Easily Implemented Methods of Radiometric Corrections for Hyperspectral–UAV—Application to Guianese Equatorial Mudbanks Colonized by Pioneer Mangroves

Marion Jaud,Christophe Delacourt,Gérard Thouzeau,Guillaume Brunier,Guillaume Sicot,Philippe Grandjean,Patrick Launeau,Emma Michaud,Nicolas Le Dantec,Jérôme Ammann,Jules Fleury

doi:10.3390/rs13234792

Abstract

Hyper-DRELIO (Hyperspectral DRone for Environmental and LIttoral Observations) is a custom, mini-UAV (unmanned aerial vehicle) platform (<20 kg), equipped with a light push broom hyperspectral sensor combined with a navigation module measuring position and orientation. Because of the particularities of UAV surveys (low flight altitude, small spatial scale, and high resolution), dedicated pre-processing methods have to be developed when reconstructing hyperspectral imagery. This article presents light, easy-implementation, in situ methods, using only two Spectralon® and a field spectrometer, allowing performance of an initial calibration of the sensor in order to correct “vignetting effects” and a field standardization to convert digital numbers (DN) collected by the hyperspectral camera to reflectance, taking into account the time-varying illumination conditions. Radiometric corrections are applied to a subset of a dataset collected above mudflats colonized by pioneer mangroves in French Guiana. The efficiency of the radiometric corrections is assessed by comparing spectra from Hyper-DRELIO imagery to in situ spectrometer measurements above the intertidal benthic biofilm and mangroves. The shapes of the spectra were consistent, and the spectral angle mapper (SAM) distance was 0.039 above the benthic biofilm and 0.159 above the mangroves. These preliminary results provide new perspectives for quantifying and mapping the benthic biofilm and mangroves at the scale of the Guianese intertidal mudbanks system, given their importance in the coastal food webs, biogeochemical cycles, and the sediment stabilization.

Highlights

Mangroves are highly productive ecosystems, which dominate the intertidal zone of tropical and subtropical coasts
To calculate a and b coefficients, we considered the radiance measured by the field spectrometer above both white and grey Spectralon panels, and the digital numbers (DN) measured simultaneously by the hyperspectral camera
The quality of the results is assessed in two ways: (i) Firstly, by comparing the spectra obtained from Hyper-DRELIO imagery with spectra, measured by the field spectrometer on the same types of substrates or sediment coverage

Summary

Introduction

Mangroves are highly productive ecosystems, which dominate the intertidal zone of tropical and subtropical coasts. Mangroves grow on fine sand to silty sediments in areas protected from high-energy wave action, mainly found in depositional coastal environments, such as the deltaic, lagoon, or mudflat systems These ecosystems are more and more threatened by global changes, which include anthropogenic pressures They include a variety of geomorphological forms, such as the creeks, ridges, runnels, sediment platforms, and depressions—that can be observed from aerial view—and, depending on pixel resolution of the imaging sensors, the spatial coverage and density of the trees and the age of the forests This spatial heterogeneity of habitats increases the complexity in signal processing of remote sensing data, concerning the identification and quantification of the trees and intertidal benthic constituents

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