DRELIO: An Unmanned Helicopter for Imaging Coastal Areas

Very high spatial resolution remote sensing images and Digital Elevation Models (DEM) are widely used in coastal management applications. For example, they are used for the quantification of morphosedimentary changes of the coastal fringe, including cross-shore and longshore sediment transport. They are also used as input in hydrodynamics numerical modelling. Spatial resolution, precision and accuracy are critical parameters of the DEM. Presently, most of DEM are built using aerial or satellite images with a spatial resolution coarser than 50cm is not accurate enough for most of applications. An unmanned photogrammetric helicopter (DRELIO) has been developed. It is equipped with an autopilot system. After loading the flight plan, no ground communications are needed from take off to landing. The fly altitude can reach 100m above the ground. DRELIO can operate in windy conditions up to 50km/h and it is able to make stationary flights. A reflex camera with high quality interchangeable optics is onboard. Depending on the focal length and flying altitude, the resolution of the images varies from 1 to 5cm with a ground coverage of 50 by 75m up to 250 by 375m. Due to specific flight conditions and image acquisitions, a photogrammetric toolbox has been developed. Using stereoscopic images and GPS positioning of reference points on the images, it allows building DEM and an orthorectified image with a spatial resolution better than 5cm. In this study, we present an example of an acquisition realzed on the beach of Porsmillin (French Brittany) and we discuss the precision and accuracy obtained by this method. The DRELIO system, which produces DEM concurrent to LIDARs, appears now to be more flexible and efficient than UAV (Unmanned Aerial Vehicle) helicopters equipped with electric engines, UAV planes and less expensive than LIDAR.