Simple methods for interactive 3D modeling, measurements, and digital databases of coral skeletons

Abstract

Coral studies rely on comparative research between several reference coral skeletons and living organisms imaged on reefs. However, many of these skeletons are part of collections, making extensive comparative works difficult. Nowadays, imaging technologies and image processing approaches allow acquisition of three‐dimensional (3D) datasets that can be converted to virtual models. These models can be made easily accessible and shared among collaborators or researchers using the Internet. Here, we compare high cost and low cost technologies on coral skeletons as well as imaging phantoms for accurate reference. We generated 3D models that were compared. Even though CT scanning and photogrammetry are the most accurate methods the latter is far less costly. It also allows for texture mapping, an essential tool in the field, and easy dataset handling. We submitted these 3D virtual models to experts for taxonomical identification, and they identified all specimens accurately. Finally, we further discuss applicability of our approach on museum collections, living corals, and field work.

Quantifying Multiscale Habitat Structural Complexity: A Cost-Effective Framework for Underwater 3D Modelling

“…We present three examples where this framework was used to quantify surface area, height profiles, volume and surface rugosity of coral reefs, across three different spatial extents and with high-resolution. Additionally, the validation analyses showed accuracies ranging from 79% (reefscape) to 90% (coral colony), in agreement with previous studies assessing accuracies for photogrammetric measures of coral colonies [28,31,43,[66][67][68]. These results show evidence of the utility of this framework to coral reef ecology and monitoring, in particular given the rapid degradation of coral reefs worldwide, for example they could enable the monitoring of coral reef flattening after a bleaching event [69].…”

Accuracy and Precision of Habitat Structural Complexity Metrics Derived from Underwater Photogrammetry

“…As a result, SR values varied by between 10% and 40% from the reference model. While few other studies have looked at accuracy of SR, these values are actually consistent with the range in bias in estimates of both surface area (1%-78%) and volume (2%-24%) from other studies using photogrammetry to model a variety of different coral morphotypes [25][26][27][28][29]. An important question in any assessment of bias is the accuracy of the reference value itself.…”

“…Underwater photogrammetry has proven to be a relatively accurate technique for morphological measurements of hemispherical scleractinian corals with studies demonstrating mean differences in 3D surface areas of 0.85% [25], <5% [27], 13% [26], and 1%-17% [28] and differences in volume of 1.7% [26], 3% [29], to 5%-9% [28]. Accuracy does seem to decrease for more complex branching and disc growth forms with errors in volume of 8% [28] and 17%-24% [29] and in surface area of 2% [28] and 12%-78% [27].…”

“…Accuracy does seem to decrease for more complex branching and disc growth forms with errors in volume of 8% [28] and 17%-24% [29] and in surface area of 2% [28] and 12%-78% [27]. Comparisons of model geometry accuracy are less common although McKinnon et al [30] demonstrated that photogrammetry can produce models of corals which are >90% similar to those of reference models derived from laser scans.…”

“…While the accuracy of conventional metrics like volume and surface area have been well considered at small (coral colony) spatial extents (as outlined above), there has been little work comparing either the geometry (3D structure) of derived models with those of highly accurate references (but see [30]) or any derived metrics of structural complexity. Aside from limited work using reference models [28] there has also been little assessment of the precision (repeatability) of photogrammetry approaches at small scales (but see [29]). At spatial extents larger than an individual coral colony it becomes quite difficult to have any true reference model against which to compare those generated from photogrammetry approaches.…”