POTENZIALITÀ E PROBLEMATICHE NELL’UTILIZZO DELLA FOTOGRAMMETRIA SUBACQUEA PER LA MISURA DELL’ACCRESCIMENTO DI COLONIE CORALLIGENE [EVALUATION OF UNDERWATER PHOTOGRAMMETRY’ PERFORMANCES IN THE ASSESSMENT OF CORAL REEF 3D CHANGES]

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 level of accuracy of 10-15 mmyears. An approach based on processing of underwater imagery by the well-known Structure from Motion method and ground control points derived from the adjustment of underwater topographic measurements allowed the three-dimensional quantification of ecological characteristic at patch scale and an assessment of the reef growth and its structural complexity. To guarantee the repeatability of surveys over time at required level of accuracy and spatial resolution, the paper introduces issues related to reference 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 measuring distances and height differences, provided an average precision of 1.2 mm and 2.9 mm in the horizontal and vertical component respectively. The final positional precision of photogrammetric reconstructions was in 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 cm which represents a factor to be considered in the successive interpretations of coral growth or dissolution phenomena. The direct comparison of the two subsequent point clouds is effective in the evaluation of growth rates and in the successive morphometric analyses. For highly accurate quantitative assessment of local changes, an expert operator can create and analyze specific 2D profiles that easily come from the point clouds.