## 2 Materials and Methods
### Satellite Imagery Selection and Processing  
#### 2.5.1 Selection of Satellite Images  
To produce a water and land mask to extract the intertidal area, compute a DEM, and undertake the supervised classification of the intertidal habitats, we used the Sentinel-1 and Sentinel-2 satellite imagery freely available from the Copernicus Open Access Hub ([[https://scihub.copernicus.eu/](https://scihub.copernicus.eu/)]([https://scihub.copernicus.eu/](https://scihub.copernicus.eu/)); last accessed on 30 October 2021).  
Sentinel-1 carries an active synthetic aperture radar (SAR) instrument that acquires images of the earth's surface by emitting C-band microwaves (at 5.405 GHz) and reading their backscattering reflections in dual-polarisation modes (vertical and horizontal waves). This allows this instrument to acquire images in all weather conditions, during the day and night. Sentinel-2 delivers a multispectral imaging of the earth's surface by passively reading the reflection of the sun light [64]. This limits the usability of these images only to daylight and according to cloud cover at the time of acquisition. Sentinel-2 samples 13 spectral bands: four bands at a 10 m \(\times\) 10 m pixel spatial resolution (covering visible and near infrared regions), six bands at a 20 m resolution (red-edge and short-wave infrared regions), and three bands at a 60 m resolution (coastal aerosol, water vapor and cirrus). Further technical details can be found at [[https://sentinels.copernicus.eu/](https://sentinels.copernicus.eu/)]([https://sentinels.copernicus.eu/](https://sentinels.copernicus.eu/)); accessed on 10 December 2021.  
For the Sentinel-1 image, we used a Level-1 Ground Range Detected image acquired in the Interferometric Wide Swath mode, with a high geometric resolution (pixel size) of 5 m \(\times\) 20 m, resampled to a pixel size of 10 m \(\times\) 10 m. Sentinel-1 VH/VV dual-polarisation images were the only options available for our study area. For the Sentinel-2 images, we used Level-2A Bottom-of-Atmosphere reflectances acquired directly from Copernicus Open Access Hub. The Level 2A images were atmospherically corrected, using the algorithm ATCOR-2/3 [65]. Coastal aerosol, water vapor, and cirrus bands were not used in this study because they have no land use applications. Sentinel-1A had a revisitation period for our study area of 12 days, and the sensing time was at ca. 19h17. Sentinel-2 satellites revisited the study area every 5 days and acquired images between 11h21 and 11h23. For the supervised classifications, we chose a Sentinel-1A scene sensed on 28 January 2020 and two Sentinel-2A adjacent scenes (T28 PCT and T28 PDT, to cover the full extent of the study area), both from 4 February 2020, with less than 10% of cloud cover at the acquisition time. These dates are within (in the case of Sentinel-2A) or very close to (in the case of Sentinel-1A) the period of the collection of the field reference data. The sensing time captured the period closest to the low-tide peak. To produce the intertidal mask and the digital elevation model, we selected a time series of Sentinel-2 images covering as best as possible the ebbing and rising tide periods, resulting in 29 images for each of the two different scenes (T28 PCT and T28 PDT), from April 2019 to May 2021 (Table S1).