A drone’s eye view: a preliminary assessment of the efficiency of drones in mapping shallow-water benthic assemblages

Abstract

This study represents a preliminary assessment of the efficiency of consumerlevel drones to survey shallow-water benthic cover (0-5m depth). We hypothesised that the use of a drone to map benthic assemblages would reduce the duration, cost, and manpower requirements, while increasing accuracy, relative to manual survey techniques. A OJI Mavic 2 Pro drone was used to survey four bays in Malta by obtaining a highaltitude photo for each bay. This was then processed via k-means clustering to generate a pseudocolour image (PCI). The value of k corresponded to the number of benthic cover classes (BCCs ), which was determined upon inspection of the original aerial image. Since k was dependent on the respective benthic complexity of each bay, the k value varied from site to site. Each site was also mapped using manual survey techniques to enable comparison of the relative representation of BCCs between manual and drone-based methods. Data from manual surveys were obtained from transects spaced ca.10 m apart, where the number of transects taken was dependent on the size of the respective site. The correspondence between the two survey methods was determined using Principal Component Analysis (PCA) on the BCC relative cover of each site. Results obtained indicated a statistically significant positive correlation between the relative cover ofBCCs in maps produced through drone and manual surveys (r=0.845, p<0.0001). The relative efficiency of the two survey methods was assessed by comparing the area surveyed per man hour (m2h-1), where the automated drone survey method was significantly more efficient in all four sites. The drone survey was also more accurate than the manual survey, in that it mapped the entire area without the need for any interpolation between transects. This suggests that while manual surveys are a good approximation of the field situation, PCis are capable of analysing benthic cover to give results of superior accuracy and coverage, but in a much shorter time, and without bias. The only real limitation with regards to using drones for mapping purposes is the weather, since the drone cannot be flown in rainy conditions, and waves caused by strong winds obscure the benthos. The time of day at which the drone is flown is also a factor due to the sun's glare on the water's surface, which also obscures the benthos beneath. In addition, aerial imagery can only be used for mapping of benthic assemblages in very shallow waters and requires high water transparency.