2.1.2. Litto3D® Reference Dataset

The French Oceanographic and Hydrographic Marine Service (SHOM) and the French National Institute for Geographical and Forest Information (IGN) conducted a joint altimetric and hydrographic survey of Mayotte from 2003 to 2010. Most of the island was mapped using the airborne topographic and bathymetric LiDAR and multibeam echosounder. The resulting Litto3D® product provides soundings located in a three-dimensional geometric reference system with high spatial resolution and a land-sea continuum (data are available for free from https://diffusion.shom.fr/, last accessed: 28 December 2021). Data extracted from this dataset, corresponding to the ICESat-2 ground track, and used for comparison, include bathymetric points acquired using the bathymetric LiDAR and multibeam echosounder.

Litto3D® soundings are provided in a cartesian coordinate system in the horizontal plane and with orthometric heights. The point cloud density is constrained by the acquisition method over a specific area and the gridded model is provided with a spatial spacing of either 1 m or 5 m. Specification regarding the positioning and the geodesy of the Litto3D® dataset are presented in Table 1 [34]. A local geoid, RGM04, was used as a reference for this dataset.


**Table 1.** Specifications of the Litto3D® dataset acquired over Mayotte.

This dataset was used during the validation phase to measure the accuracy of the SDB, but Litto3D® data were not used as calibration points for the models.
