Wildlife survey; Unmanned Aerial Systems; Aerial inventory; Direct georeferencing
Abstract :
[fr] La protection et la gestion raisonnée des écosystèmes naturels passent par la nécessité de quantifier l'importance des populations animales. Les inventaires de la grande faune, traditionnellement des inventaires aériens par échantillonnage, pourraient être avantageusement remplacés par des inventaires utilisant de petits avions sans pilote (avions ou hélicoptères téléguidés avec appareil photo embarqué). Bien que l'utilisation de drones comme outils d'inventaire des grands mammifères ait déjà été mis en œuvre par le passé, certaines questions persistent, notamment la manière la plus adéquate de déterminer la surface de la bande inventoriée. La surface inventoriée est en effet une information capitale pour le calcul de la densité d'animaux ainsi que de leurs effectif total. La surface de la bande photographiée peut se calculer selon différentes approches en fonction des informations et outils utilisés pour le géoréférencement du bloc d'images. Deux méthodes utilisant les données de navigation du drone (GPS et attitude) sont comparées dans cet article, l'une utilisant les équations de colinéarité afin de projeter individuellement l'emprise (fauchée) de chaque image sur le sol et l'autre utilisant des outils de vision par ordinateur et de photogrammétrie afin de déterminer l'orientation du bloc d'images. Les surfaces estimées selon ces méthodes furent comparées à une surface de référence, calculée au moyen du géoréférencement du bloc d'image avec des points d'appuis. La méthode de projection des fauchées, bien que plus simple à mettre en œuvre, s'avère à la fois la plus rapide et la plus précise en terrain à faible dénivelé et répond correctement aux attentes relatives aux inventaires faunique [en] Conservation of natural ecosystems requires regular monitoring of biodiversity, including the estimation of wildlife density. Recently, unmanned aerial systems (UAS) have become more available for numerous civilian applications. The use of small drones for wildlife surveys as a surrogate for manned aerial surveys is becoming increasingly attractive and has already been implemented with some success. This raises the question of how to process UAS imagery in order to determine the surface area of sampling strips within an acceptable confidence
level. For the purpose of wildlife surveys, the estimation of sampling strip surface area needs to be both accurate and quick, and easy to implement. As GPS and an inertial measurement units are commonly integrated within unmanned aircraft platforms, two methods of direct georeferencing were compared here. On the one hand, we used the image footprint projection (IFP) method, which utilizes collinearity equations on each image individually. On the other hand, the Structure from Motion (SfM) technique was used for block orientation and georeferencing. These two methods were compared on eight sampling strips. An absolute orientation of the strip was determined by indirect georeferencing using ground control points. This absolute orientation was considered as the reference and was used for validating the other two methods. The IFP method was demonstrated to be the most accurate and the easiest to implement. It was also found to be less demanding in terms of image quality and overlap. However, even though a flat landscape is the type most widely encountered in wildlife surveys in Africa, we recommend estimating IFP sensitivity at an accentuation of the relief.
Disciplines :
Life sciences: Multidisciplinary, general & others
Author, co-author :
Lisein, Jonathan ; Université de Liège - ULiège > Forêts, Nature et Paysage > Gestion des ressources forestières et des milieux naturels
Linchant, Julie ; Université de Liège - ULiège > Forêts, Nature et Paysage > Laboratoire de Foresterie des régions trop. et subtropicales
Lejeune, Philippe ; Université de Liège - ULiège > Forêts, Nature et Paysage > Gestion des ressources forestières et des milieux naturels
Bouché, Philippe
Vermeulen, Cédric ; Université de Liège - ULiège > Forêts, Nature et Paysage > Laboratoire de Foresterie des régions trop. et subtropicales
Language :
English
Title :
Aerial surveys using an Unmanned Aerial System (UAS): comparison of different methods for estimating the surface area of sampling strips
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