Abstract
This paper presents a multi-temporal underwater photogrammetric survey of a reef patch located in Moorea, French Polynesia, designed to detect a coral growth of 10–15 mm/year. Structure-from-Motion photogrammetry and underwater imagery allows the three-dimensional quantification of reef structural complexity and ecologically relevant characteristics at the patch scale. A high degree of accuracy and fine resolution are required in order to guarantee the repeatability of surveys over time within the same reference system, meaning a proper geodetic network and acquisition scheme are mandatory. Measuring tools and reference points were properly designed in order to constrain the photogrammetric reconstruction. The network adjustment, performed with distance and height difference observations, provided an average accuracy of ± 1.2 mm and ± 2.9 mm in the horizontal and vertical components, respectively. The final accuracies of photogrammetric reconstructions are on the order of 1 cm and few millimeters for the 2017 and 2018 monitoring campaigns, respectively. This results in realized errors in the comparison of about ± 1 cm. Coordinate variations larger than this magnitude can be reasonably interpreted as coral growth or dissolution. The direct comparison of the two subsequent point clouds is effective in order to evaluate trends in growth and perform morphometric analyses. For highly accurate quantitative assessment of local changes, an expert operator can create and analyze specific 2D profiles that are easily produced from the point clouds.
Highlights
Visualizations using underwater robotic vehicles (Kocak and Caimi 2005; Johnson-Roberson et al 2010), and in situ operations based on underwater photogrammetric surveying (Burns et al 2015; Royer et al 2018) have been used to document declining trends in coral reef cover
Field surveys of coral reefs carried out by robotic vehicles (Johnson-Roberson et al 2010) or divers equipped with consumer-grade cameras are allowing the three-dimensional (3D) quantification of ecologically relevant characteristics at the patch scales and have opened the way to obtaining detailed and accurate surveys for the monitoring of coral growth, reef structural complexity (Ferrari et al 2017), and as predictors of reef organismal abundance, biomass, and diversity (Burns et al 2015)
This study presents photogrammetric, multi-temporal surveys of a patch reef located in Moorea, French Polynesia, with the specific aim of providing a workflow for the 3D monitoring and mapping of coral reefs at the level of accuracy required to detect annual changes in the 3D structure of the reef
Summary
A range of survey methodologies including satellite sensing, airborne investigations, vessel-based acoustic and light detection and ranging (LIDAR) technologies (Collin et al 2018), visualizations using underwater robotic vehicles (Kocak and Caimi 2005; Johnson-Roberson et al 2010), and in situ operations based on underwater photogrammetric surveying (Burns et al 2015; Royer et al 2018) have been used to document declining trends in coral reef cover. Field surveys of coral reefs carried out by robotic vehicles (Johnson-Roberson et al 2010) or divers equipped with consumer-grade cameras are allowing the three-dimensional (3D) quantification of ecologically relevant characteristics at the patch scales and have opened the way to obtaining detailed and accurate surveys for the monitoring of coral growth, reef structural complexity (Ferrari et al 2017), and as predictors of reef organismal abundance, biomass, and diversity (Burns et al 2015)
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