Mapping coral reefs using consumer-grade drones and structure from motion photogrammetry techniques
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We propose a novel technique to measure the small-scale three-dimensional features of a shallow-water coral reef using a small drone equipped with a consumer-grade camera, a handheld GPS and structure from motion (SfM) algorithms. We used a GoPro HERO4 with a modified lens mounted on a DJI Phantom 2 drone (maximum total take-off weight <2 kg) to perform a 10 min flight and collect 306 aerial images with an overlap equal or greater than 90%. We mapped an area of 8380 m2, obtaining as output an ortho-rectified aerial photomosaic and a bathymetric digital elevation model (DEM) with a resolution of 0.78 and 1.56 cm pixel−1, respectively. Through comparison with airborne LiDAR data for the same area, we verified that the location of the ortho-rectified aerial photomosaic is accurate within ~1.4 m. The bathymetric difference between our DEM and the LiDAR dataset is −0.016 ± 0.45 m (1σ). Our results show that it is possible, in conditions of calm waters, low winds and minimal sun glint, to deploy consumer-grade drones as a relatively low-cost and rapid survey technique to produce multispectral and bathymetric data on shallow-water coral reefs. We discuss the utility of such data to monitor temporal changes in topographic complexity of reefs and associated biological processes.
KeywordsDrone mapping Coral reefs Bathymetry from drones Structure from motion underwater Bathymetry from photogrammetry
This research was supported by the Institutional Strategy of the University of Bremen, funded by the German Excellence Initiative (ABPZuK-03/2014) and by ZMT, the Leibniz Center for Tropical Marine Ecology, Bremen. We acknowledge the Centre de Recherches Insulaires et Observatoire de l’Environnement (CRIOBE) for support during the field data collection. The airborne LiDAR measurements and analysis were supported by the US National Science Foundation grants OCE1435133 (Physical Oceanography) as well as OCE1236905 and OCE1637396 (Moorea Coral Reef LTER). We acknowledge useful assessments and corrections from two anonymous reviewers as well as the journal editor.
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