3D reconstruction of marble shipwreck cargoes based on underwater multi-image photogrammetry
Introduction
Since the 1960–70's archaeologists have adapted the aerial surveying method to underwater conditions, using photogrammetry with the first stereo-pair cameras to document underwater sites (Bass, 1966, Hohle, 1971, Leatherdale and Turner, 1991, Capra, 1992). These cameras offered high accuracy in recording, measuring and interpreting photographic images, but imposed some operating constraints, such as parallel optical axes required for stereo-vision conditions. Another disadvantage was the high degree of technical knowledge required to produce relatively few measurements.
The necessity of producing accurate detailed three-dimensional mapping, a product of stereo-photogrammetry, combined with the necessity of reducing underwater work time for hyperbaric reasons, has driven researchers to look for a fast documentation technique. At the same time, this technique had to be low-cost and accessible, considering the limited budgets of the discipline. Image-based technique using digital photogrammetry is now recognized as a powerful and accessible tool for non-destructive archaeology all around the world, and also for underwater archaeological sites (Green et al., 2002, Canciani et al., 2003, Bass, 2006, Green, 2004, Drap et al., 2007, Drap et al., 2013).
McCarthy and Benjamin (2014) present research using standard cameras and automated processing data with special software employed to capture 3D models of underwater archaeological features. Furthermore, the possibility of checking the results in the field is certainly one of the main advantages.
In recent years, Ca' Foscari University and University IUAV of Venice have been conducting research into the application of integrated techniques to support underwater measurement documentation. Recently published papers, such as Menna et al. (2011), Skarlatos et al. (2012), Eric et al. (2013), Henderson et al. (2013) and Demesticha et al. (2014), show how multi-image photogrammetry is now an effective technique for mapping and retrieving the shape and geometry of completely submerged objects. Underwater photogrammetry has thus been transformed from a highly technical and costly process to a much more powerful and accessible tool (McCarthy and Benjamin, 2014).
The production of a precise 3D model offers many opportunities from both measurement and recording points of view, because it represents a realistic model. It is possible to verify directly many kinds of data: measurements, shapes, colours, locations, etc., without the necessity of returning to the site.
This paper describes the method employed in two underwater sites in 2014, with image acquisition, topographical measurement and data processing to produce 2D drawings and 3D models of the shipwreck cargoes.
Section snippets
The archaeological sites
The research on the photogrammetric technique began some years ago in Crotone, southern Italy, with an experiment. Using some images not intended for a final 3D representation, but only for a simple photomosaic, we evaluated the perspectives of research of a 3D representation model from point clouds (Fig. 1).
The department of Studi Umanistici of Ca’ Foscari University (headed by Carlo Beltrame) and the Circe Laboratory of Photogrammetry of IUAV University of Architecture of Venice (headed by
Photogrammetrical survey
In recent years, image-based surveying and 3D modelling can offer results comparable to those of laser-scanning (range-based survey) for many terrestrial and aerial applications, thanks to significant progress in hardware and innovative image-matching algorithms, as described by Remondino et al. (2104).
Nowadays, in the field of cultural heritage documentation and recording, photogrammetry is considered a competitive technology. Image-based measurement tools allow obtaining realistic 3D models,
Conclusions
The experience described in the recent studies of Balletti et al. (2015a) highlights the importance of this approach to the documentation. Costa et al. (2015) also confirmed the potential of the 3D dimensional method for affording the public an underwater experience of the archaeological sites 'in dry suits'.
Regarding the survey, the study demonstrates the technological advances achieved in recent years with multi-image photogrammetry, already a largely automatic approach to processing,
Acknowledgements
We want to thank the Superintendent of the Soprintendenza del Mare, Sebastiano Tusa, for the permissions to dive on the sites and Nicolò Bruno for the collaboration in the organization of the missions. The team of Ca' Foscari University included the photographer Duilio Della Libera. The team of IUAV University included.
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