HMS Colossus: debris field survey 2004
The principal aim of this year’s survey was to investigate each of the magnetic anomalies identified in the ADU magnetic data. To achieve this the position of each anomaly was entered by USB link directly from computer to a hand held Garmin 76C GPS unit3, which avoided any possible keying errors in entering the positions manually. In practice the GPS unit proved to be extraordinarily accurate, the mean distance from shot to located iron object being only 5.5m over all dives where iron objects were detected. A validation dive was also made by dropping the shot onto one of the positions reported by Wessex Archaeology for their acoustic beacon array, deployed by them during their work on site in June-July this year. The acoustic beacons were anchored in position with sand bags, which remained on the seabed after the beacons were recovered, so the position was still apparent from the sandbags. The shot line was deployed at the position of one of these beacons4 - it was found to be only 0.75m from the sandbags on the seabed.
A 25kg shot line was dropped into the water when the GPS unit indicated it was 2m or less from the target position. A pair of divers descended the shot line and conducted a circular search of the seabed using a distance line marked in meters attached to the shot line. The position of any artefacts located was recorded by the measurement indicated on the distance line and a compass bearing taken with a hand-held compass back along the distance line to the shot. Any artefacts were also sketched, measured and described. All recording was done on pre-printed underwater sheets to assist volunteers in the recording process (see record sheet example appendix II). The resulting records were entered daily into the computerised record using 3H Site Recorder and AutoCAD for the master site plan. The mechanics of the search technique were perfected in two practice searches undertaken in Mounts Bay prior to the start of the project.
Great emphasis was placed on the need to search each area thoroughly. Seabed searches can be very difficult to undertake effectively. This is especially important when using volunteer recreational divers; each diver was repeatedly briefed on the need to search thoroughly and systematically. An illustration of the difficulties of searching is given by a dive by professional archaeological divers (Wessex Archaeology dive 205) in the Colossus debris field this year, where gun 7 and a 4m long iron spar were both missed. This is all too easy to do as objects are often obscured by weed and kelp.
3 The unit is EGNOS enabled (the European version of WAAS); when the EGNOS satalite is functioning the unit is said to be capable of accuracy of 2-3m.
4 Wessex Archaeology Dive 192, beacon B position 260038.83E 5535519.86N UTM zone 30, WGS84 datum. CISMAS dive DFS17.