A drone-based method for mapping the coral reefs in the shallow coastal waters – case study: Kish Island, Persian Gulf

Abstract

The present study aimed to examine the capabilities of a low-cost and standard drone (namely, the DJI™ Phantom 4 Pro) for mapping of coral reefs in shallow coastal waters. To this end, a coral site in Kish Island, located in the northern Persian Gulf, was selected as the study area, wherein other methods had been previously practiced to generate maps from the corals. The drone flight was operated by the Pix4Dcapture® mobile application, due to its simplicity and high capabilities for performing an automated flight based on user’s settings. The flight altitude was set at 50 m, covering the ~6 ha (200 × 300 m) area in ~5.5 min. Prior to the drone flight, 11 diving buoys were positioned as ground control points (GCPs) on the study area and their coordinates were subsequently measured three times via a handheld GPS. Afterwards, the Agisoft™ Metashape software was utilized to create the orthophoto mosaic from the total number of 121 overlapped taken photos. The spatial resolution of the final orthophoto mosaic was by ~2 cm, contributing to the identification of the features with the minimum size of 20 cm. Although the produced mosaic was geo-registered, it was geo-referenced once again based on the above-mentioned GCPs in the ENVI™ 5.3 software to increase planimetric accuracy. Thereafter, the corrected mosaic was digitized by on-screen digitizing in the Autodesk® AutoCAD Map software, so that an identifier (ID) was assigned to each polygon with reference to the types of corals or other substrate features such as the rocks. Consequently, a topological map was generated in the format of the ESRI™ shapefile. The results demonstrated that the proposed technique was capable to differentiate the types of corals (including bleached ones) and other substrate features. In comparison with other alternative methods, the given technique cost lower than field observations and its results were much more accurate compared with those from satellite imagery. The capabilities of the proposed technique may be significantly improved if drones equipped with a multispectral camera were used, but then again, the costs will be increasingly higher.