## 4 Discussion
### Multi-Sensor Approach and Variable Importance  
We showed that the combination of Sentinel-1 and Sentinel-2 imagery with a detailed DEM achieves higher classification accuracies than without the association of the SAR and DEM. The SAR is useful in helping to retrieve geophysical traits in intertidal areas, and in particular, the C-band is more effective for sensing the roughness of finer sediments [110]. In our study, S1-VH, a C-band information layer, was among the most relevant in classifying different types of exposed sediments, namely in separating bare sediment and areas with high densities of fiddler crabs and, within these, distinguishing between finer (mixed) and coarser (sandy) sediments. Multispectral imagery, on the other hand, is especially useful in assessing moisture and water content [107; 111] and in mapping intertidal vegetation features [27]. In our study, high water film cover was a characteristic trait in several habitat types, particularly for bare sediment areas and macroalgae, whereas the FCB areas and rocks had little to no water content. Consequently, Sentinel-2-derived moisture indexes, particularly nNDMI and NDWI, were very important in the classification of these habitat types, effectively complementing the SAR information layers.  
The variable selection procedure allowed for a much clearer interpretation of the predictors and of the specific characteristics distinguishing each habitat type. The most important predictor of intertidal habitats in our study area was the DEM. Elevation models are used in land cover classification exercises to indicate topographic features, and in intertidal landscapes; they have been shown to be spatially related to sediment grain size distributions [112; 113]. [PERSON] et al., (2011) showed that fine-grained sediments tended to occur in elevated areas, while coarser sandy sediments would occur in lower areas. They also reported a strong correlation between satellite reflectance (from Landsat ETM+ imagery) and the DEM, supporting the argument that, in intertidal flats, more elevated areas (with higher exposure times) have less water content and finer grain sizes. In our study, we only found similar results in this regard within bare sediment areas, where mixed sediments (with finer grain sizes) were on average more elevated than sandy sediments and also displayed less water content, according to their reflectance in water and moisture content indexes. However, in the FCB areas the pattern was the opposite, with sandy sediment displaying higher mean elevations and with similar water and moisture contents in the sandy and mixed sediments. Similarly, we found a significant relationship between elevation and sediment mud content in both the bare sediment and the FCB areas, but again with opposite patterns, whereas in the bare sediment areas this relationship was positive but weak (simple linear regression, \(p>0.001\), R\({}^{2}\) = 0.002) and in the FCB habitats, it was negative and stronger (\(p<0.001\), R\({}^{2}\) = 0.21). Further experimental studies analysing the role of fiddler crabs in changing intertidal sediment characteristics and topography in this area would help in clarifying these results. While the combination of Sentinel-1 and Sentinel-2 (excluding the DEM) also produced similarly high Overall Accuracy, the inclusion of the DEM was important in vastly improving the classification of the bare sediment mixed habitats (+9.9% User's Accuracy and +3.3% Producer's Accuracy) and performed better overall for most of the remaining habitat types. This supports our recommendation to also integrate a detailed DEM, together with multispectral and SAR predictors, in intertidal habitat classification exercises.