# Comparing Sentinel-2A and Landsat 7 and 8 Using Surface Reflectance over Australia
## 4 Results
### Surface Reflectance  
The scatter plots in Figures 2 and 3 show that there are consistent differences between Sentinel-2A and both of the Landsat sensors. The differences appear to be proportional to the reflectance magnitude, with minimal constant offset. Depending on which band, the ODR slope statistic ranges from 0.93 to 1.09 and the RMSE ranges from 0.005 to 0.021.  
There is a certain amount of scatter evident in all scatter plots, but the regions shown as darker, representing the majority of the pixels, are much less scattered. This is supported by the Pearson correlation values, which are all in the range 0.97 to 0.99.  
Figure 2 shows the direct comparison of corresponding bands between Sentinel-2A and Landsat-7. Figure 3 shows the equivalent comparison of Sentinel-2A with Landsat-8. Very similar patterns appear, although the amount of difference is slightly dependent on which Landsat is being compared. This is consistent with earlier work suggesting that Landsat-7 and Landsat-8 are not identical instruments but exhibit systematic variation between the two sensors [7; 25; 26].  
Figures 4 and 5 show the same reflectance comparisons, after applying the fitted adjustment using Equation (1), with coefficients from Table 4. Unsurprisingly, the adjustment brings the data back to the 1-to-1 line, removing systematic between-sensor variation in either direction. All the ODR slope statistics are equal to 1 (within two decimal places). The RMSE values  
\begin{table}
\begin{tabular}{l c c c c c c c} \hline \hline  & **Coefficient** & **Blue** & **Green** & **Red** & **NIR** & **SWIR1** & **SWIR2** \\ \hline Landsat-7 & \(c_{0}\) & \(-\)0.0022 & 0.0031 & 0.0064 & 0.012 & 0.0079 & \(-\)0.0042 \\  & \(c_{1}\) & 0.9551 & 1.0582 & 0.9871 & 1.0187 & 0.9528 & 0.9688 \\ \hline Landsat-8 & \(c_{0}\) & \(-\)0.0012 & 0.0013 & 0.0027 & 0.0147 & 0.0025 & \(-\)0.0011 \\  & \(c_{1}\) & 0.963 & 1.0473 & 0.9895 & 1.0129 & 0.9626 & 0.9392 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Coefficients for Equation (1), to adjust Sentinel-2A reflectance to the corresponding Landsat reflectance. Fitted from the training data described in Table 3.  
Figure 2: Comparison of surface reflectance from Sentinel-2A and Landsat-7, for same-day acquisitions. Darker regions represent larger numbers of points. Solid line is 1-to-1, dashed line is the ODR regression fit showing proportional bias.  
adjustment, except for Landsat-8 Red band, which remains the same. It is worth emphasizing that the coefficients used for the adjustment were fitted on a separate set of points. As discussed in Section 2.1, the extracted point data was randomly split into training and validation subsets. The coefficients were fitted on the training points, but the comparisons presented here are from the validation points, which are quite distinct, while still coming from the same regional population.  
Figure 4: Comparison of surface reflectance from Sentinel-2A and Landsat-7, for same-day acquisitions after adjusting Sentinel-2A to estimate Landsat-7, using Equation (1). Darker regions represent larger numbers of points. Solid line is 1-to-1, dashed line is the ODR regression fit showing proportional bias.  
Figure 3: Comparison of surface reflectance from Sentinel-2A and Landsat-8, for same-day acquisitions. Darker regions represent larger numbers of points. Solid line is 1-to-1, dashed line is the ODR regression fit showing proportional bias.