Coastal sand dunes monitoring using synchronized hyperspectral and full-waveform LiDAR remote sensing

Abstract:

Due to the coastal morphodynamic impacted by climate changes (Luijendijk et al., 2018 ; Juigner et al., 2012) there is a need for a systematic and large scale monitoring (Douglas and Crowell, 2000 ; Deboudt, 2010 ; Kerguillec et al.,2019). The monitoring of sandy dunes in Pays-de-la-Loire (France) requires a simultaneous mapping of i) its morphology allowing an evaluation of the sedimentary stocks and ii) its low vegetation cover constituting a significant proxy of the dune dynamic.

The synchronization of the hyperspectral imaging (HSI) with the full-waveform (FWF) LiDAR is possible with an airborne plateform (Asner et al., 2007). For more intimate combination, we aligned the 1064 nm laser beam of a bi-spectral Titan FWF LiDAR with 401 bands and 15 cm range resolution on the Hyspex VNIR camera with 160 bands and 4.2 nm spectral resolution, making both type of data following the same emergence angle. A ray tracing procedure allows to associate both type of data while conserving the acquisition angles. The stacking of multiple shifted FWF associated to the same pixel allows to reach a 5 cm range resolution grid. The objectives are i) the accuracy improvement of the Digital Terrain Models (DTM) obtained from FWF analysis by calibrating it on dGPS field measurements and correcting it from local deviations induced by the vegetation and ii) in combination with airborne reflectance obtained with PARGE and ATCOR-4 corrections, the implementation of a supervised hierarchic classification of the main foredune vegetation proxies whatever the acquisition the year and the physiological state. The normalization of FWF LiDAR range segments to dry sand reference waveform and the centering on their top canopy echoes allows to isolate Ammophilia arenaria from other vegetation types by two FWF indices, without confusion with slope effects (Mallet, 2009). 14 HSI reflectance indices, 19 HSI spectral angle mapping (SAM of Kruse et al. 1993) indices base on 2017 spectral field measurement performed with the same Hyspex VNIR camera were stacked with both FWF indices into a single co-image for each acquisition year. A simple straightforward hierarchical classification of all 35 pre-classified co-image bands was successfully applied along 20 km of coastline from 2017 to 2019 prefiguring the systematic larger study up to 250 km every year.