Proceedings of the CIPA WG 6 International Workshop on Scanning for Cultural Heritage Recording

Böhler, Wolfgang

Bibliographic data

Monograph

Persistent identifier:: 830281592

Title:: Proceedings of the CIPA WG 6 International Workshop on Scanning for Cultural Heritage Recording

Sub title:: September, 1 - 2, 2002, Corfu, Greece

Scope:: 159 Seiten

Year of publication:: 2002

Place of publication:: Thessaloniki

Publisher of the original:: Publ. ZITI

Identifier (digital):: 830281592

Illustration:: Illustrationen

Signature of the source:: T 15 B 1303

Language:: English

Additional Notes:: Literaturangaben

Usage licence:: Attribution 4.0 International (CC BY 4.0)

Editor:: Böhler, Wolfgang

Corporations:: International Workshop on Scanning for Cultural Heritage Recording, 2002, Kerkira; ICOMOS/ISPRS Committee for Documentation of Cultural Heritage

Adapter:: International Workshop on Scanning for Cultural Heritage Recording, 2002, Kerkira; ICOMOS/ISPRS Committee for Documentation of Cultural Heritage

Founder of work:: International Workshop on Scanning for Cultural Heritage Recording, 2002, Kerkira; ICOMOS/ISPRS Committee for Documentation of Cultural Heritage

Other corporate:: International Workshop on Scanning for Cultural Heritage Recording, 2002, Kerkira; ICOMOS/ISPRS Committee for Documentation of Cultural Heritage

Publisher of the digital copy:: Technische Informationsbibliothek Hannover

Place of publication of the digital copy:: Hannover

Year of publication of the original:: 2015

Document type:: Monograph

Collection:: Earth sciences

Chapter

Title:: 5. PROJECTS AND EXPERIENCES

Document type:: Monograph

Structure type:: Chapter

Chapter

Title:: EXPERIENCES WITH LASER SCANNING AT i3mainz. Andreas MARBS

Document type:: Monograph

Structure type:: Chapter

- 112 - the facades is not sufficient. Laser scanning provides a fast and easy way to get a detailed model of the surface for the creation of orthophotos. 3.3 Cenotaph The cenotaph (empty tomb) of Emperor Maximilian I. is situated in the Hofkirche church in Innsbruck, Austria. It was created by various artists during the 16 th century and is known as the largest and most important mausoleum of the Western World. The sarcophagus itself (2.5 x 4.5 x 2.0 metres) is made of black marble and bronze. It is ornamented with 24 finely carved alabaster reliefs (80 x 50 cm 2 ) depicting scenes of Maximilian's life. A life-size bronze statue of the emperor is kneeling on top of the sarcophagus as well as four smaller bronze statues (the four cardinal virtues), which are situated near the comers. The first restoration attempt since the completion of the cenotaph (around 1568) started in January 2001 and is still in progress. During May 2002 there was a one-week break between two phases of restoration where the cenotaph was freely accessible, without any lattice or glass, for the first time ever. It had to be made the most of these few days by documenting this unique cultural heritage with state-of-the-art technology, before the cenotaph was hidden behind lattice and glass again, probably for the next 500 years. Figure 4. Left: Cenotaph in the Hofkirche church of Innsbruck. (Photo: AEIOU) Right: Uncovered sarcophagus with reliefs Measurements: In addition to traditional close range photogrammetry, carried out by an Austrian company specialised on this matter, i3mainz was engaged to record the cenotaph’s shape with laser scanning methods. For this purpose two scanners were used. The idea was to use the Mensi SOISIC scanner to record the plain geometry, including marble parts (steps, frame and lid) and bronze statues. Scanning the alabaster reliefs with details smaller than one millimetre by using this scanner is not sensible. Instead, the GOM ATOS II scanner was brought into action. A common coordinate system had to be created for the three measurements (photogrammetry, Mensi, GOM). Therefore, tacheometric measurements were carried out. Twenty special red spheres were placed on the cenotaph as connecting points to register the single point clouds from each scanner viewpoint. The spheres were measured by aiming at the left, right and upper contour and calculating the direction to the centre (the lower margins could not be measured due to the spheres’ bases). The photogrammetrists placed their adhesive reflecting targets on the cenotaph. Eight observation points for the total station were selected around the cenotaph. After measuring angles to all targets (observation points, spheres, photogrammetric targets) and using a calibrated invar rod as a scale, a common net adjustment was computed, which resulted in 3D coordinates with a standard deviation of less than 0.5 mm. Problems: In the planning phase of the measurements it was not clear, to what extent the material and colour of the object would influence the results of scanning. It was considered to spray the crucial parts with some kind of chalk or powder. The best conditions for both scanners, Mensi and GOM, are a dark environment and an object surface, which is bright and smooth. On site first the black parts of the cenotaph (marbled frame) and the bronze statues were tested with the Mensi scanner - with acceptable results. Some points were missed, but altogether the scanner worked very well as long as the sun was not shining directly onto the object. The GOM scanner had more problems with daylight and the black surface. The projected light pattern was completely absorbed by the marble, but the alabaster reliefs could be scanned properly, of course. Scanning with Mensi SOISIC: The whole cenotaph was scanned with an average point width of 2 mm. For each pair of reliefs one scan was applied including the neighboured parts of the black marbled frame and the steps below the sarcophagus. By using this method, the important parts of the marbled frame were always scanned from two directions which leads to less hidden areas. Moreover, the scanner was placed on a scaffold to scan the upper part of the cenotaph with the lid and the five statues. Each scan contained at least four spheres for registration. All in all, 20 scans were applied during four days and nights. One single scan took up to six hours (3-4 hours as an average) depending on the size of the scanned area. Figure 5 shows the registered point cloud of the whole ceno taph. Altogether, more than 10 million points were recorded, which is a large amount of data even for powerful PC’s. Figure 5. Registered point cloud of the cenotaph (shaded view) Scanning with GOM ATOS II: Scanning the reliefs with ATOS II turned out more difficult than originally expected. To reach a maximum of accuracy and detail, the scanning volume has to be very small. In order to maximise the precision of the model, the reliefs had to be divided into several tiles - each part had to be scanned separately. This was possible by sticking special reflecting targets to the object and using those to stitch

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