Abstract
Abstract. Recent advances in automation of photogrammetric 3D modelling software packages have stimulated interest in reconstructing highly accurate 3D object geometry in unconventional environments such as underwater utilizing simple and low-cost camera systems. The accuracy of underwater 3D modelling is affected by more parameters than in single media cases. This study is part of a larger project on 3D measurements of temporal change of coral cover in tropical waters. It compares the accuracies of 3D point clouds generated by using images acquired from a system camera mounted in an underwater housing and the popular GoPro cameras respectively. A precisely measured calibration frame was placed in the target scene in order to provide accurate control information and also quantify the errors of the modelling procedure. In addition, several objects (cinder blocks) with various shapes were arranged in the air and underwater and 3D point clouds were generated by automated image matching. These were further used to examine the relative accuracy of the point cloud generation by comparing the point clouds of the individual objects with the objects measured by the system camera in air (the best possible values). Given a working distance of about 1.5 m, the GoPro camera can achieve a relative accuracy of 1.3 mm in air and 2.0 mm in water. The system camera achieved an accuracy of 1.8 mm in water, which meets our requirements for coral measurement in this system.
Highlights
The Moorea Island Digital Ecosystem Avatar (IDEA) project was recently developed as a joint initiative by an interdisciplinary group of experts to create a digital Avatar of the Pacific island of Moorea, French Polynesia (Cressey, 2015, Davies 2016)
These were further used to examine the relative accuracy of the point cloud generation by comparing the point clouds of the individual objects with the objects measured by the system camera in air
The accuracy assessment issue of underwater 3D modelling is quite challenging, because the precise measurement of GCPs and check points in the unconventional environment of underwater is difficult, and water body--lens port--air in waterproof lens housings form a complicated multiple media environment that greatly alters the path of optical rays
Summary
The Moorea Island Digital Ecosystem Avatar (IDEA) project was recently developed as a joint initiative by an interdisciplinary group of experts to create a digital Avatar of the Pacific island of Moorea, French Polynesia (Cressey, 2015, Davies 2016). Risk (1972) was among the first to report a strong positive correlation between fish species diversity and substrate topographic complexity, an observation that has been repeated for both abundance and species richness in a variety of coral reef systems including Moorea (Holbrook et al 2002, 2003, Schmitt and Holbrook 2007). This is an important (albeit not the only) reason why the topography of coral habitats is so much of interest. A decisive step forward in methodology was introduced by Du Preez and Tunnicliffe (2012), who used a laser point profiler together with a single video camera to reconstruct height profiles from moving platforms like ROVs, scuba divers, This contribution has been peer-reviewed
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