Boulders were either moved to an adjacent area away from the excavation, or, in some situations, depending on the available space, they were returned to the excavated area on completion of excavation work in order to backfill the excavated holes. These were removed by placing them into one-tonne-bulk-container bags and then moving these with the use of air bags, or, in the case of large boulders, by straps connected directly to the air bags. The site contained many rocks of boulder size between 25 kg and 1500 kg in weight. Spoil was removed by airlifts and by hand in areas with delicate material. Surveys of this nature should achieve RMS residuals below 30 mm under normal diving conditions, so the results are better than expected given the considerable wave surge experienced on site.Įxcavation involved the controlled removal of all sediment and finds within the area down to bedrock.
After the adjustment, the measurements fit together to a precision of 15 mm (RMS residuals), with 51 distance and depth measurements used to calculate 15 CP positions. The distance and depth measurements were adjusted using Site Recorder 4 software, which calculated the best estimate of position for each survey control point. The position of each excavation frame was then calculated by taking three or four distance measurements and a depth measurement from the top of two diagonally opposite corner posts. The CPs were 8 mm-diameter mild-steel reinforcing bar, hammered into cracks in the rock and marked with yellow plastic discs. The position and orientation of the frames was determined using conventional 3D tape trilateration (Direct Survey Method) referenced to a network of 15 fixed survey control points (CPs) installed on the seabed and positioned relative to each other, and in real world coordinates (WGS 84), using a combination of distance and depth measurements plus position measurements derived from GPS. All finds recovered from each square were individually numbered in relation to their grid position. The frames were positioned as close to the seabed as possible and supported by adjustable legs. The frames resulted in a series of 3 x 3 m grids that were further separated into nine 1 m squares using rope lines fixed with elastic cords.
The wreck site was divided into a series of adjoining 1 metre grid squares using frames made up on location from 3 m-long, 48 mm-diameter steel pipes joined at the corners by cross Kee Klamp® frame joints.
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