Written by Simon Brown, Managing Director of AccuPixel Limited
Orkney’s Scapa Flow, a massive natural harbour, played a crucial role as a naval base during both World Wars. Renowned for its stunning collection of German First World War battleships and cruisers, all intentionally scuttled in June 1919, these wrecks now serve as world class dive sites, significantly boosting the local economy through the influx of recreational divers.
Beyond the famous large wrecks, Scapa Flow is littered with smaller, yet equally intriguing sites. Some remnants from salvage activity that saw many German vessels raised and scrapped, are battleship-sized debris fields. Others are minute anomalies, only known through side scan sonar survey discoveries. Often overshadowed by the iconic wrecks, these sites are often found in pristine and undisturbed conditions, offering unique insights into historical events.
The risks to such sites are multifaceted. Commercial fishing activities pose inadvertent damage, while visiting divers sometimes remove non-ferrous artifacts. Much material is lost or damaged before recording and with this the opportunity to record, interpret, and understand the history and context of these sites, and the people who are connected to them, is lost.
Enter underwater photogrammetry, a revolutionary technique to record in exceptionally high detail what lies on the seabed. Underwater photogrammetry is a sophisticated method of mapping and recording underwater sites with stunning accuracy and detail. By taking a series of overlapping photographs of an object or site from different angles, scientists can create precise, high-resolution 3D models and 2D maps. This technique allows researchers to study and document underwater artifacts as if they were viewing them on land. The magic begins with a diver equipped with a high-resolution camera. During a dive, hundreds of photographs are taken from various perspectives, ensuring every angle is covered. Back on the surface, these images are fed into specialized software that stitches them together, creating a comprehensive 3D model. Small sites can be scanned in minutes to be studied at length from the comfort of the office. The method of rapid collection and post-fieldwork study can also be applied to considerably larger sites [1].
Among the myriad sites in Scapa Flow that have been scanned into 3D using photogrammetry, three small but significant sites stand out. All have been subject to damage or loss and two have been recorded prior to disturbance. One particularly important site, despite suffering from artifact removal and context loss, was recorded pre-disturbance and the details digitally preserved for future study.
Grumman Wildcat JV751
In the depths of Orkney’s Scapa Flow, a remarkable story of wartime aviation emerged from the sea when a section of a rudder was recovered in a trawl net. This fragment belonged to a Grumman Wildcat JV751, a World War II aircraft lost due to a steam catapult malfunction on the aircraft carrier HMS Trumpeter [2]. Detailed sonar surveys and ROV investigations pinpointed the wreckage, revealing the partial remains of the aircraft. Surprisingly, the fuselage aft of the cockpit was missing, likely damaged by the trawl net that recovered the rudder section.
Despite the damage, the site still contains significant artifacts, including non-ferrous and valuable components such as the oil cooler and several 0.50 calibre machine guns. To preserve the site in detail, a photogrammetric survey was conducted capturing 767 high resolution images in a single dive. This meticulous work has created a persistent digital record of the site, evidencing the physical presence, context and location of the artefacts themselves.
Using a full frame DSLR camera with strobe lighting offers significant advantages over the more automated consumer level or action type cameras. The manual control and larger sensor size reduced digital noise and avoided distortions caused by rolling shutters, ensuring the accuracy of the photogrammetric model. This method allowed for precise and clear documentation of the underwater heritage site, offering a deeper understanding of its historical significance.
21” Torpedo
In another fascinating discovery in Scapa Flow, a 21-inch torpedo was located in 2019 during a sonar survey. Substantially intact and still armed with its warhead, the presence of this live torpedo was promptly reported to HM Coastguard. The Royal Navy diving unit based at Faslane were tasked with disposing of the weapon, a process involving detonating the warhead using an explosive charge. Due to the high explosive nature of the site, the destruction was considerable, and little remained once the disposal had taken place.
Before its destruction, an opportunity arose to document this piece of history. A rapid photogrammetric survey was conducted, capturing 422 individual frames in just eight minutes. This efficient process covered an area of 33 square meters, creating a detailed 3D model of the torpedo. The use of control points ensured the scale and accuracy of the final reconstruction.
