UAS-derived multispectral habitat mapping for nearshore isolated oceanic islands: the example of Henderson Island, Pitcairn Group, South Pacific

Abstract

ABSTRACT Henderson Island (Pitcairn Group) is a highly remote raised atoll system where conventional nearshore hydrographic surveys are difficult to conduct. However, baseline seabed information is increasingly important for conservation planning and for understanding processes associated with extreme beach plastic accumulation. This study presents a rapid, low-logistics workflow for shallow coastal seabed mapping based on a multispectral uncrewed aerial system (UAS) survey. The acquisition covered ~10 ha, from the beachface to ~10 m water depth (120 m flight altitude; ~6 cm ground sampling distance; 15 shore-parallel flight lines; ~85% overlap; 1,758 images) over a representative segment of the northwestern coast and was complemented by four towed-camera transects and legacy BlueROV imagery to support interpretation. Two independent depth-proxy estimators were derived from the same dataset: (i) a geometric surface generated via Structure-from-Motion/Multi-View Stereo (SfM/MVS) reconstruction and (ii) a spectrally derived relative-depth surface based on spectrally derived bathymetry (SDB) using visible-band ratios. The SfM and SDB surfaces were empirically matched by dense sampling, showing the strongest agreement at intermediate depths and enabling a depth-dependent weighting scheme to fuse both estimators into a single bathymetry/geomorphology proxy digital elevation model (DEM). The fused surface was stacked with the multispectral bands and classified using a supervised support vector machine to map six geomorphology/habitat classes, interpreted using towed-camera and ROV observations. Although the fused DEM is not a calibrated bathymetry model in the absence of acoustic soundings, it provides a robust and replicable morphology proxy suitable for rapid geomorphological segmentation, habitat reconnaissance, and prioritization of sampling targets (e.g., potential depocenters) in remote, clear-water coastal settings.