Full-Waveform LiDAR Pixel Analysis for Low-Growing Vegetation Mapping of Coastal Foredunes in Western France

Patrick Launeau,Marc Robin,Erwan Le Menn,Saïd Moussaoui,Manuel Giraud,Françoise Debaine,Dimitri Lague,Antoine Ba

doi:10.3390/rs10050669

Abstract

The monitoring of coastal sand dunes requires regular high-resolution aerial photography along hundreds of kilometers of coastal strips. Light detection and ranging (LiDAR) is now the most widely used method for detailed topographic and vegetation studies. The aim of this work is to show how the full-waveform shapes returned from single or multiple targets can carry information relating to low-vegetation cover and ground roughness of dunes. This work focuses on marram grass, widely involved in the development of mobile dunes. Low-growing plants often exhibit identical pigmentary composition and can only be distinguished by the height of their foliage, which modifies the shape of the LiDAR waveform around the main returns at the top of the foliage. We show that ray tracing of full LiDAR waveforms on the regular grid of pixels of hyperspectral images, acquired synchronously, can resolve the confusion between low-vegetation gradients and bare sand by analyzing the waveform damping induced by cumulating microdiffusion on foliage height, but also with glint effects on the surface roughness of compact materials. Analysis of successive shorelines of wet to dry sand, sand to pioneer couch grass, and couch grass to consolidating marram grass can thereby be conducted routinely.

Highlights

The French coastal dunes are classified as a natural habitat by the European Community (Council directive 92/43/EEC of 21 May 1992)
Instead of mapping the vegetation with full waveform (FWF) Gaussian decomposition [12] retrieving echoes, we propose an alternative method by transforming digitized FWF into 400 band FWF ray tracing pixels (FWFp) in images to be processed without echo segmentation
FWFp samples corresponding to 50–300 pixels identified as the regions of interest (ROI) were collected in two areas of interest (AOIs, Figure 1e,d)

Summary

Introduction

The French coastal dunes are classified as a natural habitat by the European Community (Council directive 92/43/EEC of 21 May 1992). The foredune is partially covered with marram grass (Ammophila arenaria), which is described as “foredune engineers” [5,6], has a height between 0.3 to 0.5 m, and often obscures the underlying topography [7]. Such vegetation has a strong impact on the dune morphology and dynamics, because it influences sediment transport by trapping dune sand [8]. Classical LiDAR survey records discrete echoes in real-time but may not be able to distinguish targets that are too close to each other. Discrete echo classification of aboveground vegetation of coastal dune cover is usually classified as low (0–0.3 m), medium (0.3–2 m), and high (>2 m) [11] (Doyle, 2017)

Objectives

Methods

Results

Conclusion