Detecting change in coral reef 3D structure using underwater photogrammetry: critical issues and performance metrics.


Rossi, P.Castagnetti, C.Capra, A.Brooks, A. J.Mancini, F.


Applied Geomatics


This paper presents a multi-temporal underwater photogrammetric survey of a reef patch located in Moorea Island to detect a coral growth of 10-15 mm\years. Structure from Motion photogrammetry and underwater imagery allow the three-dimensional quantification of ecological characteristic at patch scale and reef structural complexity. High accuracy and resolution are required in order to guarantee the repeatability of surveys over time within the same reference system; a proper geodetic network and acquisition scheme are mandatory as well. Measuring tools and reference points were properly designed in order to constraint the photogrammetric reconstruction. The network adjustment, performed with distance and height difference observations, provides an average accuracy of ±1.2 mm and ±2.9 mm in the horizontal and vertical component respectively. The final accuracies of photogrammetric reconstructions are in the order of 1 centimetre and few millimetres for 2017 and 2018 monitoring campaigns respectively; this results in errors in the comparison of about ±1 cm, so, coordinate variations bigger 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 growth main trends and perform morphometric analyses. For highly accurate quantitative assessment of local changes, an expert operator can create and analyse specific 2D profiles that easily come from the point clouds.





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Journal Article

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