Accuracy Assessment of Underwater Photogrammetric Three Dimensional Modelling for Coral Reefs

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.

Monitoring Coral Growth – The Dichotomy Between Underwater Photogrammetry and Geodetic Control Network

“…A further degradation by approximately a factor two can be considered for underwater photogrammetry (Maas, 2015), leading to a final theoretical accuracy of about 1.6 mm, which is still an order of magnitude better than the accuracy provided by the underwater geodetic network (short poles). Also this theoretical value correlates well with a past empirical analysis performed with the same camera system (Guo et al, 2016).…”

Cost and time-effective method for multi-scale measures of rugosity, fractal dimension, and vector dispersion from coral reef 3D models

“…Our lower accuracy was to be expected, as we did not calibrate cameras and used a considerably less time-consuming rendering process compared to other methods: e.g., methods that include manually removing outlier points on 3D models, manually identifying ground control objects, and/or using PhotoScan’s high or ultra-high quality settings. [55] showed that models from calibrated GoPro footage can achieve RMSE errors of 0.40 mm; users requiring models accurate at scales finer than 1.5 cm should consider a method requiring higher hardware/software effort and cost or at minimum calibrate their cameras.…”

Camera Calibration Techniques for Accurate Measurement Underwater

“…Two different types of underwater cameras are evaluated in a preliminary study of accuracy for the monitoring of coral reefs (Guo et al 2016). In-air and underwater calibrations were undertaken, validated by an accurately known target fixture and 3D point cloud models of cinder blocks.…”