UAV LiDAR for below-canopy forest surveys

Remote sensing tools are increasingly being used to survey forest structure. Most current methods rely on GPS signals, which are available in above-canopy surveys or in below-canopy surveys of open forests, but may be absent in below-canopy environments of dense forests. We trialled a technology that facilitates mobile surveys in GPS-denied below-canopy forest environments. The platform consists of a battery-powered UAV mounted with a LiDAR. It lacks a GPS or any other localisation device. The vehicle is capable of an 8 min flight duration and autonomous operation but was remotely piloted in the pre- sent study. We flew the UAV around a 20 m × 20 m patch of roadside trees and developed postprocessing software to estimate the diameter-at-breast-height (DBH) of 12 trees that were detected by the LiDAR. The method detected 73% of trees greater than 200 mm DBH within 3 m of the flight path. Smaller and more distant trees could not be detected reliably. The UAV-based DBH estimates of detected trees were positively correlated with the human- based estimates (R 2 = 0.45, p = 0.017) with a median absolute error of 18.1%, a root-mean- square error of 25.1% and a bias of −1.2%. We summarise the main current limitations of this technology and outline potential solutions. The greatest gains in precision could be achieved through use of a localisation device. The long-term factor limiting the deployment of below-canopy UAV surveys is likely to be battery technology. Resume : Les outils de teledetection sont de plus en plus utilises pour analyser la structure forestiere. La plupart des methodes courantes se servent de signaux GPS qui sont percepti- bles au-dessus du couvert forestier ou en dessous du couvert forestier dans les forets ouvertes, mais qui ne sont pas captables en dessous du couvert forestier lorsque la foret est dense. Nous avons mis a l'essai une technique qui facilite la recherche mobile en dessous du couvert forestier ou les signaux GPS ne sont pas captables. Notre plateforme consiste en un vehicule aerien sans pilote (UAV), alimente par piles et muni d'un LiDAR. Il n'est equipe ni de GPS ni d'autre dispositif de localisation. Le vehicule a une capacite de vol et d'opera- tion autonome d'une duree de huit (8) minutes, bien qu'il ait ete teleguide aux fins de la pre- sente etude. Nous avons fait voler l'UAV au-dessus d'un terrain de 20 m × 20 m dote d'arbres d'alignement et avons elabore un logiciel post-traitement afin d'evaluer le diametre a hau- teur d'homme (DHH) de 12 arbres detectes par le LiDAR. Cette methode nous a permis de detecter 73 % des arbres dont le DHH etait plus grand que 200 mm dans un rayon de 3 m de la trajectoire de vol. Les arbres plus petits et plus eloignes n'ont pu etre detectes efficace- ment. Les estimations du DHH des arbres detectes a partir du UAV ont ete positivement cor- relees avec les estimations effectuees par les humains (R 2 = 0.45, p = 0.017), prenant en

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