Repeated object reconstruction of the Bremen Hanse Cog

Abstract

Today the Bremen Hanse Cog is on display in the German Maritime Museum (Deutsches Schifffahrtsmuseum) in Bremerhaven. The cog was found in October 1962 in the river Weser and was prepared in conservation liquid between 1982 and 1999 (DSM, 2003b; Hoffmann, 1981). In 1982 the Institute of Photogrammetry and GeoInformation (IPI) used a metric camera SMK 120 to take about 80 analogue images of the cog from several directions. With additional geodetic measurements, signalised vertical lines and tie points the three-dimensional point cloud was calculated and line drawings from three directions of the cog were plotted. After 17 years of conservation distortions in the range of +/-10 - 25 cm of several parts of the wooden cog were detected. In 2003 over 100 images were taken with digital cameras to carry out the photogrammetric object reconstruction and calculation of a point cloud of the cog again. To reconstruct the profiles measured in 1982 the profile lines have been visualized with white tape comparing the historic analogue drawings with the current surface of the ship. In 1982 the three dimensional data was calculated from overlapping stereo image pairs. Today the point cloud has been calculated by image coordinate measurements in convergent digital images and the application of the adjustment software PhotoModeler 5.0 Pro from Eos Systems Inc. This paper deals with the documentation of the photogrammetric measurement and adjustment process, the visualisation of results and the comparison of the measured profiles of the cog after 21 years.