3D modeling

Ancient shipwrecks provide the most direct and primary class evidence of seaborne trade and seafaring in the Mediterranean during antiquity. Their location, though, in an unfriendly environment for humans, makes their excavation very demanding in human resources and equipment. Therefore, it is essential to make the investigations cost- and effort- effective by using the best of the available tools, technical and technological. As good documentation (UNESCO, 2001) is a particularly crucial process for the post-excavation archaeological study and assessment of a shipwreck site, the use of elaborate mapping techniques is indispensable during the excavation.
Photogrammetry is a well adopted method for underwater mapping, since it is a non contact and non destructive technique. On the other hand, it is not a trivial task at all (Canciani et al., 2002; Ludvigsen et al., 2006; Drap et al., 2007; Chapman et al., 2010). By definition “Photogrammetry is the art, science and technology of obtaining reliable information about physical objects and the environment through the process of recording, measuring, and interpreting photographic images and patterns of electromagnetic radiant energy and other phenomena” (Mc Glone, 2004). It is quite clear, that photogrammetry it is not a real-time or automated process. In fact most photogrammetric tasks are laborious and tedious. The much younger field of machine vision, takes advantage from computer vision algorithms and focuses on real time image exploitation for controlling a specific process or activity. Since photogrammetry’s entrance in the “digital era” it was a matter of time before embracing computer vision algorithms towards complete post- processing mapping automation.

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Archeolie 2014

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Work Program

Lost Island project will start in a few days and here in the beautiful island of Lipari everything is quite ready. Now we will go back to the North for some scientific meeting and we will bring to the archipelago our big rubber boat and Helium.
Together with the Soprintendenza del Mare, Sicily region, we have made a complete and detailed  Work Program, divided by divers experience with a list of targets.
Let's give you a short summary of our program, for more info please email us at projects@gue.com

Fundy: There are several shallow water Archaeological targets that we have to survey. From a large amount of amphoras to ancient columns and capitals. We will realize a complete video survey of these artefacts. After this first phase, called observation, we will choose the best target to realize a fine scale pohotogrammetry detection in order to create a 3D reconstruction of the target. Fundy program is based on two main actions: Surevy and Photogrammetry. We will dive in shallow water, very clear and with the direct support of archaeologists. We will work not only in water but also in laboratory. In September we will have an important archaeological event, organized in the Aeolian Islands, and we will work also on the making of a video documentary that will be showed to the public during this event. Lot of work but also lot of fun.
Tech1: We will explore one of the most important archaeological submerged site of the Aeolian Islands, a roman wreck sunked at 45 mt. We will find a large amount of amphoras laying on white sand. The wreck represents a very important archaological document of ancient history and during the times it has been object of several archaological research made by famous scientists. We will have the chance to realize a complete photogrammetry survey of the wreck. There are only few images from this wreck, using hd video cameras we will realize the first video documentation of the site to get a complete and detailed overview of the wreck. Using photogrammetry and 3D software we will create a Virtual reconstruction of the wreck and once again we will work together with archaeologists to realize a real scientific work.
Tech2: for T2 and RB divers we have a big sourprise...due the importance of our work we will update only by mail...top secret.

Don't forget to bring with you your family or frineds, Aeolian Islands are a small paradise in the South Mediterranean Sea, 7 volcanic islands full of history and nature...a perfect place to Explore, Dream, Discover and have fun!

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what we will do: Photogrammetry

Operative considerations

The principle of underwater photogrammetry does not differ from that of terrestrial or aerial photogrammetry, but it is necessary to take into account certain elements that may cause disturbances; in particular, the refraction of the diopter water-glass and the presence of the camera housing [39].

The specific constraints of the underwater medium (turbidity of water, presence of suspended particles) force the operators to work on a large scale, close to the objects (between 0.5 and 2 to 3 meters, depending on the water quality). This apparently constraining aspect imposes having to produce a great quantity of stereo pairs, but on the other hand it offers a very high degree of accuracy.

The important advantage of using photogrammetry in underwater surveys in comparison with the use of other techniques consists in its simplicity of implementation and the diversity of potential results (3D measurements, 3D reconstruction, orthophotography, and vector restitution).

