Underwater RTI HMS Invincible

David Selmo — Fri, 21 Jun 2013 16:13:49 +0000

On February 19^th, 1758 the HMS Invincible set sail out of Portsmouth Harbor for Canada to join the British fleet to fight the French. Unfortunately, due to a domino effect of extraordinary bad luck, she ran aground on Horse Tail Sand not far from the harbor. Invincible was a French ship launched in 1744 and captured by the British in 1747. At that time the British out-number the French on the sea. The French compensated with a new 74 gun class built specially for speed and maneuverability. The Invincible was the first of this class and ‘state of the art’ technology of her day. While in the service of the Royal Navy her advantages were quickly recognized and she became the template that British ship design immediately began to emulated. Her maritime cultural significance, therefore, earned her a position on English Heritage’s list of 46 protected wreck sites in British waters. I had the privilege to dive her this week under the leadership of Dan Pascoe of the Nautical Archaeology Society, my advisor Dr. Fraser Sturt, and some of my maritime masters archaeology classmates. My personal objective was to attempt the first ever successful PTM file from a URTI dataset collected on an actual underwater archaeological site. To honor my instructor who taught me wooden boat part recognition, Dr. Julian Whitewright (of the HWTMA and University of Southampton CMA) I selected an Invincible ‘trenail’ to RTI.

Invincible’s outer planking is significantly thick; more resembling ‘timbers’ than planks. These huge planks were held to the frame timbers by cylindrical billets (‘trenails’) that were hand-hewn with a draw knife to a dowel shape, split at one end, and then driven with a mallet through a pre-bored hole that went clear through the planking into the frame member. Once the trenail was driven flush pinning the plank to the frame, it was then secured with a wedge of hardwood (usually English Oak) in much the same fashion an iron hammer head is secured to a Hickory handle.

Carrying a Fuji Finepix F200EXR auto-focusing underwater camera, a Bembo Trekker Mk3 tripod, a Diverite 15w 1000 lumen HID wreck diving light, and a Cultural Heritage Imaging RTI reflectance ball on the end of a threaded rod, Amy Astley (PhD student studying the taphonomy of historic shipwreck sites with the University of Southampton’s Department of Ocean and Earth Science ) and myself exited the stern of the Shogun (http://www.msdsmarine.co.uk/) into the 16c water of the Solent and made our way to a part of the wreck that has a large section of exposed planking. It did not take long to locate a trenail and setup up for its URTI dataset capture. In c. 8 meters of water with a southeasterly rising tide bringing dirty water in from the Solent at a rate of half a knot (approximately 1 foot per second), I struggled to stay in position while making this data capture. A half knot current is enough to gently blow an inexperienced diver off the site. I was entirely surrounded by smooth planking and even though I wore extra lead weight to help keep me down for the task, it was all I could do to hold onto an archaeological marker tag with one finger and freehand the light positions with the other hand. Periodically, I had to pause while debris passed by or momentarily got stuck on the trenail. In the end, and much to our amazement the camera did not move and the capture was a success.

These JPEG images are screen captures from the first ever PTM file generated from an underwater archaeological field environment. This trenail has been submerged for 255 years. It is c. 3 cm diameter. You can easily discern the longitudinal grain patterns of the wedge, the radial grain pattern in the trenail, and the clear signs of site degradation from marine wood boring micro-organisms on a surface that Dan Pascoe testifies was ‘prestine, just one season ago.’ A big thank you to Amy, the crew of the Shogun, Dan Pascoe, Dr. Sturt and my classmates for a fantastic and memorable day of diving. One more small step forward for URTI.

R. Dan Pascoe. C. Dr. Fraser Sturt. L. David Selmo on board the Shogun anchored near the HMS Invincible.

Raw image in the PTM file dataset. The reflectance ball is on a threaded rod held down with a soft-pack diver lead weight. HID hand-held light is oriented from the upper left corner of the photo.

Screen JPEG capture of the finished PTM file with slight specular enhancement transformation applied.

HMS Invincible

Show embedded map in full-screen mode

Location of HMS Invincible

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Underwater Reflectance Transformation Imaging…a success.

David Selmo — Tue, 28 May 2013 15:14:25 +0000

I’ve been working on Reflectance Transformation Image capture in a sub-aquatic environment. On 2 May, 2013 the first ever PTM file from an RTI dataset captured entirely underwater was successfully processed in the Archaeological Computing Research Group computer lab at the University of Southampton using RTIBuilder software. Images were captured in 1.5 meters of water at the campus Jubilee pool using a common 12mpix digital camera and a 15 watt 1000 lumen HID dive-light. The object photographed was a chunk of 18^th century wooden ship ‘deadeye’ rig blocking found in the Itchen River in downtown Southampton. The experiment was the first in a research design I’ve authored to assess the viability of ‘Underwater RTI’…or what I’ve dubbed ‘URTI’. Approximately 1540 photographs were taken over a 3-hour period in the pool. The light radius was controlled by a piece of string spinning freely under the object via a heavy steal cylinder, and arc positions (4-6 per section, 13 sections) were entirely free-handed. Seven independent sets of data were captured, and although only one of the sets produced acceptable results, the experiment proved the viability of RTI underwater. Further research is forthcoming to continue the viability of RTI as an underwater diagnostic tool. This summer I will be conducting baseline feasibility studies for the further development of technology and methodology in support of Reflectance Transformation Imaging in sub-aquatic environments.

Dave Selmo notes camera settings and lighting configuration.

Raw image capture from ‘set 4 of 7′ used in the processed PTM file.

Camera mount improvised from dive shed rubbish and a 30″ Stilson pipe wrench.

JPG capture of the PTM file, diffused gain transformation applied.

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David Selmo – Shipwrecks and Submerged Worlds

Underwater RTI HMS Invincible

Underwater Reflectance Transformation Imaging…a success.