[Coral-List] new tool/technique: Accurate coral reef maps from underwater digital imagery without ground control

[Storlazzi, Curt D]

Colleagues,

Whether it be post storm damage assessments or attempts to quantify the success of coral reef restoration activities, there is a growing need for accurate mapping....and re-mapping....of reefs at:

(1) across areas larger than what divers can cover on scuba tanks;
(2) at spatial resolutions finer than can be attained through aerial- or space-borne remote sensing imagery; and
(3) in not relatively placid waters that characterize most coral reefs.

To address this need and operational considerations, the US Geological Survey developed a new system that can measure mm-to-cm scale changes in coral reefs over time without pre-existing ground control and over large (10s of kilometers per day) spatial scales to far surpass what can be done with a single camera system (e.g., doi: 10.1007/s00338-016-1462-8) in terms of accuracy and precision. Please see below for more information, and feel free to contact the authors with any questions.

Curt
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Abstract:

Structure-from-Motion (SfM) photogrammetry can be used with digital underwater photographs to generate high-resolution bathymetry and orthomosaics with millimeter-to-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. Survey-to-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.

Keywords:

Underwater photogrammetry, Structure-from-Motion, synchronized cameras, coral reef, digital surface model, orthomosaic, post processed kinematic GNSS, ground control

Citation:

G.A. Hatcher, J.A. Warrick, A.C. Ritchie, E.T. Dailey, D.G. Zawada, C. Kranenburg, and K.K. Yates, 2020. Accurate bathymetric maps from underwater digital imagery without ground control. Frontiers in Marine Science, 7:525, https://www.frontiersin.org/articles/10.3389/fmars.2020.00525/full

This imagery and other georeferenced seafloor imagery are available online through the USGS video and photography portal at: https://video.ioos.us/

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Curt D. Storlazzi

U.S. Geological Survey
Pacific Coastal and Marine Science Center
2885 Mission Street
Santa Cruz, CA 95060

Staff web page: https://www.usgs.gov/staff-profiles/curt-d-storlazzi
Coral Reefs: https://coralreefs.wr.usgs.gov/<http://coralreefs.wr.usgs.gov/>