Abstract
Visual 3D reconstruction techniques provide rich ecological and habitat structural information from underwater imagery. However, an unaided swimmer or diver struggles to navigate precisely over larger extents with consistent image overlap needed for visual reconstruction. While underwater robots have demonstrated systematic coverage of areas much larger than the footprint of a single image, access to suitable robotic systems is limited and requires specialized operators. Furthermore, robots are poor at navigating hydrodynamic habitats such as shallow coral reefs. We present a simple approach that constrains the motion of a swimmer using a line unwinding from a fixed central drum. The resulting motion is the involute of a circle, a spiral‐like path with constant spacing between revolutions. We test this survey method at a broad range of habitats and hydrodynamic conditions encircling Lizard Island in the Great Barrier Reef, Australia. The approach generates fast, structured, repeatable, and large‐extent surveys (~110 m2 in 15 min) that can be performed with two people and are superior to the commonly used “mow the lawn” method. The amount of image overlap is a design parameter, allowing for surveys that can then be reliably used in an automated processing pipeline to generate 3D reconstructions, orthographically projected mosaics, and structural complexity indices. The individual images or full mosaics can also be labeled for benthic diversity and cover estimates. The survey method we present can serve as a standard approach to repeatedly collecting underwater imagery for high‐resolution 2D mosaics and 3D reconstructions covering spatial extents much larger than a single image footprint without requiring sophisticated robotic systems or lengthy deployment of visual guides. As such, it opens up cost‐effective novel observations to inform studies relating habitat structure to ecological processes and biodiversity at scales and spatial resolutions not readily available previously.
Highlights
Effective techniques to quantify underwater benthic community composition and physical habitat structure are of importance to marine ecologists and resource managers
The physical habitat structure built by benthic communities provides diverse niches and supports and array of associated organisms
Several recent studies have used to off-the-shelf Structure from Motion (SfM) software such as Photoscan to build 3D models of colonies and broader reef patches, and characterize the quality of these reconstructions (Burns, Delparte, Gates, & Takabayashi, 2015; Figueira et al, 2015; Leon, Roelfsema, Saunders, & Phinn, 2015; Storlazzi, Dartnell, Hatcher, & Gibbs, 2016), establishing confidence in the use of visual reconstructions to address ecological questions (Burns et al, 2016)
Summary
Several recent studies have used to off-the-shelf Structure from Motion (SfM) software such as Photoscan to build 3D models of colonies and broader reef patches, and characterize the quality of these reconstructions (Burns, Delparte, Gates, & Takabayashi, 2015; Figueira et al, 2015; Leon, Roelfsema, Saunders, & Phinn, 2015; Storlazzi, Dartnell, Hatcher, & Gibbs, 2016), establishing confidence in the use of visual reconstructions to address ecological questions (Burns et al, 2016) These techniques rely on combining overlapping images into a composite 3D reconstruction, and while they can scale to areas of tens to thousands of square meters consisting of tens of thousands of images, they need a systematic way of covering the survey site.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
