Understanding satellite-derived bathymetry using Sentinel 2 imagery and spatial prediction models

Abstract

Optical satellite data is an efficient and complementary method to hydrographic surveys for deriving bathymetry in shallow coastal waters. Empirical approaches (in particular, the models of Stumpf and Lyzenga) provide a practical methodology to derive bathymetric information from remote sensing. Recent studies, however, have focused on enhancing the performance of such empirical approaches by extending them via spatial information. In this study, the relationship between multibeam depth and Sentinel-2 image bands was analyzed in an optically complex environment using the spatial predictor of kriging with an external drift (KED), where its external drift component was estimated: a) by a ratio of log-transformed bands based on Stumpf's model (KED_S) and b) by a log-linear transform based on Lyzenga's model (KED_L). Through the calibration of KED models, the study objectives were: 1) to better understand the empirical relationship between Sentinel-2 multispectral satellite reflectance and depth, 2) to test the robustness of KED to derive bathymetry in a multitemporal series of Sentinel-2 images and multibeam data, and 3) to compare the performance of KED against the existing non-spatial models described by Stumpf et al. and Lyzenga. Results showed that KED could improve prediction accuracy with a decrease in RMSE of 89% and 88%, and an increase in R 2 of 27% and 14%, over the Stumpf and Lyzenga models, respectively. The decrease in RMSE provides a worthwhile improvement in accuracy, where results showed effective prediction of depth up to 6 m. However, the presence of higher concentrations of suspended materials, especially river plumes, can reduce this threshold to 4 m. As would be expected, prediction accuracy could be improved through the removal of outliers, which were mainly located in the channel of the river, areas influenced by the river plume, abrupt topography, but also very shallow areas close to the shoreline. These areas have been identified as conflictive zones where satellite-derived bathymetry can be compromised.

Space‐Borne Cloud‐Native Satellite‐Derived Bathymetry (SDB) Models Using ICESat‐2 And Sentinel‐2

“…Next, a multi‐temporal mosaic was derived using four Sentinel‐2 bands; B1‐coastal aerosol (resampled to 10 m), B2‐blue, B3‐green, and B4‐red, as these wavelengths are less susceptible to light attenuation than longer wavelength bands. Based on our work within both temperate and tropical optical conditions (Poursanidis et al., 2019b, 2020), the use of the first four Sentinel‐2 bands yields increased accuracies in comparison to the exclusion of one or more bands. This is particularly pertinent to the first 5 m of depth within which one can observe the minimum light attenuation in the water column.…”

Satellite-derived bathymetry in optically complex waters using a model inversion approach and Sentinel-2 data

“…9) obtained from the 'optimal' images with either C2RCC or DWC correction were representative of the actual known bathymetric features described by the INFOMAR programme and also by other studies carried out in the same areas (e.g. Monteys et al, 2015;Cahalane et al, 2017;Casal et al, 2019;Casal et al, 2020).…”

“…Only the 10 m resolution bands were included in the analysis. Band 1 (444 nm) was ruled out due to its low spatial resolution (60 m), and for consistency with previous studies (Casal et al, 2019(Casal et al, , 2020.…”

Examining the Links between Multi-Frequency Multibeam Backscatter Data and Sediment Grain Size

“…The RMSE values were specifically designed for model validation and selection, especially where problems of overfitting and underfitting were expected in a multivariate regression modelling framework [50]. Therefore, lower RMSE values favoured models that better fit the data, and achieved the highest prediction of the response variable [51]. In addition, residual plots of fitted verses residuals for each of the models are given in the appendix (Appendix A, Figure A1).…”