Accurate Bathymetric Maps From Underwater Digital Imagery Without Ground Control

Abstract

Structure-from-Motion (SfM) photogrammetry can be used with digital underwater photographs to generate high-resolution bathymetry and orthomosaics with millimeterto-centimeter scale resolution at relatively low cost. Although these products are useful for assessing species diversity and health, they have additional utility for quantifying benthic community structure, such as coral growth and fine-scale elevation change over time, if accurate length scales and georeferencing are included. This georeferencing is commonly provided with "ground control," such as pre-installed seafloor benchmarks or identifiable "static" features, which can be difficult and time consuming to install, survey, and maintain. To address these challenges, we developed the SfM Quantitative Underwater Imaging Device with Five Cameras (SQUID-5), a towed surface vehicle with an onboard survey-grade Global Navigation Satellite System (GNSS) and five rigidly mounted downward-looking cameras with overlapping views of the seafloor. The cameras are tightly synchronized with both the GNSS and each other to collect quintet photo sets and record the precise location of every collection event. The system was field tested in July 2019 in the U.S. Florida Keys, in water depths ranging from 3 to 9 m over a variety of bottom types. Surveying accuracy was assessed using pre-installed stations with known coordinates, machined scale bars, and two independent surveys of a site to evaluate repeatability. Under a range of sea conditions, ambient lighting, and water clarity, we were able to map living and senile coral reef habitats and sand waves at mm-scale resolution. Data were processed using best practice SfM techniques without ground control and local measurement errors of horizontal and vertical scales were consistently sub-millimeter, equivalent to 0.013% RMSE relative to water depth. Surveyto-survey repeatability RMSE was on the order of 3 cm without georeferencing but could be improved to several millimeters with the incorporation of one or more non-surveyed marker points. We demonstrate that the SQUID-5 platform can map complex coral reef and other seafloor habitats and measure mm-to-cm scale changes in the morphology and location of seafloor features over time without pre-existing ground control.

Which Method for Which Purpose? A Comparison of Line Intercept Transect and Underwater Photogrammetry Methods for Coral Reef Surveys

“…To this end, new methods adopting the FAIR principles (findability, accessibility, interoperability, and reusability) for stewardship of reef data can improve the monitoring of these ecosystems (Rossi et al, 2021). Among them the development of artificial intelligence for automated analysis of images (Williams et al, 2019;González-Rivero et al, 2020) and photogrammetry outputs (e.g., Hopkinson et al, 2020;Mohamed et al, 2020), multispectral and hyperspectral imagery (e.g., Parsons et al, 2018;Bajjouk et al, 2019;Li et al, 2019), and the improvement of diver-based or automated data acquisition from Remotely Operated Vehicles and Autonomous Underwater Vehicles (Friedman et al, 2012;Obura et al, 2019;Hatcher et al, 2020;Rossi et al, 2021) will likely revolutionize this field. Finally the integration of all existing reef habitat mapping data in common and open source data bases could help to overcome the lack of transferability across systems/scale, provide more accurate and representative assessments of habitats, and generate conservation measures and habitat rehabilitation actions (Madin et al, 2019;Rossi et al, 2021).…”

Classification of underwater photogrammetry data for temperate benthic rocky reef mapping

“…More recently the photogrammetry technique has been developed and is now widely deployed in subtidal environments. This technique allows a 3D reconstruction with a more detailed characterisation of benthic rocky substrates and at larger scale of their associated landscapes (Hatcher et al, 2020). Based on multi-view optical information (either photographic or videographic), the photogrammetry has been mainly used in clear water environments to describe coral reefs structure at various biological levels from individuals to reefs (Burns et al, 2015;Figueira et al, 2015;Leon et al, 2015;Burns et al, 2016;Ferrari et al, 2016;Anelli et al, 2017;Carlot et al, 2020;Fukunaga et al, 2020).…”

Assessing Seagrass Restoration Actions through a Micro-Bathymetry Survey Approach (Italy, Mediterranean Sea)

“…Compared to the traditional monitoring of P. oceanica transplants, underwater photogrammetry provides high-resolution information from which microscale information can be extracted [22,23,34,[83][84][85][86][87][88]. As highlighted in the literature, many restoration actions have failed, and this is often due to the erroneous choice of the intervention site and its characteristics, e.g., depth of the site and wave exposure, currents, and occurrence of boat anchoring [8].…”