## 2 Data and Methods
### Data Sets
#### 2.2.2 Landsat/OLI and MODIS Satellite Data  
Over test site 2 (mouth area), atmospherically corrected reflectances from Pleiades were compared to MODIS water reflectance values, used here as a reference to compensate for the lack of matchup with in situ data. MODIS/Terra and MODIS/Aqua images recorded on 7 April 2017 (Table 2) were corrected for atmospheric effects using the SeaDAS software [37], applying the NIR-SWIR atmospheric correction algorithm [38]. The time differences between Pleiades (11:15 UTC), MODIS/Terra (12:00 UTC) and MODIS/Aqua (13:40) data acquisitions on this day were 45 minutes and 2 hours 25 minutes, respectively.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Characters** & **Pleiades** & **SPOT-4** & **OLI** & **MODIS** \\ \hline Blue & 450–520 nm & / & 450–515 nm & 459–479 nm \\ Green & 520–600 nm & 500–590 nm & 525–600 nm & 545–565 nm \\ Red & 630–690 nm & 610–680 nm & 630–680 nm & 620–670 nm \\ NIR & 760–900 nm & 780–890 nm & 845–885 nm & 841–876 nm \\ Spatial resolution & 2 m & 20 m & 30 m & 500/250 m \\ Revisit time & 1–3 days & 1–3 days & 16 days & Daily \\ \hline \hline \end{tabular}
\end{table}
Table 1: Specifications of Pleiades, Satellite Pour I’Observation de la Terre (SPOT-4), Landsat8-Operational Land Imager (OLI) and MODerate resolution Imaging Spectroradiometer (MODIS) satellite sensors.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Test Sites** & **Image Date** & **Image Time** & **Time of High** & **Tidal Range** & **Satellite/Sensor** \\  & & & **Tide** & **(m)** & \\ \hline Test site 1 & 03/05/2017 & 11:15 & 10:57 & 3.7 & Pleiades \\ Test site 1 & 24/05/2017 & 11:04 & 16:00 & 5.0 & Pleiades \\ Test site 1 & 01/06/2017 & 10:52 & 10:24 & 3.6 & Pleiades \\ Test site 1 & 10/06/2017 & 11:23 & 05:22 & 4.6 & Pleiades \\ Test site 1 & 14/06/2017 & 10:52 & 07:38 & 3.9 & Pleiades \\ Test site 1 & 19/09/2018 & 10:50 & 13:22 & 2.5 & Pleiades \\ Test site 2 & 03/04/2017 & 10:56 & 08:48 & 3.1 & Pleiades \\ Test site 2 & 06/04/2017 & 11:22 & 13:00 & 2.7 & Pleiades \\ Test site 2 & 07/04/2017 & 11:15 & 13:57 & 3.0 & Pleiades \\ Test site 2 & 19/04/2017 & 11:22 & 09:05 & 1.8 & Pleiades \\ Test site 2 & 20/04/2017 & 11:15 & 11:01 & 1.7 & Pleiades \\ Test site 2 & 21/04/2017 & 11:08 & 12:12 & 1.9 & Pleiades \\ Test site 2 & 07/04/2017 & 10:53 & 13:57 & 3.0 & Landsat/OLI \\ Test site 2 & 07/04/2017 & 12:00 & 13:57 & 3.0 & Terra/MODIS \\ Test site 2 & 07/04/2017 & 13:40 & 13:57 & 3.0 & Aqua/MODIS \\ \hline \hline \end{tabular}
\end{table}
Table 2: Satellite data acquisition dates, times (Coordinated Universal Time, UTC), corresponding tidal conditions and satellite/sensor types. Tidal information is provided by the Service Hydrographique et Oceanographique de la Marine (SHOM) in Pauillac and Cordouan, respectively, for test sites 1 and 2.  
The Pleiades water reflectances at 655 nm were compared to those retrieved from the 250 m MODIS band at 645 nm after spatial and spectral resampling. In the same way, Pleiades reflectance at 558 nm was compared to MODIS reflectance at 555 nm, with the original 500 m spatial resolution resampled to 250 m.  
Moreover, Landsat8/OLI satellite data with a spatial resolution of 30 m for multispectral bands was used. On 7 April 2017, one Landsat8-OLI image was recorded over test site 2 at 10:53 UTC, i.e., only 22 minutes before the Pleiades image. OLI data were processed with the ACOLITE software selecting (for comparison) both the DSF and shortwave-infrared (SWIR) atmospheric correction methods [33; 39] to retrieve water reflectance values.