C. Atzeni

3D digitizing of cultural heritage

Abstract In this paper, the authors briefly review the state of the art of the 3D acquisition and digitizing techniques applied to heritage. The main focus is on motivations, issues and technical specification of the 3D digitizing of heritage artworks. Different digitizing technologies currently available for this specific application have been evaluated and tested, with application to a pair of case studies, showing that 3D digitizing technologies are sufficiently developed for extensive application in the field of cultural heritage.

High-accuracy 3D modeling of cultural heritage: the digitizing of Donatello's "Maddalena"

Three-dimensional digital modeling of heritage works of art through optical scanners, has been demonstrated in recent years with results of exceptional interest. However, the routine application of three-dimensional (3D) modeling to heritage conservation still requires a systematic investigation of a number of technical problems. The paper describes the process of acquiring a 3D digital model of the Maddalena by Donatello, a wooden statue representing one of the major masterpieces of the Italian Renaissance which was swept away by the Florence flood of 1966, and subsequently restored. The paper reports all the steps of the acquisition procedure, from the project planning to the solution of the various problems due to range camera calibration and to optically noncooperative material. Since the scientific focus is centered on the 3D models overall dimensional accuracy, a methodology for its quality control is described. Such control has demonstrated how, in some situations, the ICP-based alignment can lead to incorrect results. To circumvent this difficulty, we propose an alignment technique based on the fusion of ICP (iterative closest point) with close-range digital photogrammetry and a noninvasive procedure in order to generate a final accurate model. Detailed results are presented, demonstrating the improvement of the final model, and how the proposed sensor fusion ensures a prespecified level of accuracy.

Ground-based radar interferometry for landslides monitoring: atmospheric and instrumental decorrelation sources on experimental data

The application of ground-based radar interferometry for landslide monitoring is analyzed: a case study based on an experimental campaign carried out in Italy during 2002 is discussed. Interferometric data obtained from coherent synthetic aperture radar (SAR) images acquired by means of C-band ground-based equipment are analyzed. The campaign was aimed at retrieving potential terrain movements of a small landslide observed hundreds of meters away. Critical aspects related to spatial and temporal decorrelation are discussed: the use of optical photogrammetry as a technique for evaluating mechanical stability and correcting geometric distortion is presented. Results also confirmed that the application of ground-based radar interferometry can be attractive and effective if the acquired SAR images maintain an adequate coherence on different dates.

Permanent scatterers analysis for atmospheric correction in ground-based SAR interferometry

Ground-based synthetic aperture radar (GB-SAR) interferometry has already been recognized as a powerful tool, complementary or alternative to spaceborne SAR interferometry, for terrain monitoring, and for detecting structural changes in buildings. It has been noted that, in spite of the very short range, compared with the satellite configuration, in GB-SAR measurement the disturbances due to atmospheric effects cannot be neglected either. The analysis of the interferometric phases of very coherent points, called permanent scatterers (PSs), allows the evaluation of the atmospheric disturbance and the possibility of removing it. In this paper, the PS analysis is carried out both on a test site facility and on a real campaign (Citrin Valley, Italy) that provided data with a temporal baseline of about ten months.

Landslide monitoring by ground-based radar interferometry: A field test in Valdarno (Italy)

A ground-based Interferometric Synthetic Aperture Radar (InSAR) was installed to monitor a landslide in Valdarno (Italy). The aim was to field-test an innovative remote sensing instrument able to provide distributed information over sliding slopes with a rate of several images a day. Radar images and interferometric displacement maps projected on the Digital Elevation Model (DEM) of the test site are reported.

Remote monitoring of buildings using a ground-based SAR: Application to cultural heritage survey

In this Letter, the authors describe an innovative application of radar interferometric techniques aimed to monitor structural deformations of buildings. The proposed application is based on the use of ground-based instrumentation able to operate as Interferometric Synthetic Aperture Radar (InSAR). Preliminary experimental results on a model of an historical building encourage the development of this technique for use in architectural heritage survey.

Terrain mapping by ground-based interferometric radar

The authors propose a ground-based interferometric synthetic aperture radar (SAR) technique for terrain mapping. It is based on a coherent continuous-wave step-frequency (CW-SF) radar moved along a linear horizontal rail. It works synthesising microwave holographic images taken from different points of view to obtain elevation maps by phase comparison. The focusing algorithm for imaging synthetic holograms and digital elevation models (DEM) is able to correct topographic distortion and phase wrap through an iterative multi-baseline procedure.

Fusion of range camera and photogrammetry: a systematic procedure for improving 3-D models metric accuracy

The generation of three-dimensional (3-D) digital models produced by optical technologies in some cases involves metric errors. This happens when small high-resolution 3-D images are assembled together in order to model a large object. In some applications, as for example 3-D modeling of Cultural Heritage, the problem of metric accuracy is a major issue and no methods are currently available for enhancing it. The authors present a procedure by which the metric reliability of the 3-D model, obtained through iterative alignments of many range maps, can be guaranteed to a known acceptable level. The goal is the integration of the 3-D range camera system with a close range digital photogrammetry technique. The basic idea is to generate a global coordinate system determined by the digital photogrammetric procedure, measuring the spatial coordinates of optical targets placed around the object to be modeled. Such coordinates, set as reference points, allow the proper rigid motion of few key range maps, including a portion of the targets, in the global reference system defined by photogrammetry. The other 3-D images are normally aligned around these locked images with usual iterative algorithms. Experimental results on an anthropomorphic test object, comparing the conventional and the proposed alignment method, are finally reported.

Dynamic Monitoring of Bridges Using a High-Speed Coherent Radar

Remote dynamic monitoring of bridges by a high-speed interferometric radar is proposed. The equipment is a continuous-wave step-frequency radar with very fast frequency hopping that is capable of sampling the structure at a rate high enough for transient analysis of motion through phase comparison of successively acquired images. An experimental test carried out on a highway bridge forced by vehicular traffic is presented

Monitoring of an Alpine Glacier by Means of Ground-Based SAR Interferometry

Spaceborne differential synthetic aperture radar (SAR) interferometry has been proven to be a powerful tool in monitoring environmental phenomena and, in particular, in observing glaciers and retrieving information about their surface topography and dynamics. In the last decade, the use of this technique has been successfully extended from space to ground-based observations as a tool for monitoring, on a smaller scale, single landslides, unstable slopes, and more recently, areas covered by snow but not yet glaciers. In this letter, the results of an experimental activity carried out to evaluate the potential of ground-based microwave interferometry to estimate the velocity of an unstable area belonging to a glacier is reported. This experiment demonstrated the possibility of remotely monitoring surface displacements of the monitored glacier up to a distance of about 3 km even if, due to the lack of ground truths on the observed area, the data interpretation must be carefully worked out.