Evaluation of underwater photogrammetry’ performances in the assessment of coral reef 3D changes

Abstract

This paper presents a multi-temporal underwater photogrammetric survey of a reef patch located in Moorea Island (French Polynesia) to detect a coral growth at a required levelof accuracy of 10-15 mm\years. An approach based on processing of underwater imagery by the well-known Structure from Motion methodand ground control points derived from the adjustment of underwater topographic measurements allowedthe three-dimensional quantification of ecological characteristic at patch scale and an assessment of the reefgrowth anditsstructural complexity. To guarantee the repeatability of surveys over time at required level of accuracy and spatial resolution, the paper introduces issues related toreference frame, network adjustment and design of photogrammetric surveying. Measuring tools and reference points were properly designed and presented by the authors. The network adjustment performed after measuringdistancesand height differences, providedan average precisionof 1.2 mm and 2.9 mm in the horizontal and vertical component respectively. The finalpositionalprecision of photogrammetric reconstructions wasin the order of 1 centimeter and few millimeters for 2017 and 2018 monitoring campaigns respectively. Then, the accuracy of the comparison resulted of about 1 cmwhich representsa factor to be considered in the successive interpretations of coral growth or dissolutionphenomena. The direct comparison of the two subsequent point clouds is effective in theevaluationof growth rates and in the successive morphometric analyses. For highly accurate quantitative assessment of local changes, an expert operator can create and analyzespecific 2D profiles that easily come from the point clouds