## Materials and Methods
### Correlation Analysis  
The backscatter values from both PALSAR-2 and Sentinel-1A were extracted from the images, which represented HH, HV, VV, and VH polarizations. The AGB at the sample plots on the ground was then correlated with the corresponding backscatter of these polarizations by using linear regression method. Instead of using the single polarization as a variable, several other variables have been derived by manipulating these single polarizations. This manipulation was performed to produce variety of image variables and that to examine the roles of polarization in estimating AGB. Table 4 lists the variables that have been derived from the individual PALSAR-2 and Sentinel-1A and combination of polarizations from both sensors. These variables act as predictors to the AGB at the sample plots. Simple linear regression method was used to investigate the relationship between the AGB and the image variables. Multiple linear regressions were also applied to the variables to observe whether the combination of polarizations from both PALSAR-2 and Sentinel-1A able to improve strength of the correlations. Manipulation of polarization of individual PALSAR-2 and Sentinel-1A as well as combination of both sensors were tested and multiple variable equations have been produced. The relationship between backscatter and AGB is represented in a common linear function as \(y=ax+b\), \(x\) and \(y\) denote image variables and AGB, respectively and \(a\) and \(b\) are the equation coefficients. The strength of the relationship was measured by the derived coefficient of correlation (\(R^{2}\)). The greater \(R^{2}\) indicates a stronger relationship between two variables; the value \(R^{2}\) of 0 means no correlation and 1 is \begin{table}
\begin{tabular}{c c c} \hline \hline
**Variable** & **PALSAR-2** & **Sentinel-1A** \\ \hline \hline \multirow{2}{*}{Single polarization} & HH & VV \\  & HV & VH \\ \hline \multirow{2}{*}{\begin{tabular}{c} Polarization multiplicative \\ \end{tabular} } & HH \(\times\) HV & VV\(\times\)VH \\ \hline \multirow{2}{*}{\begin{tabular}{c} Simple polarization ratio \\ \end{tabular} } & HH \(-\) HV & VV \(-\) VH \\  & HV \(-\) HH & VH \(-\) VV \\ \hline \multirow{2}{*}{\begin{tabular}{c} Polarization averaging \\ \end{tabular} } & (HH + HV)/2 & (VV + VH)/2 \\ \hline \multirow{2}{*}{\
\begin{tabular}{c} Square root of multiplicative \\ \end{tabular} } & HH \(\times\) HV)\({}^{1/2}\) & (VV \(\times\) VH)\({}^{1/2}\) \\ \hline \multirow{6}{*}{
\begin{tabular}{c} Combination of polarizations \\ \end{tabular} } & HH \(-\) VV \\  & HV \(-\) VH \\ \cline{1-1}  & (HH + HV)/(VV + VH) \\ \cline{1-1}  & (HV \(-\) VH)/(VV \(\times\) VH) \\ \cline{1-1}  & (HH \(\times\) HV) \(\times\) VV \(\times\) VH)\({}^{1/4}\) \\ \hline \hline \end{tabular} All polarizations are in sigma nought (\(\sigma^{0}\), dB).  
\end{table} 
Table 4: Variables that were derived from PALSAR-2 and Sentinel-1A. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**State**} & \multicolumn{3}{c}{**No. of Plot**} & \multirow{2}{*}{**Total**} \\ \cline{2-3} \cline{5-5}  & **Lowland** & **Hill** & **Upper Hill** \\ \hline \multirow{2}{*}{\begin{tabular}{c} Perlis \\ Terengganu \\ \end{tabular} } & 8 & 2 & 0 & 10 \\  & 48 & 18 & 4 & 70 \\ \multicolumn{1}{c}{\begin{tabular}{c} Pahang \\ Johor \\ \end{tabular} } & 46 & 18 & 8 & 72 \\ \multicolumn{1}{c}{\begin{tabular}{c} Johor \\ Negeri Sembilan \\ \end{tabular} } & 6 & 0 & 0 & 6 \\ \multicolumn{1}{c}{\begin{tabular}{c} Negeri Sembilan \\ \end{tabular} } & 40 & 14 & 0 & 54 \\ \multicolumn{1}{c}{\begin{tabular}{c} Selangor \\ Perak \\ \end{tabular} } & 42 & 18 & 8 & 68 \\ \multicolumn{1}{c}{\begin{tabular}{c} Perak \\ Kelantan \\ \end{tabular} } & 20 & 8 & 4 & 32 \\ \multicolumn{1}{c}{\
\begin{tabular}{c} Kelantan \\ \end{tabular} } & 12 & 8 & - & 20 \\ \multicolumn{1}{c}{
\begin{tabular}{c} Total \\ \end{tabular} } & 222 & 86 & 24 & 332 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Summary of distribution of sample plots within the study area. a perfect correlation.