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Bremen Cog. Three recording techniques for one object

Amandine Colson and Levente Tamas

ABSTRACT

The Bremen cog was discovered in the River Weser close to the city of Bremen in 1962. Based on dendrochronological examination, the cog was built in 1380. It was successfully conserved and restored with polyethylene glycol (PEG) at the German Maritime Museum in Bremerhaven and has been on display since 2000.

The news of the cog’s discovery was a sensation. It was the first vessel of the Hanseatic League to ever be excavated. In fact, this type of ship was only known from town seals and paintings in medieval manuscripts. Despite its age, the cog was well preserved. The steerboard side was almost complete. The reconstruction of the missing parts seemed therefore possible and of interest to the public.

At 24 m long, 7 m wide and 4 m high, this medieval vessel belongs to a group of the largest archaeological ships exhibited in Europe. It is an object of great significance for archaeologists, but also for conservation scientists and conservators worldwide. It attracts many visitors.

The German Maritime Museum in Bremen opened some 40 years ago and is now competing against newer models of tourist attractions in the town. As a member of the  Leibniz Association research network, the Museum actively follows the media revolution in the museum sector. Combining research objectives with public interest is on top of the Museum agenda. But how to achieve it?

The Museum aimed to give a new lease of life to the Bremen Cog and allow the public to see it from different viewpoints, while  also communicating research results. The case study, described in this chapter, focuses on three-dimensional monitoring of the ship present condition, the understanding of wood deformation that occurred in the past and preventing future changes. Another objective was to make a 3D computer model, based on the same digital data, for participative education of the public.

Testing different methods and comparing their long-term effectiveness was regarded necessary but, in order to develop feasible protocols, a number of experts with the appropriate qualifications and equipment were required. A case study carried out through the Colour and Space in the Cultural Heritage (COSCH) involved different tests: photogrammetry and Structure for Motion acquisition (Julien Guery); 3D scanning (i3Mainz) and Total Station (Massimiliano Ditta). Data analyses were carried out by Levente Tamas.

The 2016 saw a start of an extensive renovation campaign of the Museum. A 3D model of the ship will enable its continued public presentation, through a virtual surrogate, during the building works. The case study has led to Ph.D. research of Amandine Colson.

Keywords: Deformation monitoring, dissemination, large-scale objects, conservation, restoration, terrestrial laser scanning, total station theodolite, photogrammetry, COSCH

FIGURES

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Fig. 1: Bremen Cog, in 2000 shortly after the opening. © German Maritime Museum

 

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Fig. 2: Target on the one concrete pillar, Bremen Cog, East side

 

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Fig. 3: Bremen Cog, room plan, common coordinate system, ground floor ©Amandine Colson

 

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Tab. 1: Comparison of all photogrammetry acquisition ©Amandine Colson

 

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Fig. 4: 3D Model Photogrammetry ©Julien Guery

 

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Fig. 5: Pre-alignment with key point-features and refinement with ICP using two different laser scans – © Levente Tamas

 

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Fig. 6: The whole body and the cross-section comparison for two different laser scan measurements (from 2009 and 2015) – © Levente Tamas

 

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Fig. 7: Chart “Expertise Model” – © Amandine Colson

 

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