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

Erica Nocerino,Andrew J Brooks,Alessandro Capra,Armin Gruen,Cristina Castagnetti,Russell J Schmitt,Fabio Menna,Paolo Rossi,Matthias Troyer,Sally J Holbrook

doi:10.3390/rs12183036

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.

Highlights

Underwater photogrammetry has increasingly become a popular technique for three-dimensional (3D) mapping of subaquatic environments at different spatial scales and resolution
Based on preliminary studies [9,14] and expanding the results presented in [21], here, we critically revise and expand all the steps of the developed photogrammetric approach for coral reef temporal monitoring
While these tests demonstrate the potential for very high photogrammetric accuracy, even underwater, it should be noted that our empirical error measurements are based on the use of only five reference points

Summary

Introduction

Underwater photogrammetry has increasingly become a popular technique for three-dimensional (3D) mapping of subaquatic environments at different spatial scales and resolution. Restoring the proper color balance from a picture that has been acquired underwater while not taking into account this phenomenon is equivalent to trying to color balance a picture taken above the water under a very different source of illumination than natural light (e.g., a picture taken under domestic, tungsten-filament lighting using a color camera setting for sunlight) In this situation, if the image has not been taken in raw format, a proper reproduction of colors is very difficult to achieve in post-processing, as the color content recorded by the RGB sensor lacks the signal in the corresponding wavelengths or might saturate in another channel. This agrees with the results in [8]

Object Space Analysis

Findings

Camera Network Analysis