The implementation only requires the use of a scale bar to compute the scale of the model. Moreover, if two or three synchronized cameras are used, additional equipment is not needed at the scene as the scale is computed using the calibration of the camera set. This approach also provides a relevant appreciation of the uncertainty of measurements; where, in addition, the photographs have to be taken with an important overlap. The key factor of this method is redundancy: each point of measured space must be seen in at least three photographs.

The operative advantage is related to the simplicity of the survey. Moreover, a submarine pilot can drive a remotely operated underwater vehicle (ROV) without having to undergo a long preliminary training period. This method requires little time and does not require specific personnel, thus greatly reducing the expenses in a context where time and costs of intervention are extremely high.

Camera calibration
Camera calibration in multimedia photogrammetry is a problem identified since almost 50 years [9; 23] . The problem has no obvious solution, since the light beam refraction through the different media (water, glass, air) introduces a refraction error which is impossible to express as a function of the image plane coordinates alone [37] . Therefore the deviation due to refraction is close to that produced by radial distortion even if radial distortion and refraction are two physical phenomena of different nature. For this reason, the approach described by Kwon [45] has been adopted, consisting in the use of standard photogrammetric calibration software to perform the calibration of the set housing + digital camera. This approach can indeed correct in a large part the refraction perturbation; however, it is strongly dependent on the optical characteristics of the water/glass interface of the housing. For a more rigorous approach, we can read the interesting developments made by Gili Telem, and Sagi Filin on underwater camera calibration

Automatic photogrammetry survey
The photogrammetric process is a very efficient procedure consisting mainly of three phases. The first phase is data acquisition by photographs which requires light processing. This process is non- intrusive (remote sensing), and necessitates only slightly time- consuming (only the time necessary to take pictures), and potentially a quite thorough practice. The second phase involves further data processing and is carried out in a laboratory. This phase, which is mainly automated, includes homologous point determination and pose estimation. The last phase, data interpretation and linking with domain knowledge (underwater archaeology for example) is always manual, performed by experts and very time-consuming.

SIFT algorithm is often used to determine the homologous points
[39, 40] and recently the FAST [41] algorithm is applied. Then the pose estimation process from relative orientation of stereo pair is obtained by the Stewenius algorithm [42; 46; 60] . The SBA open source software by Manolis Lourakis [51] and Noah Snavely [58] is applied for the global bundle adjustment. Finally several approaches are proposed for surface densification PMVS by Ponce and Furukawa [54; 55] . For a good overview of these techniques it is possible to refer to these paper [64] [40] . In our application, we chose three tools to solve this problem and we have developed some of them. We also develop bridge between them in order to take benefit of the best of each of them.

Underwater 3D survey merging optic and acoustic sensors
Optic and acoustic data fusion is an extremely promising technique for mapping underwater objects that has been receiving increasing attention over the past few years [53]. Generally, bathymetry obtained using underwater sonar is performed at a certain distance from the measured object (generally the seabed) and the obtained cloud point density is rather low in comparison with the one obtained by optical means.

Since photogrammetry requires working on a large scale, it therefore makes it possible to obtain dense 3D models. The merging of photogrammetric and acoustic models is similar to the fusion of data gathered by a terrestrial laser and photogrammetry. The fusion of optical and acoustic data involves the fusion of 3D models of very different densities – a task which requires specific precautions [44; 56] .

Only a few laboratories worldwide have produced groundbreaking work on optical/acoustic data fusion in an underwater environment. See for example [38] and [41] where the authors describe the use of techniques that allow the overlaying of photo mosaics on bathymetric 3D digital terrain maps [52] . In this case we have important qualitative information coming from photos, but the geometric definition of the digital terrain map comes from sonar measurements.

Optical and acoustic surveys can also be merged using structured light and high frequency sonar as by Chris Roman and his team [50]. This approach is very robust and accurate in low visibility conditions but does not carry over qualitative information.

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how to get there

Fly to Naples or to Palermo.

