SfM-Based Method to Assess Gorgonian Forests (Paramuricea clavata (Cnidaria, Octocorallia))

Abstract

Animal forests promote marine habitats morphological complexity and functioning. The red gorgonian, Paramuricea clavata, is a key structuring species of the Mediterranean coralligenous habitat and an indicator species of climate effects on habitat functioning. P. clavata metrics such as population structure, morphology and biomass inform on the overall health of coralligenous habitats, but the estimation of these metrics is time and cost consuming, and often requires destructive sampling. As a consequence, the implementation of long-term and wide-area monitoring programmes is limited. This study proposes a novel and transferable Structure from Motion (SfM) based method for the estimation of gorgonian population structure (i.e., maximal height, density, abundance), morphometries (i.e., maximal width, fan surface) and biomass (i.e., coenenchymal Dry Weight, Ash Free Dried Weight). The method includes the estimation of a novel metric (3D canopy surface) describing the gorgonian forest as a mosaic of planes generated by fitting multiple 5 cm × 5 cm facets to a SfM generated point cloud. The performance of the method is assessed for two different cameras (GoPro Hero4 and Sony NEX7). Results showed that for highly dense populations (17 colonies/m2), the SfM-method had lower accuracies in estimating the gorgonians density for both cameras (60% to 89%) than for medium to low density populations (14 and 7 colonies/m2) (71% to 100%). Results for the validation of the method showed that the correlation between ground truth and SfM estimates for maximal height, maximal width and fan surface were between R2 = 0.63 and R2 = 0.9, and R2 = 0.99 for coenenchymal surface estimation. The methodological approach was used to estimate the biomass of the gorgonian population within the study area and across the coralligenous habitat between −25 to −40 m depth in the Portofino Marine Protected Area. For that purpose, the coenenchymal surface of sampled colonies was obtained and used for the calculations. Results showed biomass values of dry weight and ash free dry weight of 220 g and 32 g for the studied area and to 365 kg and 55 Kg for the coralligenous habitat in the Marine Protected Area. This study highlighted the feasibility of the methodology for the quantification of P. clavata metrics as well as the potential of the SfM-method to improve current predictions of the status of the coralligenous habitat in the Mediterranean sea and overall management of threatened ecosystems.

Reef Rover: A Low-Cost Small Autonomous Unmanned Surface Vehicle (USV) for Mapping and Monitoring Coral Reefs

“…Upon inspection of the images acquired using both action cameras, we noticed evidence of the rolling shutter effect, which was likely the reason these data did not provide an acceptable SfM model. Although others have reported success using action cameras [10,14,21], this is likely due to the ability of a diver to hold a position and acquire a more deliberate still image. We did not attempt to stabilize the image through mechanical techniques (i.e., powered gimbal) as it would have added significantly to the cost and, given the budget restraints, was not practical in the underwater setting in which we were working in.…”

“…This technique involves capturing stereo photos acquired by scuba, snorkelers, or remotely operated vehicles (ROVs) that are processed through photogrammetry software to create high resolution orthophoto mosaics and digital surface models [12][13][14][15][16]. When geometrically corrected, these products can be used to detect centimeter changes in volume and surface area [17][18][19][20][21]. Scientists have used SfM models to accurately estimate a variety of ecological indicators such as linear rugosity, surface complexity, slope, and fractal dimension [22][23][24].…”

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

“…PixGPS (BR Software, Asker, Norway) was used to tag the recorded images with the GPS positioning and the rotation data. The coordinates for each of the frames were used to georeference (scale, translate and rotate) the imagery into the coordinate system defined by the World Geodetic System (WGS84) and to minimize geometric distortions [48]. The Wide Area Augmentation System (WS) mode at 1 Hz frequency was used to record the position of the buoy.…”

Reconstructing 3D virtual environments within a collaborative e‐infrastructure

“…Photogrammetry now finds applications in a wide range of domains, including cinematography and entertainment, cultural heritage, and many branches of science . For example, it is used to recreate movie locations in videogames, to reconstruct areas filmed by drones, and it can be combined with smart software to guide users toward the best poses to achieve optimal reconstructions .…”

“…Photogrammetry now finds applications in a wide range of domains, including cinematography and entertainment, [2][3][4] cultural heritage, [5][6][7][8] and many branches of science. [9][10][11][12][13][14][15] For example, it is used to recreate movie locations in videogames, 2 to reconstruct areas filmed by drones, 16 and it can be combined with smart software to guide users toward the best poses to achieve optimal reconstructions. 17 However, most applications FIGURE 1 Representation of our workflow with the major eight operations highlighted require specialised and expensive hardware (eg, graphic cards, multi-core processors, etc) and software.…”