Abstract
We present a method to construct and analyse 3D models of underwater scenes using a single cost-effective camera on a standard laptop with (a) free or low-cost software, (b) no computer programming ability, and (c) minimal man hours for both filming and analysis. This study focuses on four key structural complexity metrics: point-to-point distances, linear rugosity (R), fractal dimension (D), and vector dispersion (1/k). We present the first assessment of accuracy and precision of structure-from-motion (SfM) 3D models from an uncalibrated GoPro™ camera at a small scale (4 m2) and show that they can provide meaningful, ecologically relevant results. Models had root mean square errors of 1.48 cm in X-Y and 1.35 in Z, and accuracies of 86.8% (R), 99.6% (D at scales 30–60 cm), 93.6% (D at scales 1–5 cm), and 86.9 (1/k). Values of R were compared to in-situ chain-and-tape measurements, while values of D and 1/k were compared with ground truths from 3D printed objects modelled underwater. All metrics varied less than 3% between independently rendered models. We thereby improve and rigorously validate a tool for ecologists to non-invasively quantify coral reef structural complexity with a variety of multi-scale metrics.
Highlights
Using an array of metrics in studies spanning decades, ecologists have shown that structural complexity drives biodiversity [1,2,3,4]
Expanding metrics gives researchers additional tools to answer ecological questions about 3D surfaces–e.g., At what scales does the structure provide refuge spots for prey to hide? How does a surface trap particulate matter? We show how footage from a single uncalibrated GoPro camera can produce 4 m2 3D models with fine resolution and precision
The curve was created with the Rhinoceros 3D (Rhino) built-in command “MeshIntersect,” which provides a cross-sectioning tool that allows the user to select a slice of user-determined linear length of the model by intersecting a mesh plane with the 3D reef mesh
Summary
Using an array of metrics in studies spanning decades, ecologists have shown that structural complexity drives biodiversity [1,2,3,4]. Divers can visually score a range of structural variables using HAS [17] Both methods have revealed correlations, they result in a cursory understanding of complexity that is inadequate for addressing fine-scale ecological questions [20] or informing artificial reef designs [21] and, may be fundamentally misleading because of factors such as observer bias and dimensionality reduction [2, 22, 23]. Method for multi-scale measures of rugosity, fractal dimension, and vector dispersion from coral reef 3D models single GoPro camera; % $300) and approximately two hours of processing time on a standard laptop (!8 GB RAM, 600 MB free disk space). Our framework is ready-to-go to for use by non-programmers, and could be extended to gather any other conceivable structural complexity metric by a user with intermediate Python programming ability
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