*2.2. Land-, Sea- and Air-Truth Data*

The topobathymetry extraction requires XYZ control points for both land and sea realms [24]. The topographic DSM was calibrated and validated by 20 and 12 ground control points (see red spheres in Figure 1a), surveyed in September 2018 with a Mobile Mapper 120 provided with a 20 Hz Differential-GPS+GLONASS position output, ensuring a maintained 0.3 m accuracy. The sampling distribution was not optimal because it was constrained by the need to use a single pathway that ran along a crest surrounded by steep ravines. The bathymetric DSM was calibrated and validated by 20 and 15 sea control points, retrieved from the digitized French Navy chart (identified as 6657, based on a 1966-to-1972 hydrographic campaign originally referenced to the lowest astronomical tide, see green spheres in Figure 1a). Despite the gap in timing between the waterborne soundings and the spaceborne acquisition, the absence of major local events in the seascape enabled the freely available hydrographic soundings to be used, for the sake of transferability. Both topographic and bathymetric control points were then horizontally referenced to the UTM 6 South projection into the WGS-84 datum, and vertically zeroed to the mean sea level.

A suite of 13 habitats (Figure 1b) were inspected using geolocated handborne photoquadrats for land classes and geolocated airborne for sea classes. Photoquadrats were taken with an Olympus Tough TG-4 provided with BGR bands (16 million pixels each), while aerial pictures were monitored using a DJI Mavic Pro Platinum collecting BGR bands (12 million pixels) at 35 m altitude (height above the mean sea level). A series of 90 photoquadrats and 90 aerial photographs were orthorectified [8] to distinguish 10 classes, representative of an average land-sea coral reefscape (Table 1). Five land and five sea classes were each constituted of 30 and 30 seed pixels neighbourly and evenly grown to 3000 calibration and 3000 validation pixels, respectively. The three remaining classes, namely forest, roof and shadow, were straightforwardly characterized by 3000 calibration and 3000 validation pixels visually selected on the satellite imagery.


**Table 1.** Description of the 13 land use/land cover and sea use/sea cover classes.
