Coral Reef Mapping of UAV: A Comparison of Sun Glint Correction Methods

Abstract

Although methods were proposed for eliminating sun glint effects from airborne and satellite images over coral reef environments, a method was not proposed previously for unmanned aerial vehicle (UAV) image data. De-glinting in UAV image analysis may improve coral distribution mapping accuracy result compared with an uncorrected image classification technique. The objective of this research was to determine accuracy of coral reef habitat classification maps based on glint correction methods proposed by Lyzenga et al., Joyce, Hedley et al., and Goodman et al.. The UAV imagery collected from the coral-dominated Pulau Bidong (Peninsular Malaysia) on 20 April 2016 was analyzed in this study. Images were pre-processed with the following two strategies: Strategy-1 was the glint removal technique applied to the whole image, while Strategy-2 used only the regions impacted by glint instead of the whole image. Accuracy measures for the glint corrected images showed that the method proposed by Lyzenga et al. following Strategy-2 could eliminate glints over the branching coral—Acropora (BC), tabulate coral—Acropora + Montipora (TC), patch coral (PC), coral rubble (R), and sand (S) with greater accuracy than the other four methods using Strategy-1. Tested in two different coral environments (Site-1: Pantai Pasir Cina and Site-2: Pantai Vietnam), the glint-removed UAV imagery produced reliable maps of coral habitat distribution with finer details. The proposed strategies can potentially be used to remove glint from UAV imagery and may improve usability of glint-affected imagery, for analyzing spatiotemporal changes of coral habitats from multi-temporal UAV imagery.

Structure-from-Motion on shallow reefs and beaches: potential and limitations of consumer-grade drones to reconstruct topography and bathymetry

“…• Reconstructing underwater scenes suffers from light effects such as sun glint or reflections on the plain water surface as well as caustics, which are brighter areas from wave-induced fluid lenslets focussing sunlight on the seafloor (Casella et al 2017;Dietrich 2017;Overstreet and Legleiter 2017;Chirayath and Li 2019;Muslim et al 2019). If the water surface changes while recording, the incident and reflection angle change in Snell's Law.…”

“…Sun glint is another error source in aerial images, impairing the three-dimensional reconstruction of the scene (Overstreet and Legleiter 2017). A polar filter on the camera lens can reduce the effect of sun glint on water surfaces and several techniques for high-resolution UAV imagery exist, using near-infrared wavelengths (700-1000 nm) to account for this effect (Muslim et al 2019). However, recording information from these wavelengths requires special sensors, usually not being deployed on user-grade UAVs.…”

“…Through-water photogrammetry depends on calm water surfaces, as surface undulations further disturb the SfM-MVS process beyond the aforementioned effects. Further adverse effects from surface undulations include unwanted reflection of sunlight on the water surface-also known as sun glare- (Overstreet and Legleiter 2017;Muslim et al 2019) and light effects on the seafloor. These light effects are caused by wave crests, which act as fluid lenslet.…”

Seamless Mosaic Using Removal Sunglint to Support a Large-Scale Mapping Acceleration

“…The result of the mosaic without removing the sunglint will show that the land is darker on the data that contains the sunglint, this is because the water surface that contains the sunglint is very bright, which when the mosaic is done, the color balancing process will be carried out so that the land is sacrificed so that it appears darker. Unlike the case with the deglint process which is mostly done in research on objects under the surface of the water, where underwater objects that will indeed be preserved and sacrificed are land objects as in many studies that have been done (Bernardo et al, 2018;Chami et al, 2020;Martin et al, 2016;Muslim et al, 2019;Traganos et al, 2018). The result of the mosaic by removing the sunglint will produce a seamless mosaic, this is because the water surface that contains the sunglint is removed so that when the mosaic is done, the color balancing process can be done well.…”

Living up to the Hype of Hyperspectral Aquatic Remote Sensing: Science, Resources and Outlook

“…Researchers also used a drone to track and quantify an intense phytoplankton bloom in Weitou Bay in the western Taiwan Strait (Shang et al, 2017). Similarly, several studies have used RGB imagery from drones to map intertidal coral reefs (Murfitt et al, 2017;Muslim et al, 2019). Application of hyperspectral UAS for aquatic environments, including water quality (Zeng et al, 2017), HABs (Becker et al, 2019), shallow water benthic mapping (Parsons et al, 2018), and marine fauna surveys (Colefax et al, 2018).…”