Improvement of metric accuracy of digital 3D models through digital photogrammetry. A case study: Donatello's Maddelena

In spite of an undoubted richness of information produced by 3D optical technologies, in some cases, the method for generating a digital model from single 3D acquisitions involves the propagation of errors. These errors limit the overall metric accuracy attainable with such procedure. This happens when small 3D images are assembled together in order to model a large object. The authors present a procedure by which the metric reliability of the 3D model, obtained through iterative alignments of single range maps, can be guaranteed to an acceptable level. For this purpose, non-impeding optical targets were specifically designed for placement around the object. These are measured using a close range digital photogrammetry technique and by the 3D range camera system. From these measurements, transformation matrices have been calculated. Each matrix allows for the roto-translation (pose) of the 3D images from the local coordinate system of the range camera to an accurate global coordinate system determined by the digital photogrammetric procedure.