The first PLP was conducted in 2018 (PLP2018) as an innovative exploratory application of open-access satellite imagery and unmanned aerial systems (UAS) data for the remote detection of floating marine plastics in natural waters. The PLP2018 was designed as a test project to detect artificial plastic targets on the sea surface using satellite and UAS imaging technology. Three large artificial 10×10 m plastic targets were designed and constructed, matching the Sentinel-2 RGB and NIR bands spatial resolution.
Our controlled experiment was conducted on 06 and 07 June 2018 close to Tsamakia beach of Mytilene on Lesvos Island, Greece. A trial run based on our plan of action was completed on 06 June 2018 followed by the fine-tuned execution of the experiment on 07 June 2018. The controlled targets used in PLP18 were (i) plastic bottles, (ii) plastic bags and (iii) plastic fishing net. Each one of these targets was held together by a 10 x 10 m frame, matching the Sentinel-2 spatial resolution. PVC pipes with a 2-inch diameter were used to construct the frame and plastic net with 1.5 cm mesh size was attached to create the base of the frame. The three targets were positioned at least 30 m away from the coastline to mitigate adjacency and mixed pixel detection from spaceborne data. The full deployment of all the three targets was materialized with the assistance of about 30 students, divers and inflatable boats.
A total of 3600 polyethylene terephthalate (PET-1) 1.5 L bottles were arranged into 120 strips of 30 bottles each to construct the first target. Fishing line was used to attach the bottles in line and a wooden stop every 10 bottles was tied to provide bottle stability. Labels on the bottles were not removed, but the uncovered parts were transparent. The closed bottles were empty, not filled with any fluid, but some little amount of seawater trickled in through the holes that were used for fishing line during the experiment. The second target was made up of 138 large blue coloured low-density polyethylene (LDPE) bags. The bags were attached to the PVC frame using a thin fishing line and they covered the total area of the frame. The frame made it possible to keep the plastics afloat although some portions were slightly submerged. The third target was constructed using 200 m2 of abandoned yellowish nylon fishing nets. We considered this net as a representative example of a ghost net. The fishing net was attached to a white plastic net using fishing line, but without being able to cover all the white background. Furthermore, the optical density of the fishing net was not homogenous providing a near representation of a slightly submerged ghost net. During the experiment, we assumed the contribution by the background material of the net was negligible for analyses conducted in this study.
Very high geospatial resolution orthophotos were generated from the aerial survey conducted on 07 June 2018 using multiple sensors integrated on UAV. To quantitatively asses the targets we looked at the spectra after deriving percentage pixel coverages. Each target ended up covering 4 adjacent pixels after geometric and resampling corrections using a high resolution optical camera as master input.
At least 3 drones will be used for mapping containing, RGB, musti-spectral and thermal cameras. Dates will be selected according to weather status and Sentinel-1 / Sentinel-2 satellites acquisition times.
More on the PLP2018 experiment is described on:
Topouzelis, K., Papakonstantinou, A., Garaba, S.P. (2019). Detection of floating plastics from satellite and unmanned aerial systems ( Plastic Litter Project 2018 ). Int J Appl Earth Obs Geoinf. 79, 175–183. https://doi.org/10.1016/j.jag.2019.03.011
Marine Remote Sensing Group (MRSG)
Department of Marine Sciences
University of the Aegean
81100, Mytilene, Greece