Photogrammetry offers significant advantages over traditional site surveys, particularly in challenging underwater environments. Firstly, it provides rapid and efficient site capture. Secondly, it offers greater and more accurate analysis opportunities. By isolating sections and creating closed 3D models, estimates of volumes of interest can be made with high accuracy. In the case of the torpedo, the volume of explosive material (Torpex) was determined to be 0.1144 cubic meters. For professionals assessing the potential impact of explosives, applying the density of Torpex would reveal the mass and potential "bang" on the seabed. Geo-referencing this 3D model with real-world coordinates ensures accurate orientation and alignment, and depth measurements can be integrated into global information systems like Global Mapper for further analysis.
First World War German Diesel Pinnace
The discovery of a small but intact artifact from the German High Seas Fleet was a truly exciting discovery. In April 2017 a dive vessel echo sounder located an anomaly, leading to the discovery of a diesel pinnace dating from 1919 [3]. Although nothing remained of the wooden hull, the pinnace engine and non-ferrous fittings – helm-parts and and portholes – were recorded and left in situ. Nearby, a brass shell case, thought to be British and not contemporary of the wreck, was also recorded.
The detailed photogrammetric survey not only preserved the site but also allowed for opportunities for analysis of the marine life that inhabits the wreck. The density of welk egg distribution and presence of sea urchins on the wreck itself contrasts sharply with their absence on the surrounding seabed. While photogrammetry will always struggle with moving subjects, static creatures between frames, such as crabs and squat lobsters, can be recorded.
Unfortunately, shortly after the site was documented, a group of divers removed all non-ferrous objects. To date, no recovery report has been submitted to the appropriate authorities, highlighting the issue of unreported recovery and the loss of cultural heritage. The loss of cultural heritage is a real issue, and without reporting, recovery is a crime. Digital preservation through orthophotos not only records but also has the potential to provide quantitative evidence of such activities.
The availability of cost-effective, time efficient, accurate, and repeatable photogrammetry has been a result of two technological advances. Firstly, the availability of high-resolution consumer grade digital cameras, and secondly the advances in desktop computing power, and specifically the processing available in Graphical Processing Units (GPU), have revolutionalised the field of Blue-Tech. Commercially available photogrammetry software, like 3DF Zephyr, leverages these advancements to deliver 3D and 2D outputs with exceptional productivity and efficiency. This technology is transforming the preservation, analysis, and interpretation of underwater sites, unlocking new insights that were previously unattainable.
The significance of underwater photogrammetry extends beyond the preservation and protection of artifacts; it enriches our understanding of history itself. Each sunken ship, lost aircraft, or ancient relic documented through this technique adds a new chapter to the story of humanity. These submerged sites are time capsules, offering unique insights into the lives, technologies, and events of bygone eras. By recording and studying them, we gain a deeper appreciation of our collective past and the interconnectedness of our global heritage.
As we continue to explore the depths with the aid of photogrammetry, we are not just preserving objects; we are capturing moments in time, safeguarding the narratives that have shaped our world. This meticulous documentation allows us to share these discoveries with the world, fostering a greater appreciation for the richness of our underwater cultural heritage.
Ultimately, underwater photogrammetry is a vital tool in the quest to preserve, protect, and understand our submerged heritage. It provides a means to document our history with unparalleled accuracy, deters heritage crime, and enhances our knowledge of the past. As we look to the future, embracing and advancing this technology will be crucial in ensuring that the stories hidden beneath the waves continue to be told, enriching our understanding of the human experience and preserving our shared legacy for generations to come.
References
[1] Brown and Henderson; Recording the SS Thistlegorm: Rapid Multi-Image Underwater Photogrammetric Survey of a Large Second World War Wreck
[2] http://crashsiteorkney.com/wildcat-jv751-scapa-flow.html
[3] https://www.bbc.co.uk/news/uk-scotland-north-east-orkney-shetland-39745240
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