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3D photogrammetric restitution of carvings and GIS applies: The case of the Fontana of Nettuno at Conegliano Veneto
Full Professor, DIIAR Director
2nd year Course of Doctorate
DIIAR Dipartimento di Ingegneria Idraulica Ambientale e del Rilevamento
Sezione Rilevamento -
Politecnico di Milano
Laboratory of "Rilievo, Cartografia Numerica e GIS"
P.za Leonardo da Vinci 32 - 20133 Milano
It is described the methodology to make the survey on a carving group chosen as a case-studio, the Fontana of Nettuno at Conegliano Veneto, with the most advanced instruments and technologies in order to acquire, to process and to manage data on the carving heritage which has a widespread diffusion on the Italian territory.
It has been used modern topographic instrument, SOKKIA Power SET2000 with target plate, to acquire control point on the carving, and the Digital Camera NIKON E2 to acquire the digital taken all around the carving. It has been compared the DTM generation by auto-correlation process with the model produced by laser scanning methodology using total station reflectorness no prism (TCRM 1101 Leica). Here are related the problems and results obtained building 3dmesh model (Digital Surface Model) to process the digital 3D orthophoto (software StereoView produced by Menci srl, distributed by NIKON). Today is mandatory to set up a project of conservation which satisfy the necessity to relate heterogeneous information, which come from different disciplines, to the decision processes: GIS can be really used as decision support system in the planning of ordinary maintenance and during the yard activity.
Particularly it has been studied how to use GIS technology to relate on a three dimensional object the analysis of the materials, the state of the art, the state of degree of the marbles and of the components of the carving, the analysis of the causes of degree, disruption, decay, weathering and deterioration.. Another topic of the research deals with the problem of the metric and costs computation, managed by GIS software (ArcInfo and ArcView - ESRI), in the cases of intervention in complex shapes like it is a carving, through 3D mapping of the degree.
The survey of the carvings constitutes from always one of the most complex problems than survey and of representation, than still today they ask the search solutions more advanced to the aim to concur the generation in times and sustainable costs of a product always better usable.
Normally carvings have been rendered with photogrammetric restitution to points. For example the Marco Aurelio (Campidoglio - Rome). This process is but much expensive and long one. This methodology is adapted to being used in extensive way only in special cases.
This methodology has but a great limit also in the degree of usability why it cannot constitute the base and the support to the specialists: it is unavoidably necessary to arrive to the generation of models in a position to integrating geometry with the semantic content of the photography.
Survey and 3D representation of the case-studio.
The research here related has been carried out in the Laboratory of "Cartografia Numerica e GIS" of the DIIAR in the Polytechnic of Milan. It is described the methodology to make the survey, on a carving chosen as a case-studio, with the most advanced instruments. It has been used modern topographic instrument (SOKKIA Power SET2000 with target plate) and the Digital Camera NIKON E2 to acquire the digital taken all around the carving.
If we have superficial not flat, in order to obtain metric photographies it is necessary to proceed to the construction of ortophoto maps. The oriented photograms, for being projected and being rendered metric have need of the construction of a three-dimensional model on whose to carry out this projection. The difficulty for a sculpture is just in the construction of the model: or it is returned to the manual restitution for points, too much expensive, or the various ways already used in several fields for the automatic generation of models are experienced. The study of the sculpture has evidenced the limits of the research that has been started, near the Polytechnic of Milan - DIIAR and is from considering not ended as far as the generation of ortophoto digital 3D on complex models.
The algorithm of the auto-correlation that is used in the modules of the software of digital photogrammetry and that the automatic generation of leaving from the oriented models concurs has given to outcome negative in this case limit: currently on a complex model therefore it becomes impossible to auto-correlate automatically corresponding points not identifiable; it is possible only for the more formalized parts, as the flank of the horses and as the test-model wire-frame inserted in the pictures. The illustrations document to all the phases of the digital photogrammetric restitution (inner guideline, external guideline, bundle-block adjustment, model 3D and ortophoto - Figg. 1/6).
The only photogrammetric way of the generation of the surfaces has been therefore placed side by side from the experimentation and topographical implementation and currently ulteriorly developable with the technology laser-scanning, that it is giving good results.
Also in the field of the representation new experimentations are opened: given a great amount of points all round as in the case of a sculpture the programs do not succeed to construct in automatic a complex surface, as instead they make for construction of the DSM (Digital Surface Model). On the statue this operation has been partially made manually on the left side, the head and the chin. This has concurred the creation of a partial digital ortophoto.
Technical Appendix: Phases of the survey
I. Topographic network: it has been made a external topographical network (Fig. 1)
composed from 4 points ground-marked along 2 main radials regarding the carving group.
Used instrument: total station T2000 Wild with high-precision EDM Wild DISTOMAT 2000 and prism reflector.
Precision: angular measurement 0.1 mgon (0.3"); distance 1 mm + 1 ppm (with prism).
II. Control points to support photogrammetric survey: approximately 300 control points have been taken from vertex of network to support operations of photogrammetric restitution: internal and external orientation of model, creation of rectified digital images.
Used instrument: total station SOKKIA Power set 2000 with 1 cm target plate.
Precision: angular measurement 0.5 mgon (1.5"); distance 1 mm ± 3 ppm (with target),
2 mm ± 2 ppm (with prism).
SQM control points (with target plate): "x" 0.17 mm, " y" 0.9 mm, "z" 0.41 mm.
III. Carving group topographical survey with laser scanning device: points on head of Nettuno have been taken with a 5 cm grid pass; approximately 2500 points have been taken from 4 control points of principal network (fig 3); XYZ coordinates have been referred to a single coordinate system and loaded to a CAD as point entity to obtain a "three-dimensional cloud" of carving group (Fig. 4).
Used instrument: motorized total station reflectorness TCRM 1101 1100 (TPS 1100).
Precision: angular measurement 0.5 mgon (1.5"); distance 3 mm ± 2 ppm (no prism),
2 mm ± 2 ppm (with prism).
SQM surveyed points (standard deviation): "x" 0.18 mm, " y" 2.87 mm, "z" 0.98 mm.
IV Photogrammetrical survey: digital calibrated camera Nikon E2 (resolution 1280 x 1000 pixel - optical aperture f/6.7 f/38) has been used to the takes of tank of fountain and carving group.
For all the 12 sides of fountain has been realized a rectified digital images (omographic processing - figg. 5/6). Also has been realized a partial digital ortophoto of left side of the carving (Fig. 7).
Used software: Menci - Nikon Stereoview (old release).
Today is increasing the opportunities offered in the data acquisition and in the processing of the same thank to the use of the advanced methodologies and technology: topographic survey of vertical sections and surfaces of the manufacts with total station no prism, the agile acquisition of images (metric, semi-metric and not calibrated cameras, digital and not digital), the simplified methodologies of survey and the digital photogrammetric restitution obtained through digital Software. It has been explained the logical structures and models set up through relational data bank in order to create different archives of data: the aim is the data management to support the different phases of the project of conservation and of the Cataloguing of the monuments or objects in the museums: The intervention become the natural outcome of the levels of the single and related knowledge, and the GIS can be the natural basin to document the history of the carving, not only during the past but also in the future, recording the phases of the restoration process, the alteration and changes in the time.