Coral Reef Monitoring by Scuba Divers Using Underwater Photogrammetry and Geodetic Surveying

Abstract

Underwater photogrammetry is increasingly being used by marine ecologists because of its ability to produce accurate, spatially detailed, non-destructive measurements of benthic communities, coupled with affordability and ease of use. However, independent quality control, rigorous imaging system set-up, optimal geometry design and a strict modeling of the imaging process are essential to achieving a high degree of measurable accuracy and resolution. If a proper photogrammetric approach that enables the formal description of the propagation of measurement error and modeling uncertainties is not undertaken, statements regarding the statistical significance of the results are limited. In this paper, we tackle these critical topics, based on the experience gained in the Moorea Island Digital Ecosystem Avatar (IDEA) project, where we have developed a rigorous underwater photogrammetric pipeline for coral reef monitoring and change detection. Here, we discuss the need for a permanent, underwater geodetic network, which serves to define a temporally stable reference datum and a check for the time series of photogrammetrically derived three-dimensional (3D) models of the reef structure. We present a methodology to evaluate the suitability of several underwater camera systems for photogrammetric and multi-temporal monitoring purposes and stress the importance of camera network geometry to minimize the deformations of photogrammetrically derived 3D reef models. Finally, we incorporate the measurement and modeling uncertainties of the full photogrammetric process into a simple and flexible framework for detecting statistically significant changes among a time series of models.

Underwater Hyperspectral Imaging (UHI): A Review of Systems and Applications for Proximal Seafloor Ecosystem Studies

“…We further noticed time-consuming survey procedures in UHI diver surveys, such as gray reference boards that need to be positioned underwater before acquiring data along transects. However, this differs slightly from standard diver-based RGB imaging benthic surveys, as these are already integrating seafloor control points (i.e., geodesic networks) where local coordinates (e.g., latitude, longitude, depth) are annotated to geo-reference benthic topographic models [132]. Similarly, [67] placed and photographed (as an RGB image) four wooden frames (40 × 40 cm) before the UHI survey being completed to validate the hyperspectral images, as well as three white metal sheets, which were fixed to the seafloor to be used as a reference spectra and to delimit the study area.…”

Bundle Adjustment With Polynomial Point-to-Camera Distance Dependent Corrections for Underwater Photogrammetry

“…Only a highly redundant and geometrically strong camera network including nadir and oblique images would mitigate residual systematic errors. A less robust image configuration could result in a considerable increase in deformations in object space (Nocerino et al, 2020).…”

“…The images were acquired by SCUBA divers with a Nikon D750 24Mpx full frame DSLR camera (pixel size ≈ 6 µm) mounting a 24 mm prime lens in a NiMAR pressure housing. The surveyed reef plot measures about 20x10m 2 , featuring a rough and hilly surface, at an average depth of about 12 m and with a depth variation of about 1.6 m. Results of the photogrammetric processing aiming at monitoring across time the reef evolution are discussed in Nocerino et al (2019Nocerino et al ( , 2020.…”

Accurate Scaling and Levelling in Underwater Photogrammetry With a Pressure Sensor

“…shipwrecks), provides very uncertain Z coordinates (Skarlatos et al, 2019). More accurate direct measurements by divers have been recently demonstrated (Nocerino et al, 2020) combining trilateration and geometric levelling underwater with lasers for collecting sub-centimeter accuracy GCPs for 3D photogrammetric measurements aimed at multitemporal analysis for ecological research. Yet, the even greater complexity of these underwater surveys carried out by divers restricts its use only to areas of limited extension and at shallow depths (<20m).…”