Coastal Sand Dunes Monitoring by Low Vegetation Cover Classification and Digital Elevation Model Improvement Using Synchronized Hyperspectral and Full-Waveform LiDAR Remote Sensing

Giovanni Frati,Patrick Launeau,Manuel Giraud,Françoise Debaine,Martin Juigner,Cyril Michon,Marc Robin

doi:10.3390/rs13010029

Abstract

Due to the coastal morphodynamic being impacted by climate change there is a need for systematic and large-scale monitoring. The monitoring of sandy dunes in Pays-de-la-Loire (France) requires a simultaneous mapping of (i) its morphology, allowing to assess the sedimentary stocks and (ii) its low vegetation cover, which constitutes a significant proxy of the dune dynamics. The synchronization of hyperspectral imaging (HSI) with full-waveform (FWF) LiDAR is possible with an airborne platform. For a more intimate combination, we aligned the 1064 nm laser beam of a bi-spectral Titan FWF LiDAR with 401 bands and the 15 cm range resolution on the Hyspex VNIR camera with 160 bands and a 4.2 nm spectral resolution, making both types of data follow the same emergence angle. A ray tracing procedure permits to associate the data while keeping the acquisition angles. Stacking multiple shifted FWFs, which are linked to the same pixel, enables reaching a 5 cm range resolution grid. The objectives are (i) to improve the accuracy of the digital terrain models (DTM) obtained from an FWF analysis by calibrating it on dGPS field measurements and correcting it from local deviations induced by vegetation and (ii) in combination with airborne reflectances obtained with PARGE and ATCOR-4 corrections, to implement a supervised hierarchic classification of the main foredune vegetation proxies independently of the acquisition year and the physiological state. The normalization of the FWF LiDAR range to a dry sand reference waveform and the centering on their top canopy echoes allows to isolate Ammophilia arenaria from other vegetation types using two FWF indices, without confusion with slope effects. Fourteen HSI reflectance indices and 19 HSI Spectral Angle Mapping (SAM) indices based on 2017 spectral field measurements 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, out of the 250 km of coastline acquired from 2017 to 2019, prefiguring its systematic application to the whole 250 km every year.

Highlights

The coastline is an interface between sea and land [1]
The comparison between the 2019 and 2020 white dune crest differential GPS (dGPS) field samplings shows a 0.01 m mean deviation and a 0.028 m RMSE, showing that the dune is stable enough to be taken as a calibration reference for the 2017–2019 data
The first echoes of the d3NCFWF of Figures 9a are more scattered than the pixels of the discrete digital terrain models (DTM) in Figure 9b but both show the same trend of elevation underestimation compensated in marram grass areas

Summary

Introduction

Coastal habitats shield the hinterland from marine hazards [2,3,4]. Sandy coasts are constantly evolving [5] under the Remote Sens. Climate change increasingly and directly affects coastal morphology [13,14,15,16]. (2018) [17], around 24% of the world’s sandy beaches are eroding (28% are in accretion and 48% are stable). In France, as in many other places, the concentration of people and activities in coastal zones is increasing [18,19,20,21]. The perspective of higher sea levels due to climate change increases the risk that these territories will be exposed to natural coastal hazards [22]. Systematic and large-scale monitoring is needed in order to implement adapted protection measures [23,24,25]

Objectives

Methods

Results

Discussion

Conclusion