*4.1. Land-Sea Coral Reefscape Mapping with a Multispectral WorldView-3 Stereo-Imagery*

Together with its predecessor the 2009 WorldView-2, the 2014 WorldView-3 remains the state-of-the-science sub-metre spaceborne optical sensor, leveraging two extra spectral bands in the VIS and two supplementary spectral bands in the NIR gamut, compared to all other sub-metre competitors (see Introduction). The use of the five-band VIS and eight-band optical data showed a greater classification performance of the land-sea coral reefscape than did the basic BGR, namely a gain of 2.69% and 8.79%. These results are in strong agreement with the WorldView-2 multispectral Moorea land-sea mapping project [11].

Even if the satellite-based topography, on the one hand, and bathymetry, on the other hand, are commonly studied for sub-metre optical sensors [12,15], it is still innovative to create satellite-based merged topobathymetry: Pleiades-1 [25], WorldView-3 [24]. The RMSE validation accuracies of the photogrammetry-based topography and ratio-transformed bathymetry of 1.11 m (R<sup>2</sup> = 0.99, min = 20.26 m, max = 370.25 m, *N* = 12) and 0.89 m (R<sup>2</sup> = 0.73, min = −12.49 m, max = 0 m, *<sup>N</sup>* = 15), computed here, are also in concordance with the stereo-WorldView-3 multispectral Moorea land-sea mapping [24]. Even if the topographic accuracy was lower than the bathymetric one, an in-depth examination showed

that the accuracy rapidly diminished with depth, with a break at -5 m. In contrast, the accuracy of the altitude estimation remains constant even for higher values around 400 m. The bathymetry modeling could be ameliorated by using more soundings from a more recent survey, such as the topobathymetric LiDAR campaign locally operated in 2015. Further investigation will assess the influence of various pansharpening methods on those relief accuracies. Like the current work, this previous research highlighted that the addition of the topobathymetric DSM augmented the classification accuracy derived from the WorldView-3 basic BGR (2.74%), VIS (5.44%), and optical (9.15%) datasets. It is worth emphasizing that the DSM contribution:

