Multi-Temporal UAV Data and Object-Based Image Analysis (OBIA) for Estimation of Substrate Changes in a Post-Bleaching Scenario on a Maldivian Reef

Abstract

<jats:p>Coral reefs are declining worldwide as a result of the effects of multiple natural and anthropogenic stressors, including regional-scale temperature-induced coral bleaching. Such events have caused significant coral mortality, leading to an evident structural collapse of reefs and shifts in associated benthic communities. In this scenario, reasonable mapping techniques and best practices are critical to improving data collection to describe spatial and temporal patterns of coral reefs after a significant bleaching impact. Our study employed the potential of a consumer-grade drone, coupled with structure from motion and object-based image analysis to investigate for the first time a tool to monitor changes in substrate composition and the associated deterioration in reef environments in a Maldivian shallow-water coral reef. Three key substrate types (hard coral, coral rubble and sand) were detected with high accuracy on high-resolution orthomosaics collected from four sub-areas. Multi-temporal acquisition of UAV data allowed us to compare the classified maps over time (February 2017, November 2018) and obtain evidence of the relevant deterioration in structural complexity of flat reef environments that occurred after the 2016 mass bleaching event. We believe that our proposed methodology offers a cost-effective procedure that is well suited to generate maps for the long-term monitoring of changes in substrate type and reef complexity in shallow water.</jats:p>

Spatial Structure, Short-temporal Variability, and Dynamical Features of Small River Plumes as Observed by Aerial Drones: Case Study of the Kodor and Bzyp River Plumes

“…Airborne remote sensing of sea surface is constantly expanding during the last ten years due to significant progress in development of aerial drones, especially low-cost quadcopters [1][2][3][4][5][6]. Many previous works used airborne data to study various marine processes including mapping of coastal topography [7,8] and bathymetry [9][10][11][12], surveying of marine flora and fauna [13][14][15][16][17][18][19][20][21][22], and monitoring of water quality and anthropogenic pollution [23][24][25][26][27][28]. Several works used airborne data to study physical properties of sea surface layer including estimation of turbulence [29] and reconstruction of surface currents [30,31].…”

Mapping and evaluating kinematics and the stress and strain field at active faults and fissures: a comparison between field and drone data at the NE rift, Mt Etna (Italy)

“…The 144 opening directions measured along the extension fractures on the images obtained by drone surveys and SfM, indicate a clear homogenous dominant extension vector trending N105.7 • . This vector is perpendicular to the largest slope of the area that is represented by the scarp of the PF and coincides with the direction of the maximum slope gradient of this part of Mt Etna (Favalli et al, 1999). As a consequence, we retain that the opening vector of the NE rift is strongly influenced by gravity effects linked to the shape of the Mt Etna edifice.…”

Mapping and evaluating kinematics and stress/strain field at active faults and fissures: a comparison between field and drone data at NE Rift, Mt Etna (Italy)

“…ratio of 85% and 80 %, along the flight path and in lateral direction, respectively (Gerloni et al, 2018;Antoniou et. al., 2019;Bonali et al, 2019Bonali et al, , 2020Fallati et al, 2020). The constant speed velocity was set up by the app considering all above settings, and pictures have been captured using equal time interval modality.…”