# Comparing Sentinel-2A and Landsat 7 and 8 Using Surface Reflectance over Australia
## 5 Discussion  
The comparisons shown in Figures 2 and 3 demonstrate that the agreement between the corresponding bands of Sentinel-2A and Landsat sensors is very good, but there are small consistent variations between them. In all bands, for both Landsat sensors, the bulk of the data (shown as the darker areas of each scatter plot) is very close to the 1-to-1 line, but does not lie exactly along it, with with ODR slope statistics in the range 0.93 to 1.09, depending on the band. For many purposes, this would be quite sufficient agreement to work with combinations of both instruments, or to use with existing Landsat-derived models. However, in some contexts, the differences could be sufficient to create small persistent variations between the Landsat and Sentinel data. Applying an adjustment with Equation (1) to these reflectance values brings the between-sensor ODR slopes to 1.00, in all cases. This adjustment of reflectances also removes the systematic variations in the outputs of the fractional vegetation cover model, and from the NDVI values calculated from the surface reflectance values. This makes it possible to take models and thresholds which were  
Figure 9: Comparison of surface reflectance from Sentinel-2A and Landsat-8, for same-day acquisitions after adjusting Sentinel-2A to estimate Landsat-8, using NASA’s HLS adjustment. Darker regions represent larger numbers of points. Solid line is 1-to-1, dashed line is the ODR regression fit. The plot for the NIR band is marked as “Not Applicable” because of the HLS choice of Sentinel-2 band 8A for their NIR band. fitted using Landsat data, and apply them to the reflectance values from Sentinel-2A, and have some confidence that the new sensor is not introducing a systematic offset or scaling difference. The particular methods used here to estimate surface reflectance will have the same strengths and weaknesses for all the sensors. As mentioned in Section 2.1, the correct spectral response functions for Sentinel-2A were added to the system, and all other factors were kept the same. For the purposes of comparison, the main point is that it provides a consistent framework in which to compare surface reflectance between the sensors. The comparisons of surface reflectance do show a certain amount of scatter. There are a number of possible causes for this, including image mis-registration, and viewing geometry effects such as BRDF and atmospheric path length, which might not be fully accounted for by the transformation to surface reflectance. The viewing geometry can be markedly different between sensors, due to the different swath arrangements and resulting view angles. However, it must be accepted that one probably significant factor is that, on any given pixel, the response to the different sensors differs in ways which are specific to that pixel. This would involve details of the pixel's spectral characteristics in regions of the spectral window where the two sensors have different spectral response, and is not directly predictable from the sensor response functions alone. This component of the between-sensor difference cannot be removed using the sorts of methods described and tested here, and is likely to remain a part of any attempts to combine imagery from multiple sensors. These per-pixel differences also raise the possibility that consistent regional differences may also exist. The work presented by [PERSON] (2007) showed that this was true for the case of comparing Landsat-7 against Landsat-8, but that the differences within the Australian landscape appear to be smaller than the average between-sensor differences across the whole landscape (Table 3 in [PERSON] (2007)). The comparison of the adjustments fitted here against those fitted for the HLS project also raises the question of regional differences, on a larger scale. The most likely explanation for the substantial difference in the adjustments is that the HLS coefficients were fitted on a global dataset, potentially dominated by land surfaces not representative of those common in Australia.