The BOSS has two variants: a stereo system (Fig. 2) and a lighter-weight mono system (Fig. 3). Both consist of a sturdy aluminium frame to secure and protect the camera equipment, a flotation compartment at the top and a bolt-on base weight. The buoyancy and weighting counteract to create a self-righting action, with flotation provided by compression-resistant syntactic foam or subsurface floats. The weighting and compression-resistant buoyancy means that no adjustments are necessary to work, up to the limits of the camera housings and buoyancy (i.e. 1,000 m). When weights are removed, either system can be safely carried by two people (i.e. <35 kg). In the stereo version, eight horizontally-facing cameras are secured to brackets aligned in four stereo pairs at 90-degree intervals (Fig. 2c), and an optional downward-facing camera can be mounted within the buoyancy compartment to collect more traditional imagery (Fig. 4a). Brackets are provided for four lights. In the stereo version, camera brackets are secured to a common central column (Fig. 2a and 4a) and removed from the outer frame to reduce the risk of any physical impacts on the outer frame compromising the stereo calibration. By using small-form action cameras with external battery packs and large capacity memory cards, it is possible to film continuously for 12 hours and not require the camera housings to be opened until the end of the day, thus reducing risks to equipment, calibration stability, and substantially increasing efficiency in the field. Further information on cameras and photogrammetry are provided in the Camera and Photogrammtery section below. In the stereo system, each pair of cameras is separated by 500 mm, with the top camera in each pair angled 8 degrees downward and the bottom camera horizontal (Fig. 1) to provide adequate separations and overlap of imagery (Langlois et al. 2020).
Figure 2. BOSS design. a) stereo configuration with camera pairs mounted on internal base bar cassette, showing camera housings (grey) and lights (black), b) specifications of the stereo camera separation and angle of convergence, c) overhead field of view showing the wide 270o field of view, and d) lighter weight mono configuration.
Figure 3. BOSS equipment required for deployment. a) Stereo camera frame with an additional downward facing camera mounted in buoyancy compartment, b) rope and floats, c) synchronisation diodes, d) detachable ballast and gloves, e) lights and batteries, f) cameras, battery packs, SD cards and spare O-rings, g) field metadata sheet, whiteboard and marker, h) charging equipment and downloading footage, and i) tools including silicone grease.
m but have limited mobility at these depths.
* Planning and field work only. Purchase of ROV, consumables, processing of samples and reporting are not included.
^^ Estimates include basic positioning systems (such as USBL), grabber/manipulator and depth rated stereo cameras. Based on quotes from the companies as well as catalogue entries.
Camera and Photogrammetry
Camera specifications can influence the accuracy of taxonomic identification, and stereo-measurements require careful adherence to camera alignment and calibration protocols. In our system, we record video with Sony FDR-X3000 cameras filming at 1920 x 1080 pixels, a frame rate of 60 frames per second and using the ‘medium’ field of view setting (~67.5˚). We recommend the use of cameras with a minimum resolution of 1920 × 1080 pixels (Langlois et al. 2020; Harvey et al. 2010) and a minimum capture rate of 30 frames per second, with all settings standardised across cameras. Higher camera resolution will generally improve taxonomic identification, but all systems should be thoroughly tested before deployment for overheating issues or write speed limitations at higher-quality settings.
To maintain stereo-calibrations, cameras must have video stabilisation disabled, and a fixed focal length can allow measurements both close to and far from the camera when correctly calibrated (Boutros, Shortis, and Harvey 2015; Shortis, Harvey, and Abdo 2009). Field of view settings should be chosen to limit distortion in the image rather than maximise the field of view. White lights (550 - 560 nm) are recommended for low-light conditions (Birt et al. 2019). We recommend seeking manufacture and calibration advice for the frame from recognised providers, to ensure that the tight tolerances for effective stereo-vision are met. Each housing and camera set should be uniquely identified to ensure that individual cameras are used only in the housing they are calibrated in. Any changes to camera positions (e.g. if a camera is dismounted during battery replacement) will disrupt the calibration, increasing error in length measurements.
Video cameras can skip or lose frames, disrupting synchronisation among cameras and requiring the use of manual reference points such as a clapper board shown at the start of each take. The wide-field stereo-video drop camera system is designed to record many successive deployments but requires manual synchronisation at regular intervals. We use a flexible strip of waterproof LED lights to generate a simultaneous flash at all nine cameras. The chosen camera model should be tested to determine how often resynchronisation needs to occur to maintain accurate stereo measurements.