Photogrammetry and 3D scanning: assessment of metric accuracy for the digital model of Donatello's Maddalena

The deployment of 3D optical scanning technologies for measuring three-dimensional shapes of sculptures of high morphological complexity, have seen recently a development so noticeable that it emerges as one of the most promising methods for the analysis and preservation of the Cultural Heritage. In spite of an undoubted richness of information produced with this technique, the method for generating a digital model from single 3D acquisitions involves errors propagation that may deteriorate the actual metric accuracy attainable with such procedure. The activity reported in this paper describes a recent experiment in 3D scanning of a highly complex sculpture: the wooden statue Maddalena by Donatello, kept in the museum of the Opera del Duomo, in Florence. The acquisitions, taken with a commercial system based on fringe projection, give a local measurement uncertainty of about 70mm, but when the complete model is generated by automatic alignment of the raw 3D images in a common coordinate system, possible scale variations might be involved, especially along the height of the statue. With this preliminary work, the authors want to verify the metric reliability of the three-dimensional model, obtained through iterative alignments of single 3D acquisitions. For this purpose, a measurement over well-identified features of the sculpture have been performed and the distances between couples of significant points have been calculated with photogrammetric operations. By repeating the same measurements on the 3D model, a corresponding set of point-to-point distances have been evaluated and compared with the photogrammetric results.