Assessing the Potential of Remotely-Sensed Drone Spectroscopy to Determine Live Coral Cover on Heron Reef

Abstract

Coral reefs, as biologically diverse ecosystems, hold significant ecological and economic value. With increased threats imposed on them, it is increasingly important to monitor reef health by developing accessible methods to quantify coral cover. Discriminating between substrate types has previously been achieved with in situ spectroscopy but has not been tested using drones. In this study, we test the ability of using point-based drone spectroscopy to determine substrate cover through spectral unmixing on a portion of Heron Reef, Australia. A spectral mixture analysis was conducted to separate the components contributing to spectral signatures obtained across the reef. The pure spectra used to unmix measured data include live coral, algae, sand, and rock, obtained from a public spectral library. These were able to account for over 82% of the spectral mixing captured in each spectroscopy measurement, highlighting the benefits of using a public database. The unmixing results were then compared to a categorical classification on an overlapping mosaicked drone image but yielded inconclusive results due to challenges in co-registration. This study uniquely showcases the potential of using commercial-grade drones and point spectroscopy in mapping complex environments. This can pave the way for future research, by increasing access to repeatable, effective, and affordable technology.

Integrating a UAV-Derived DEM in Object-Based Image Analysis Increases Habitat Classification Accuracy on Coral Reefs

“…By using a PPK-ready Phantom 4 Pro RTK, we were able to map ~300,000 m 2 of shallow marine habitat down to 3.5 m deep. While this represents one of the most extensive marine UAV mapping efforts at this spatial resolution to date [22,24,29,40,48], it also highlights two of the main limitations faced by UAV-based monitoring of the marine environment. Firstly, even though the 300,000 m 2 area we mapped here far exceeds the area that can be surveyed through scuba diving or snorkelling, it is still nowhere near the amount of area that can be surveyed with airborne or satellite imagery [15,18].…”

“…In recent years, UAVs have revolutionised the ability to record the spatial distribution of benthic assemblages in very shallow reef habitats, but the conversion of high-resolution marine UAV imagery into accurate habitat maps is still in its infancy [22][23][24]26,28,29,41]. Here, we presented a new habitat mapping approach that incorporates not only the spectral (the orthomosaic) but also the geomorphometric (the DEM and its derivatives) output of SfM-processed UAV imagery into OBIA.…”

“…Finally, the OBIA framework and Random Forest classifiers are well suited to handle a large collection of independent input data, as is evident in this study, where we combined multiple DEMderived rasters with an orthomosaic. Recent studies have successfully used UAV-derived multispectral images for marine habitat mapping [24,26,27,94]. Besides providing spectral information of a higher spectral resolution, multi-spectral imagery also provides alternative methods to accurately retrieve bathymetry [49,50,82,94].…”

“…Remote Sens. 2022, 14, 5017 2 of 24 As such, there is a great need for accurate and ecologically informative maps of shallow coral reefs.…”

“…Unmanned Aerial Vehicles (UAVs) or 'drones' are also able to capture spatial data at very high spatial resolutions and offer great flexibility in their deployment for only a fraction of the costs, making them an effective surveying tool [15,17,18,21]. Consequently, the use of UAVs to map shallow coral reefs has taken flight in recent years [22][23][24][25][26][27][28][29][30].…”