Fairfax, Va. April 10-- A photomosaic technique developed by RSIS maritime archeologist Tane Casserley has evolved into a highly accurate supplement to the federal government's site documentation process for shipwrecks in U.S. waters.
Casserley's innovative technique, which he developed nearly five years ago, provides a method of quickly documenting submerged cultural resources and aids in their identification and management.
The History Channel recently aired the work of Casserley and a team of underwater archeologists in their month-long survey of shipwrecks in the Thunder Bay National Marine Sanctuary, near Alpena, Michigan. Casserley was supporting the National Heritage Program of the National Oceanic and Atmospheric Administration (NOAA), a customer RSIS supports on several
contracts.
Casserley's process involves a two-man dive team. One diver operates a Diver Propulsion Vehicle (DPV), a motorized vehicle used by divers to cover long distances underwater, and tows another diver behind him. That diver has a video camera pointed straight down at the shipwreck and uses a bubble level to keep the camera aimed downward. Both divers also wear depth gauges in order to maintain a pre-arranged depth during the survey.
Once back on the surface, computer software is used to pull still frames from the video which are then overlaid one by one to create an overall photomosaic of the site.
"While this generally produced an image of an entire shipwreck site, there were scaling problems if the shipwreck had differing levels of relief," Casserley explained. "To overcome this problem and to produce a more accurate photomosaic, SONAR depth sounding devices were attached to both the DPV and the video camera. The SONAR bounced sound waves off the bottom topography -- the deck of a shipwreck -- and the reflected sound waves were interpreted as a numeric readout in feet on the SONAR device. This insured that the camera remained a consistent height off the shipwreck and produced an accurate, scaled photomosaic of the site below."
Incorporating SONAR into the process yielded several photomosaics from the Thunder Bay National Marine Sanctuary, including the steamer Pewabic, which sank in 1865, and the schooner Cornelia B. Windiate, which sank in 1875. The work on the Windiate was featured in Deep Sea Detectives on The History Channel.
NOAA's Maritime Heritage Program is committed to preserving historical, cultural and archaeological resources within the National Marine Sanctuaries. The Maritime Heritage Program's mission is to protect, promote and explore the country's maritime heritage through a national program embracing heritage resources in evolving coastal, marine and Great Lakes stewardship.
