/b1), while the remaining 28 showed lowest backscatter at the second available image (8 July/b2). In any paddy rice field, lowest backscatter values are recorded at the flooding and planting stages. Hence, we first made a temporal distinction of paddy rice fields based on the apparent time of lowest backscatter as follows: Rice A as fields whose lowest backscatter at the start of the season were recorded by the 9 June image; Rice B as fields whose lowest backscatter at the start of the season were recorded by the 8 July image. In effect, Rice A are fields planted before 9 June, and Rice B are those planted after 9 June. Tables 2 and 3 show the extracted average temporal radar backscatter coefficients and their corresponding standard deviations at VH and VV polarizations for the two Rice subclasses and the broad land-cover categories of Water, Built, Trees and Others. \begin{table}
\begin{tabular}{c c c c c c c c c c c c c} \hline  & \multicolumn{2}{c}{**Rice A**} & \multicolumn{2}{c}{**Rice B**} & \multicolumn{2}{c}{**Water**} & \multicolumn{2}{c}{**Built**} & \multicolumn{2}{c}{**Trees**} & \multicolumn{2}{c}{**Others**} \\ \cline{2-13} _Image_ & \(\mu\) & \(s\) & \(\mu\) & \(s\) & \(\mu\) & \(s\) & \(\mu\) & \(s\) & \(\mu\) & \(s\) & \(\mu\) & \(s\) \\ \hline
9 June & \(-\)20.2 & 1.0 & \(-\)17.2 & 1.9 & \(-\)22.3 & 3.1 & \(-\)9.2 & 4.2 & \(-\)14.4 & 0.6 & \(-\)17.2 & 2.1 \\
8 July & \(-\)17.7 & 1.3 & \(-\)19.7 & 1.0 & \(-\)22.0 & 2.1 & \(-\)9.0 & 4.3 & \(-\)14.0 & 1.0 & \(-\)17.1 & 2.2 \\
1 August & \(-\)16.3 & 0.7 & \(-\)16.2 & 0.9 & \(-\)22.1 & 2.4 & \(-\)9.3 & 4.3 & \(-\)13.9 & 1.0 & \(-\)17.5 & 1.6 \\
25 August & \(-\)16.9 & 0.9 & \(-\)16.4 & 0.8 & \(-\)22.2 & 2.2 & \(-\)9.4 & 4.3 & \(-\)13.8 & 0.9 & \(-\)17.7 & 2.2 \\
18 September & \(-\)17.7 & 1.0 & \(-\)17.8 & 1.1 & \(-\)22.1 & 2.4 & \(-\)9.2 & 4.3 & \(-\)14.1 & 0.6 & \(-\)17.4 & 1.9 \\ \hline \end{tabular} \(\mu\) and \(s\) are the mean and standard deviation, respectively, of backscatter (dB) at corresponding class and date.  
\end{table} 
Table 2: Temporal backscatter coefficients for all land-cover classes at VH polarization. Figure 3: Methodological workflow. Figure 4 shows the average temporal backscatter profiles of Rice at both VV and VH polarizations. It can be observed from Figure 4 that the temporal backscatter at the VV channel is higher than that at VH at all imaging dates, in agreement with results obtained in [38; 45]. However, the VH polarization indicates the most consistent temporal increase in paddy rice backscatter. The consistency is here defined as the number of time points at which an increase in backscatter is recorded. For Rice A, there is an increase in backscatter up to the third image date (1 August 2015) at VH polarization, as opposed to a VV polarization in which the increase stopped at the second image date (8 July 2015). For Rice B, even though the increase in backscatter saturates at the second image for both polarizations, the VH channel shows a relatively shallow decline compared to the steep decline at VV. As stated in [38], the physical basis for the more sustained increase in rice temporal backscatter at VH polarization is attributable to the VH signal being less affected by changes in surface water and therefore could better reveal rice growth conditions. Furthermore, [PERSON] et al. [60] attributed this to the stronger attenuation of vertical cylinders (leaves and stems) at the VV than at the VH channel. These backscatter dynamics have also been observed in [23], and, hence, substantiate our results.