On the use of Sentinel-2 for coastal habitat mapping and satellite-derived bathymetry estimation using downscaled coastal aerosol band

Abstract

Coastal habitats provide a plethora of ecosystem services, yet they undergo continuous pressure and degradation due to the human-induced climate change. Conservation and management imply continuous monitoring and mapping of their spatial distribution at first. The present study explores the capabilities of the Copernicus Sentinel-2 mission and the contribution of its coastal aerosol band 1 (443 nm) for the mapping of the dominant Mediterranean coastal marine habitats and the bathymetry in three survey sites in the East Mediterranean. The selected sites have shallow to deep habitats and a high variability of oceanographic and seabed morphological conditions. The major findings of our study demonstrate the advantages of the downscaled Sentinel-2 coastal aerosol band 1 for both optically shallow habitat and satellite-derived bathymetry mapping due to its great water penetration. The use of Sentinel-2 band 1 allows detection of Posidonia oceanica seagrass beds down to 32.2 m of depth. Sentinel-2 constellation with its 10-m spatial resolution at most of the spectral bands, 5-day revisit frequency and open data policy can be an important tool to provide crucial missing information on the spatial distribution of coastal habitats and on their bathymetry distribution, especially in data-poor and/or remote areas with large gaps in a retrospective, rapid and non-intrusive manner. As such, it becomes a crucial ally for the conservation and management of coastal habitats globally.

Towards Routine Mapping of Shallow Bathymetry in Environments with Variable Turbidity: Contribution of Sentinel-2A/B Satellites Mission

“…The multi-scene compositing approach addressed limitations inherent in conventional methods and reduced the impact of turbidity, performing better than the standard "pick the best scene" method that relies on a single image (Figures 3 and 4c vs. Figure 4a in Cape Lookout; and Figure 10e vs. Figure 10a,c in Saint Joseph Bay). The final corrected SDB produced robust depths up to the limit of the lidar surveys, with typical errors ≤0.4 m. These excellent results from Sentinel-2 compared favorably with those produced in relatively low turbidity water in south Florida [22,44], and in regions with transparent waters [10,18,20,30]. Whereas some researchers suggested there is still work to be performed regarding the identification of the optimal period throughout the year where bathymetric errors are minimized [18,29], others asked for novel strategies to allow seabed mapping without the laborious analysis per image and the visual inspection of the "clearest scene" [19].…”

“…On the other scale, Landsat has a four-decade history using Thematic Mapper, and has been shown to be useful for some SDB in clear water [10,21,59,60], even with 30 m The use of the SDBred for shallow water retrieval addressed the worst of the overestimation issues with the SDBgreen in both study regions. The use of different bands to treat overestimation in shallow water (or underestimation by SDBred in deep water) has been pointed out recently by some researchers [19,30,57]. Switching to SDBred has been previously used to provide better discrimination of depths in very shallow water over bright targets like carbonate sand [48], and other studies have applied each model over different depth ranges [22,44].…”

“…These This paper proposes the first multiple-image technique with Sentinel-2A and Sentinel-2B that allows semi-automated SDB estimation in areas of variable turbidity. Although some recent works have already obtained accurate results with Sentinel-2 and similar satellites, they were implemented in regions with transparent waters [18][19][20]29,30]. We focus on application of the log ratio model [31] because it was designed to support routine mapping in areas with extremely limited calibration data.…”

FORCE—Landsat + Sentinel-2 Analysis Ready Data and Beyond

“…After having participated in the Atmospheric Correction Inter-comparison Exercise (ACIX) [38], FORCE will undergo further validation and testing in ACIX II, and the accompanying Cloud Masking Inter-comparison Exercise (CMIX) [72]. In order to support coastal aquatic applications [73], the option to output the coastal aerosol band of Landsat 8 and Sentinel-2 will be included. It is planned to implement support for Sentinel-1 data in the higher-level FORCE modules, which will need to be pre-processed similarly to the optical FORCE ARD; a fully integrated Level 2-like preprocessing tool is currently not planned by the developer, but could be contributed by interested third parties.…”

Evaluation of Posidonia oceanica Map Generated by Sentinel-2 Image: Gulbahce Bay Test Site

“…Among several classifiers, we selected the support vector machine (SVM) method as the classifier to generate the benthic habitat map. SVM is one of the classification methods used for benthic habitat mapping in recent studies (Traganos and Reinartz, 2018a, b;Poursanidis, Traganos, Reinartz & Chrysoulakis, 2019). As in all classifiers, spectral reflectance values of the ground truth pixels span the n-dimensional space where n denotes the number of bands.…”