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

Will Figueira,Augustine Porter,Renata Ferrari,Elyse Weatherby,Steven Hawes,Maria Byrne

doi:10.3390/rs71215859

Will Figueira, Augustine Porter + Show 4 more

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https://doi.org/10.3390/rs71215859

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Abstract

In tropical reef ecosystems corals are the key habitat builders providing most ecosystem structure, which influences coral reef biodiversity and resilience. Remote sensing applications have progressed significantly and photogrammetry together with application of structure from motion software is emerging as a leading technique to create three-dimensional (3D) models of corals and reefs from which biophysical properties of structural complexity can be quantified. This enables the addressing of a range of important marine research questions, such as what the role of habitat complexity is in driving key ecological processes (i.e., foraging). Yet, it is essential to assess the accuracy and precision of photogrammetric measurements to support their application in mapping, monitoring and quantifying coral reef form and structure. This study evaluated the precision (by repeated modeling) and accuracy (by comparison with laser reference models) of geometry and structural complexity metrics derived from photogrammetric 3D models of marine benthic habitat at two ecologically relevant spatial extents; individual coral colonies of a range of common morphologies and patches of reef area of 100s of square metres. Surface rugosity measurements were generally precise across all morphologies and spatial extents with average differences in the geometry of replicate models of 1–6 mm for coral colonies and 25 mm for the reef area. Precision decreased with complexity of the coral morphology, with metrics for small massive corals being the most precise (1% coefficient of variation (CV) in surface rugosity) and metrics for bottlebrush corals being the least precise (10% CV in surface rugosity). There was no indication however that precision was related to complexity for the patch-scale modelling. The 3D geometry of coral models differed by only 1–3 mm from laser reference models. However, high spatial variation in these differences around the model led to a consistent underestimation of surface rugosity values for all morphs of between 8% and 37%. This study highlights the utility of several off-the-shelf photogrammetry tools for the measurement of structural complexity across a range of scales relevant to ecologist and managers. It also provides important information on the accuracy and precision of these systems which should allow for their targeted use by non-experts in computer vision within these contexts.

Highlights

Introduction and BackgroundStructural complexity plays an important role in structuring biotic assemblages and in maintaining biodiversity of benthic marine ecosystems [1]
As marine ecosystems are threatened from many anthropogenic and natural forces, there has been an increasing focus on understanding the linkages between complexity and function but on the more basic issue of how to measure it accurately and over scales relevant to both ecological processes driving ecosystem trajectories and management [5]. This issue is of particular concern in coral reef ecosystems where the threats to biodiversity are derived from the loss of structural complexity due to the interactive effects of stressors such as coral bleaching, overfishing, cyclones, coastal development, and outbreaks of predator species [8,9]
The precision with which coral colonies were modeled with the DSLR camera system was generally high with average distances between 3D models ranging from 1–4 mm at any given point (Figures 3b and 4, Table 2)

Summary

Introduction

Introduction and BackgroundStructural complexity plays an important role in structuring biotic assemblages and in maintaining biodiversity of benthic marine ecosystems [1]. As marine ecosystems are threatened from many anthropogenic and natural forces, there has been an increasing focus on understanding the linkages between complexity and function but on the more basic issue of how to measure it accurately and over scales relevant to both ecological processes driving ecosystem trajectories and management [5] This issue is of particular concern in coral reef ecosystems where the threats to biodiversity are derived from the loss of structural complexity due to the interactive effects of stressors such as coral bleaching, overfishing, cyclones, coastal development, and outbreaks of predator species [8,9]

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