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Chapter 7 - Bremen Cog: Three Recording Techniques for One Object
- Edited by Anna Bentkowska-Kafel, Lindsay MacDonald
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- Book:
- Digital Techniques for Documenting and Preserving Cultural Heritage
- Published by:
- Amsterdam University Press
- Published online:
- 26 January 2021
- Print publication:
- 08 January 2018, pp 121-140
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- Chapter
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Summary
ABSTRACT
The Bremen Cog is a ship which 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.
Despite its age, the cog was well preserved and the steerboard side was almost complete. The reconstruction of the missing parts seemed therefore possible and of interest to the public.
The museum wanted to give a new lease of life to the 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 the three-dimensional monitoring of the ship's current 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 the participatory education of the public. This model of the ship will enable its continued public presentation, through a virtual surrogate, during the building works. The case study involved different tests: photogrammetry and Structure from Motion acquisition, 3D scanning and total station. Data analyses were also carried out.
Keywords: Deformation monitoring, large-scale objects, conservation, restoration, laser scanning, total station theodolite, photogrammetry, COSCH
Introduction
The Bremen Cog was discovered in 1962 near the city of Bremen, Northern Germany, in the river Weser. The decision was taken to salvage the ship. Most timbers, together with finds, were recovered from the river bed between 1962 and 1965. Thanks to dendrochronology analyses, the ship timbers were dated to the second half of the fourteenth century. Following a six year reconstruction at the purpose-built German Maritime Museum in Bremerhaven, the Bremen Cog was found to be 24 m long, 7 m wide and 4 m high. Its conservation took almost twenty years before it was possible to present the ship to the public in 2000. About a year later, the first signs of deformation occurred. After different trials to design a new support system for the ship, the museum decided to work on a system that would feed information to the future support girdle. At that point the relevance of designing a non-invasive, long-term deformation monitoring system became clear.
Total Station Surveying
- Edited by Anna Bentkowska-Kafel, Lindsay MacDonald
-
- Book:
- Digital Techniques for Documenting and Preserving Cultural Heritage
- Published by:
- Amsterdam University Press
- Published online:
- 26 January 2021
- Print publication:
- 08 January 2018, pp 253-256
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- Chapter
- Export citation
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Summary
COSCH Case Study that has employed this technology: Bremen Cog
Definition
A total station is a surveying instrument using laser light. The distance between the instrument and the target is measured and recorded digitally. It is considered a direct surveying technique because the operator chooses the acquired points manually and defines them in advance. The degree of accuracy of each point remains high, but the global accuracy of the acquisition varies depending on the operator's methodology applied in a given context. A multitude of points, called a point cloud, will be produced providing geometrical data. This technique is very often used on building sites, as well as on archaeological excavations to measure large distances and establish maps.
Description
The total station is a composite technology, which allows selective recording of 3D coordinate points without direct contact between the instrument and the subject. The device integrates the functions of a theodolite (transit) for measuring angles, with an electronic distance meter (EDM) and a digital recorder. Angles and distances are measured from the total station to points under survey, and the spatial coordinates (X, Y, Z) are calculated using trigonometry and triangulation. The final output is a sequence of points with three-dimensional coordinates in relation to a local or geographical reference system.
The user has no visual feedback or control of the ongoing acquisition, until the process is completed and the data stored on the internal memory. However, there is a different way of using this instrument, as demonstrated in the COSCH case study of the Bremen Cog (chapter 7). The setup introduces a new element in the workflow, consisting of data acquisition, in real time, through software which enables a direct communication between the total station and the host computer. The innovation lies in the use of 3D CAD software (Rhinoceros 3D) which can communicate directly with the total station through a plug-in, Termite, developed by Frederick Hyttel, a former student of the Maritime Archaeology Programme in Esbjerg, University of Southern Denmark. The most useful feature of Termite is the ability to resect the total station data on the fly. “Resection involves the computation of instrument position via observation of two or more reference marks or stations of known position… . Once the instrument has been moved to a new position, these marks can be re-observed to determine the new station coordinates” (Andrews et al. 2009, 9).