1.3 Logistics, objectives and choice of methods
All on-site work at Aanloop Molengat was characterised by the exposed location close to a lee shore and at two hours’ sailing distance from the nearest harbour. Underwater time was scarce because of the substantial risk of adverse conditions. In view of this, the decision was taken to make the best of the occasional optimal conditions and allow for step-by-step progress in a repeated hit-and-run strategy with a relatively small vessel. This meant accepting slow progress. The approach demanded great flexibility but was realistic in terms of staffing and direct expenditure. It tied in well with the objective of engaging the original discoverers and the local and diving communities, and with using SCUBA as the diving technique. It is a completely different approach than one would choose under time pressure or under development-led circumstances, where it would be appropriate to deploy a large support vessel or platform and be less weather dependent.
With the aim of analysing the packing and stacking of the cargo material, it was considered essential to document the position and orientation of each item in three dimensions. In view of long interruptions to the work, it was not feasible to establish stable, retraceable and reusable reference points on and away from the wreck mound as required for direct survey (Lundin 1973; Adams 1986). Methods of trilateration, or combined measuring and sketching are reliable but time-consuming (Maarleveld 1984). Tidal currents and height differences interfere with direct measuring over distances exceeding a couple of metres. Considering the occasional occurrence of relatively good underwater visibility, it was decided to structure documentation around photography instead. This would fit in well with the hit-and-run operational strategy, and photogrammetry was a developing field that showed great promise (Maarleveld & Vos 1989).
A steel frame of 32 x 11 m was lowered around the wreck mound at the start of the 1986 season (fig. 6). It was oriented along the measuring poles of the 1985 survey and served as an anchor for the shot lines at each corner, as well as for the floating crossbar along which the photogrammetric camera could be moved. The bar was inspired by the system that replaced a photo tower in the Madrague de Giens excavations (Gianfrotta & Pomey 1980), but needed to be much more robust to meet North Sea conditions. Moreover, flashlights were needed and it was decided not to rely on a single-lens camera, but to ensure the creation of a stereo-pair at each shot by using a double-lens camera (Hasselblad/Ocean Optics MC-70).
It was unclear how many layers would need to be recorded and correlated. The depth of the find layer had been established with a handheld Kyholm corer (Nørnberg & Christensen 1980). Probing into the wreck mound was not possible.
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Figure 6 The steel frame at the quay in Den Helder, prior to being sailed out and lowered around the wreck-mound. Note the staggering, colour-coded eyelets at 25 cm intervals along the side of the rectangle. These are meant for attachment and navigation of the floating crossbar that holds the camera. They also served as help in drawing the site plan, in which they are indicated with numbers. The four-legged safety booth that was to be attached to the frame can be discerned in the background (photo: A. Vos (RCE)).
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