## 3 Results
### Impact of Satellite Data Spatial Resolution on Turbidity Retrieval  
Spatial variability was also examined using resampled Pleiades imagery by comparing the original 2 m data to simulated 20 and 250 m data. The effects of spatial resolution on the correspondence with in situ measurements at the Paulilac station on 24 May 2017 show differences of 0.31% for the 2 m, 8.27% for the 20 m and 26% for the 250 m resampled Pleiades imagery. The coarser resolution shows a worst correspondence with the in situ measurements due to the increase of turbidity to the shallow shores of the estuary, and mixed pixel effects that include land and platform reflectances (Figure 11). Transects (indicated on Figure 1) of turbidity were compared at the different resolutions. Figure 12 shows the transect along the main navigation channel in test site 1 with a strong variability of turbidity. At the original resolution, the variability is sharp and complex, following the mud banks and turbulent currents, as previously observed using SPOT data [17]. At 20 and 250 m, the transect was smoothed, only preserving the main turbidity features. Good agreement between 2 m and 20 m data was found \begin{table}
\begin{tabular}{c c c} \hline
**Test Site 1** & **MAPE (\%)** & **RMSE (NTU)** \\ \hline Pleiades & 16.6\% & 107 \\ OLI & 15.0\% & 104 \\ MODIS & 15.4\% & 105 \\ \hline \end{tabular}
\end{table} 
Table 4: Validation results for Pleiades, OLI and MODIS algorithms. Figure 10: Empirical relationships established for test site 1 based on field measurements (2013 SeaSWIR survey) between water turbidity (NTU) and water reflectance in the NIR bands of Pleiades-1B (blue), OLI (red) and MODIS (grey). (MAPE 2.5%), while a much larger difference was found for the comparison between 2 and 250 m data (MAPE 10%). The transect for test site 2 (Figure 13) shows small-scale variability at 2 m data, but overall, the patterns are well preserved in the 20 and 250 m resampled data, showing low differences (MAPE of 2.8% and 4.1%, respectively). The turbidity products at 2, 20 and 250 m were compared over the full study site; example maps are shown in Figure 14 for site 1 and Figure 15 for site 2. In site 1, the turbidity features are very well captured at both 2 and 20 m, but at 250 m only the main features remain. Mixed pixels along the shores and islands contaminate the product. The MAPE are 7.8% and 18%, respectively, for the 20 and 250 m products compared to the original 2 m data. The MAPE between the 20 and 250 m products is 13%. In the MTZ (site 1), a 20 m spatial resolution seems well adapted to map most turbidity features. Figure 11: The Pauillac station area of Péiades (2 m), 20 m and 250 m satellite images (24 May 2017). Figure 12: Comparisons between turbidity in Nephelometric Turbidity Unit (NTU) retrieved from Péiades data at different spatial resolutions along a transect, from north to south in test site 1 (red line on Figure 1a) on 24 May 2017. Figure 14: Comparison between Pleiades turbidity in Nephelometric Turbidity Unit (NTU) (24 May 2017) resampled to different spatial resolutions: (**a**) maps at 2, 20 and 250 m, and scatter plots comparing (**b**) 2 and 20 m, (**c**) 2 m and 250 m, (**d**) 20 m and 250 m turbidity values. The black square shows the area of interest considered in the scatter plots and the color bar denotes pixel density. Figure 13: Comparisons between turbidity in Nephelometric Turbidity Unit (NTU) retrieved from Pleiades data at different spatial resolutions along a transect, from west to east in test site 2 (blue line on Figure 1c) on 7 April 2017. At the estuary mouth (site 2), the turbidity maps at 2, 20 and 250 m show a similar turbidity features (Figure 15). The observed differences in turbidity are 10% and 16% respectively for the 20 and 250 m products compared to 2 m. The difference between 250 m and 20 m is 9%. In this test site, the improvement from 20 m to 250 m is not as obvious as in test site 1.