# Comparison of SNAP-Derived Sentinel-2A L2A Product to ESA Product over Europe
## 4 Results
### Intercomparison of BOA Reflectances  
Figure 5 compares ESA-L2A and S2C-L2A reflectance for all dates over areas identified as clear sky land by the SLC map. Statistics are also indicated for vegetated areas only (column 2) and for bare areas only (column 3) over each site. Figure 6 summarizes the mean and the standard deviation values of \(R^{2}\) and RMSE between ESA-L2A and S2C-L2A for each MSI band across all dates for the different sites. All quantities are reported as percent reflectance (%). Tests for statistical difference between statistics are not presented since the large number of sampled pixels results in extremely small confidence intervals for each intercomparison statistic. Rather, difference statistics are interpreted considering the radiometric calibration accuracy (\(\sim\)3%) and the expected accuracy in surface reflectance (\(\sim\)5%) cited in Section 1. Over Barrax, an almost linear relationship was observed between ESA-L2A and S2C-L2A reflectance data for all bands with an RMSE \(\leq\) 1.06%; \(|\mathrm{bias}|\leq\) 0.27%; \(R^{2}\geq\) 0.97 (Figure 5). The RMSE was up to 30% (in relative terms) lower over bare areas in comparison to vegetated areas, although the absolute difference in RMSE between products was much smaller than L1C radiometric calibration uncertainty (Figure 6a). For La Crau, the scatter between ESA-L2A and S2C-L2A reflectance data was substantially greater than Barrax (Figure 5). S2C-L2A reflectance data have a small negative bias in comparison to ESA-L2A (bias from \(-\)0.02% to \(-\)0.52%), except for SWIR 2 band (bias = +0.06%). The RMSE ranges from 0.79% to 4.48%, and \(R^{2}\) ranges from 0.67 and 0.95. Blue, Green, Red, VRE 1 and SWIR 2 bands had lower RMSE and higher \(R^{2}\) (RMSE \(\leq\) 1.91%, \(R^{2}\geq\) 0.88) than VRE 2, VRE 3, NIR and SWIR 1 bands (RMSE \(>\) 3.12%, \(R^{2}\leq\) 0.60). Unlike Barrax, differences between vegetated and bare areas were only noticeable in VRE-2, VRE-3 and NIR where vegetated areas exhibited slightly lower agreement (Figure 6b). For Harth, the magnitude of scatter between ESA-L2A and S2C-L2A reflectance data was intermediate between Barrax amd La Crau (Figure 5). S2C-L2A reflectance data were negatively biased in comparison to ESA-L2A (bias from \(-\)0.02% to \(-\)0.30%), except for Blue and SWIR 2 bands (bias \(\geq\) 0.04%). The RMSE between ESA-L2A and S2C-L2A ranges from 0.50% to 3.56%, and \(R^{2}\) ranges 0.89 and 0.98. As with La Crau, the Blue, Green, Red, VRE 1 and SWIR 2 bands had lower RMSE and higher \(R^{2}\) (RMSE \(\leq\) 1.66%; \(R^{2}\geq\) 0.94) than VRE 2, VRE 3, NIR and SWIR 1 bands (RMSE \(>\) 3.12%, \(R^{2}\leq\) 0.60). In addition, lower agreement was noted over vegetated versus bare areas only for VRE-2, VRE-3 and NIR. The vertical bars included in Figure 6 indicates that RMSE exhibited temporal variation that was on the same order of magnitude as their average levels over the five dates considered. Moreover, the temporal variability was similar for both vegetated and bare areas. In contrast, \(R^{2}\) only exhibited substantial temporal variation in comparison to the temporal average for La Crau. Figure 5: Comparison of ESA-12A and SSC-12A BOA reflectance for Barrax, La Crau and Harth sites. Figure 7a presents difference maps for each MSI band over Barrax, La Crau and Harth for day of year (DOY) 230. An apparent spatial structure was noted, particularly for VRE 2, VRE 3, NIR and SWIR 1 bands at La Crau and Harth, which correlates qualitatively with the DEMs shown in Figure 1. Similar patterns were observed for all dates at La Crau and Harth (not shown). Figure 7b shows that differences between ESA-L2A and S2C-L2A reflectance tend to be proportional to local topographic slope, especially for VRE 2, VRE 3, NIR and SWIR 1 bands. Linear regressions between observed differences in reflectance and topographic slope indicated \(R^{2}\) ranging from 0.29 to 0.38 for these bands at La Crau and Harth.