Archives; XXII Congress of the International Society for Photogrammetry and Remote Sensing: Commission IV

Ogleby, Cliff

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Multivolume work

Persistent identifier:: 85670931X

Author:: Ogleby, Cliff

Title:: Archives; XXII Congress of the International Society for Photogrammetry and Remote Sensing

Sub title:: Melbourne, Australia, 25 August - 1 September 2012

Year of publication:: 2012

Place of publication:: Deakin West, ACT

Publisher of the original:: International Society for Photogrammetry and Remote Sensing

Identifier (digital):: 85670931X

Language:: English

Document type:: Multivolume work

Volume

Persistent identifier:: 856718505

Author:: Ogleby, Cliff

Title:: Commission IV

Sub title:: Melbourne, Australia, 25 August - 1 September 2012

Scope:: XIV, 588 Seiten, Seiten XV - XXIX

Year of publication:: 2012

Place of publication:: Deakin West, ACT

Publisher of the original:: International Society for Photogrammetry and Remote Sensing

Identifier (digital):: 856718505

Illustration:: Illustrationen, Diagramme, Karten

Signature of the source:: ZS 312(39,B4a)

Language:: English

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

Publisher of the digital copy:: Technische Informationsbibliothek Hannover

Place of publication of the digital copy:: Hannover

Year of publication of the original:: 2016

Document type:: Volume

Collection:: Earth sciences

Chapter

Title:: [IV/6: Global DEM Interoperability]

Document type:: Multivolume work

Structure type:: Chapter

Chapter

Title:: DTM CROSS VALIDATION AND MERGING: PROBLEMS AND SOLUTIONS FOR A CASE STUDY WITHIN THE HELI-DEM PROJECT. Ludovico Biagi, Laura Carcano, Mattia De Agostino

Document type:: Multivolume work

Structure type:: Chapter

- 141 - characteristics of the objects can be used for further analogical or numerical computer assisted interpretation and automatic features extraction. But no such device is already available now. The other principle that allows optical or optoelectronic 3D recording of archaeological objects is the triangulation, i. e. the fact that the distance between a point and an observer can be determined through the change of position caused by this point to a laterally projected light pencil. This principle is used in many relief analysis techniques, among which the very well- known linear scanning method. The linear scanning method can be summarised as follows: a laser line is projected on the studied object and appears distorted by the relief of this object when it is seen with an angle from the incident direction. By scanning the object surface with an accurate positioning system, it is possible to get the depth information line by line and then to reconstruct a 3D representation of the object. This technique may only examine one scan at a time. So it is limited by the accuracy of the positioning system and the way to create the scanning line. Many tests have revealed that, because of this limitation, it is very difficult (and sometimes impossible) to use a laser scanning system with high precision in aggressive environmental conditions like those found in many archaeological sites. The possible applications in the field of Archaeology are thus limited. The main problem of the scanning method, that is the problem of moving precisely the scanning laser line, can be eliminated by generalising the triangulation principle: instead of one single line, it is a constructed pattern or a grid that is projected. Its deformation is then used as the probe of the relief. This light pattern has to be periodically structured and static, based on a grid alternatively light and dark. One projects it on the surface to analyse. By recording the scene with a CCD camera, it is possible to superpose the image of the grating modified by the relief with the reference one without any deformation. This process, which replace the comparison between the distorted laser line and the original one, creates geometrical shapes (Figure 2); it is the moiré effect, whose fringes pattern is closely connected with the relief of the analysed object. Everyone has already seen this effect, often ignoring it: when somebody on television wears a striped or squared clothe, this pattern, modified by the anatomy of the person itself, interferes with the pixels grid of the camera and generates a moiré effect. The interpretation of these moiré pictures through the triangulation principle gives the whole relief information of the analysed object (Figure 3). The accuracy of this technique is comparable with the accuracy of the traditional laser scanning but its process is much more faster, since a surface of one square meter can be analysed in one time, with a single shot. Besides, the elimination of the problems linked to the precise and regular moving of the laser line makes it much easier to use in difficult in situ conditions. So the moiré technique perfectly fulfil the requirements of archaeological recording: fast acquisition, accuracy, robustness and flexibility, necessary to allow working on site, in aggressive environmental conditions. Moreover, by combining moiré and photogrammetric approaches, it is possible to define the recorded 2.5 surface shape in an euclidian cartesian reference system that allows metrology and merging thematic and geometric information from the object in this system. Figure 2. Moiré effect on an Ancient Egyptian Relief (copy of the relief Brussels MRAH E 2157) Figure 3. Detail of a 3D recording of the same relief with the projected moiré technique 5. CONCLUSION: THE AIMS OF THE OSIRIS PROJECT In conclusion of all this, the ICAUL (Interdisciplinary Center for Archaeometry of the University of Liège), with Hololab Laboratory and SURFACES (Service Universitaire de Recherches Fondamentales et Appliquées en Cartographie et Étude Spatiales) Laboratory, of the University of Liège, have decided to develop together a complete portable set-up (making the whole optoelectronic acquisition and the data processing) specifically dedicated to the quick and accurate numerical 3D recording of archaeological documents. It will use the projected

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fullscreen: Commission IV (Part 4)

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