The Magazine for Underwater Professionals
12th April 2019
3D at Depth, the USA-based specialist in subsea LiDAR laser technology and 3D data solutions, announced yesterday the success of an innovative underwater survey technique that integrates photogrammetry with subsea laser LiDAR technology to create accurate, precise, reverse engineered 3D models. The technique was developed during an October 2018 expedition with the Air/Sea Heritage Foundation, USA, to document the largely intact wreck of a rare and historically significant US Navy Douglas TBD-1 Devastator aircraft resting at the bottom of Jaluit Lagoon in the Republic of the Marshall Islands (RMI).
Of the 129 Devastators built, all were either lost in battle, destroyed in operational accidents, or scrapped before the end of World War II. Part of the ongoing ‘Devastator Project’, this recent expedition was developed to capture precise, repeatable, millimetric data to build an accurate 3D model as a 21st century site map for this significant cultural heritage asset; and to serve as a “pre-disturbance survey” for the proposed recovery, conservation and public exhibition of the historic plane at the National Museum of the United States Navy in Washington, DC.
Although the Devastator appeared largely intact from previous research expeditions, the sensitive nature of the wreck and often low water clarity conditions around the site provided limitations for most of the standard data collection processes and other optical imagery collection methods. In addition, as a pre-disturbance survey, it was important to capture not only the repeatable, accurate, millimetric details of the aircraft but also the substance and structures of the marine biomass on and around the aircraft. These challenges created a unique opportunity for 3D at Depth’s SL3 subsea laser LiDAR with its powerful real-time 3D data processing unit. In addition, the team used still photography, 4K video, 3D photogrammetry, and 360-degree VR.
Over a period of six days on site, dozens of dives were conducted to the wreck located at a depth of 130 feet (40 metres). Four registration markers were set in pre-approved areas to check accuracy for both laser scans and photogrammetry technology. 3D at Depth’s SL3 laser was deployed at varying ranges from the wreck site while maintaining a safe distance, so as not to disturb the surrounding area. The steerable beam allowed for flexibility and accuracy while working alongside other technology applications. The SL3 pulsed at 40,000 measurements per second for each scan position acquiring very high-density sector scans.
In total, the SL3 LiDAR laser delivered a total of 92 million points and the photogrammetric data after processing produced 5.7 million vertices. Each dense point cloud was then surfaced modelled and moved into a 3D design computer. Optical imagery from photogrammetry, 360 VR and stills were integrated into the designs to render an exact 3D model of the Devastator. Along with the laser LiDAR data, the team captured 1398 still images with photogrammetry.
“3D at Depth's underwater laser LiDAR technology utilises our patented index of refraction correction algorithms to produce a true digital twin with accurate, precise, measurement,” said Neil Manning, chief operating officer at 3D at Depth. “The fusion of those data outputs with 3D volumetric photogrammetry offers a revolutionary step towards to accurate digital preservation and visualisation of large features on the seafloor. We look forward to advancing this technique for a wide range of applications in the areas of nautical archaeological sites and wrecks, documentation of maritime accidents and baselines surveys of sensitive marine environments.”