High Resolution Orthomosaics of African Coral Reefs: A Tool for Wide-Scale Benthic Monitoring

Abstract

Abstract:Coral reefs play a key role in coastal protection and habitat provision. They are also well known for their recreational value. Attempts to protect these ecosystems have not successfully stopped large-scale degradation. Significant efforts have been made by government and research organizations to ensure that coral reefs are monitored systematically to gain a deeper understanding of the causes, the effects and the extent of threats affecting coral reefs. However, further research is needed to fully understand the importance that sampling design has on coral reef characterization and assessment. This study examines the effect that sampling design has on the estimation of seascape metrics when coupling semi-autonomous underwater vehicles, structure-from-motion photogrammetry techniques and high resolution (0.4 cm) underwater imagery. For this purpose, we use FRAGSTATS v4 to estimate key seascape metrics that enable quantification of the area, density, edge, shape, contagion, interspersion and diversity of sessile organisms for a range of sampling scales (0.5 m × 0.5 m, 2 m × 2 m, 5 m × 5 m, 7 m × 7 m), quadrat densities (from 1-100 quadrats) and sampling strategies (nested vs. random) within a 1655 m 2 case study area in Ponta do Ouro Partial Marine Reserve (Mozambique). Results show that the benthic community is rather disaggregated within a rocky matrix; the embedded patches frequently have a small size and a regular shape; and the population is highly represented by soft corals. The genus Acropora is the more frequent and shows bigger colonies in the group of hard corals. Each of the seascape metrics has specific requirements of the sampling scale and quadrat density for robust estimation. Overall, the majority of the metrics were accurately identified by sampling scales equal to or coarser than 5 m × 5 m and quadrat densities equal to or larger than 30. The study indicates that special attention needs to be dedicated to the design of coral reef monitoring programmes, with decisions being based on the seascape metrics and statistics being determined. The results presented here are representative of the eastern South Africa coral reefs and are expected to be transferable to coral reefs with similar characteristics. The work presented here is limited to one study site and further research is required to confirm the findings.

UAV-Based 3D Point Clouds of Freshwater Fish Habitats, Xingu River Basin, Brazil

“…The area is spatially explicit and provides a high level of detail from which the habitats can be studied. As shown by [4,5,12,13], in marine environments, and was also found by [1], the complexity metrics calculated from 3D surface reconstructions are more robust than conventional measures. The photographs used to generate these point clouds were collected from low altitude (30 m AGL) UAV flights.…”

“…UAV-based photography and 3D reconstruction of terrestrial environments using Structurefrom-Motion (SfM) photogrammetry have become increasingly common [7][8][9][10]. The concepts have further been applied underwater in marine ecosystems, predominantly to study coral reefs [5,[11][12][13] and deep-sea structures [14]. In a strict sense, our use of the term SfM refers to an analytical workflow that combines both SfM and Multiview Stereo (MVS) photogrammetry, as such, we refer to the methodology as SfM-MVS.…”

Reconstructing 3D virtual environments within a collaborative e‐infrastructure

“…The Green Bubbles European ITN project promotes sustainable development of scuba diving and conservation of underwater habitat through the engagement of scuba divers. This project uses underwater photogrammetry as a flexible and reliable technique for high resolution benthic habitat mapping and aims at developing new products for scuba divers suitable for citizen science programmes . In order to test our 3D reconstruction pipeline, Green Bubbles provided more than 3000 photos taken on different days covering partially overlapping coral reef areas in Indonesia.…”

Quantifying Coral Reef Composition of Recreational Diving Sites: A Structure from Motion Approach at Seascape Scale

“…High-resolution data were collected at each site in one single dive between April and June 2015 covering the entire coral reef area or surveying those areas commonly visited by divers at the larger study sites. Dive sites were mapped as in [23] using a modified Diver Propulsion Vehicle (DPV) equipped with action cameras (GoPro Hero3 Silver-Woodman Labs, Inc., San Mateo, CA, USA), a tablet (Asus Google Nexus 7 tablet-Google Inc., Mountain View, CA, USA), an illumination system and one GPS buoy (Garmin Ltd., Lenexa, KS, USA) ( Figure 2). A detailed description of the vehicle in Palma et al [23].…”