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

-14- directly on the square of the distance between the camera and the object. A high precision is therefore achieved by suitably limiting this distance, and thus the illuminated area. The acquisition of relatively large objects, as a statue of human size, requires a large number (typically hundreds) of partial views (or range maps), taken all around the object, that have to be integrated through a complex alignment procedure in order to represent the whole surface. Using portable equipment, employed to scan Heritage artworks kept in museums, such range images are taken from unknown locations, and the coordinate system of each point cloud is referred to the range camera. In order to refer all the acquisitions to the same coordinate system, adjacent range maps are acquired with partial superimposition (at least 30%), and rototranslated by means of special software algorithms that minimize the mean square deviation or find a best match between overlapping points of adjacent maps (Besl, McKay, 1992; Chen, Medioni, 1992). 2.2 TOF based 3D scanning Another class of 3D acquisition systems - often indicated in the photogrammetry world with the term “laser scanner” - is based on the simultaneous measurement of the time of flight of light pulses sent towards the object, giving the distance between the scanner and the measured surface, and the vertical and horizontal angles. The kind of output is similar to that attainable with a triangulation scanner. The main advantage is that long-range imaging is possible, thus allowing large structures to be imaged. However, the achievable accuracy is relatively low, never better than a few millimeters, but commonly resulting in the centimeter range. Also in this case a procedure of partial views alignment is required. 2.3 Digital Photogrammetry Photogrammetry is a well consolidated technique, that has a long history of developments for over a century. Digital photogrammetry is the product of an intensive research conducted for the last 20 years for the automation of information extraction from digital images, based on image analysis methods. Photogrammetry is based on the acquisition of multiple images of the object to be measured, taken from point of views sufficiently apart by a photo camera. The method requires the selection of recognizable elements on the acquired images, including reference targets of a known size. Once the selection of homologous points has been performed, the 2D coordinates on the photogram plane are processed using an algorithm of bundle adjustment that allows the corresponding 3D coordinates to be estimated. Photogrammetry results generally suitable to large scale measurements. Close-range photogrammetry cannot be used for camera-object distance lower than 1-2 meters. The process of the identification of the points to be measured, being manual or semi-automatic, requires a long a tedious work if the number of points is elevated. The available technology is largely used in the metric survey of architectures and archaeological sites (El-Hakim et al., 1996). A unique feature of photogrammetric techniques is its extremely high accuracy, that ranges from 100 ppm to 10 ppm of the overall dimension of the imaged area. 3 3. CASE STUDY: BAPTISTERY OF FLORENCE The object of our study was a Romanic mosaic and part of the surrounding area, located in the underground of the Baptistery of Florence, at the north-west comer. The measurement conditions in such environment are particularly difficult: the bad conservation state after the early dislocation to a museum of the most valuable pieces (1912-15, 1925) and the difficult access don’t make easy the monument understanding. Surveys based on overall measurements and inaccurate photography’s compositions have been made up to 1990, but due to the complexity of the archaeological site such kind of measurements and are prone to artificial regularizations, giving possible fake results. The first systematic studies over these structures were started in 1990 and published later (Marino et al., 1996). The mixing of mosaic portions with wall structures of different ages makes difficult the historical interpretation of the artifact. In this case the differences in the pavement construction techniques, such as stones shape and cutting methods or mortar characteristics and thickness, are the only elements for an appropriate reading of the monument. In the examined room a portion of a wall adjacent to the mosaic is semicircular and is sheltered by the foundation walls of the apse. All the room walls are made with three different layers: the lower one has a regular structure, the intermediate - less homogeneous - shows slid connections, while the upper one is mainly made by conglomerate. On the roof lie a mosaic fragment representing geometrical drawings and made by small black and white tesserae, of the roman period. 4. METHODS AND INSTRUMENTATIONS A well established praxis in studying an artifact like the one previously described is to make topographic surveys integrated with photographs and 2-D drawings (generally 1:10 scale) showing elevations. For the smaller details a contact relief on polyvinyl is generally used, where each mosaic tesserae or the overall mosaic drawing are taken. In this way important mosaic details are acquired. From these raw data some papers are worked out, such as thematic maps of the materials employed, of the gaps, of restorations, of the conservation state, etc. A tridimensional relief is often acquired through molds made in silicon or glass-reinforced plastic in order to accurately evaluate the presence of roof depressions. The introduction of advanced 3D measurement technologies seemed therefore very advantageous for eliminating all the manual steps typically employed for this kind of survey. 4.1 Topographic survey 11 points have been identified on the mosaic surface, and marked with reflecting targets. Their topographic coordinates have been measured with a steel band for planimetry, and with geometric leveling performed with a digital level meter Leica NA3003 and invar stadia bar for the points height. 4.2 Digital photogrammetry Photogrammetry in this application act as the “global reference” supplying a single coordinate system for repositioning piece of data coming from other sensors. Such global reference is then reoriented to the absolute reference defined by older surveys. In order to have a good level of redundancy for measurements control, it was decided to make photogrammetry with two different kind of equipment. 4.2.2 Semimetric camera: A semimetric camera Rollei 6006 (6x6) was employed with a calibrated lens with 40 mm focal lenght. Each photogram, characterized by an average scale of 1:40, was then acquired with a calibrated Mirage scanner capable of 1400 DPI.

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