British Airways (0870 850 9850; www.ba.com) flies from Gatwick to Naples. Monarch (0870 040 5040; www.flymonarch.com) flies to Naples from Birmingham, East Midlands, Glasgow and Manchester. Ryanair (0872 246 0000; www.ryanair.com) flies from Stansted to Palermo. Easyjet (0905 821 0905; www.easyjet.com) flies from Stansted to Naples.

Ferries to the islands are operated by SNAV (00 39 081 428 5111; www.snav.it) from Naples; Siremar (00 39 090 928 3242; www.siremar.it) from Milazzo; and Usticalines (00 39 090 924 9199; www.usticalines.it), from Milazzo and Palermo.

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Secca di Capistello wreck

The real excavation of the wreck took place in 1977 under the direction of Michael Katzev and Donald A. Frey (INA) with the technical support of Sub Sea Oil Co.. But sometime earlier a team of DAI (Deutsche Archäologische Institut) started the survey of the wreck, but it was stopped by the tragic death of two archaeologists.
The excavation of 1977 was the first deepwater archaeological excavation. The wreck lies at - m 59. To excavate the ship properly, saturation diving had to be done. Divers breathed a helium and oxygen mixture. Dives were 5, 7 and 9 days. This meant that the divers had to go through 38 hours decompression at the end of their shift.
On the whole, divers worked on the wreck for two hours. There were teams of four, two diving in the morning and two in the afternoon. For safety reasons, one diver always monitored the other from the on-site diving bell. For further safety, there was a continual diver-surface com and CCTV. However, there was a lot of interference.
The hull was the major artefact to be uncovered in the 1977 excavation. Of the hull, around 6 sq. m was exposed during excavation with the further indication that more lay under a layer of undisturbed sand and amphorae. The details of the uncovered hull are as follows: The main section featured eight to ten exposed strakes on which eight frames were located and overlain with three interior planks. The frames were moulded 10cm and sided 16cm. The strakes are given an average measurement of 20cm in width and 4.5cm in thickness. These were joined together by pegged mortise-and-tenon fastenings that were widely spaced - between 16cm to 18.5cm. Mortises averaged 5cm to 6cm deep.
A large interior timber was uncovered in the excavation running longitudinally across six frames. Measuring 30cm wide and 6cm thick, the timber may have acted as a clamp to strengthen the hull from the inside. Two other interior timbers were recovered which could have been the remains of ceiling planking. A wooden pole 7cm in diameter lied parallel to the main timber but was unattached to the hull, it's purpose remains unknown.
Brushwood was found beneath the amphorae and spread over the inside of the hull. The interior of the hull was coated with a thin layer of a dark tarry substance which had to be removed by the divers to get to the tenon peg outlines and the seams between the strakes. A number of copper and iron nails were also found, one complete measuring 15cm in length and retaining its original clenched shape.
The Capistello ship, indicated by the size of the hull recovered and by that remained buried or lost, is believed to be bigger than the Kyrenia ship of roughly a century earlier. The Kyrenia measured 15m by 5m. The Capistello is dated to the fifth century BC, 100 years prior to the Kyrenia.
The Capistello wreck was a small freighter carrying amphorae and Campanian ware, a fairly normal cargo for a Mediterranean freighter to carry. The Capistello wreck was dated to the IVth century, a century later than the Kyrenia.
The building technique of the ship falls into the transitional period of Mediterranean boatbuilding as shown by the Kyrenia and St Congloue ships. The Capistello wreck has mortise and tenon fastenings, but they are widely spaced and do not give the hull strength and integrity. This is carried by the frames, which were moulded to fit the hull. They are fastened by copper nails to the hull, the nails clenched over on the inside. The frames were a pattern of floor timbers and half-frames alternating with each other. Additionally, the excavation showed a logitudinal timber running over the frames. It was 30 cm wide, and was used as a stringer to further strengthen the hull.
Blanck H., 1978, Der Schiffsfund von der Secca di Capistello bei Lipari. Mitteilungen des Deutschen Archaologischen
Instituts, Romisch Abteilung 85. Mainz am Rhein.
Frey D.A. & Hentschel F.D. & Keith D.H., 1978, Deepwater archaeology. The Capistello wreck excavation, Lipari,
Aeolian Islands. IJNA, 7.4: 279-300.
Frey D.A. #et al#., 1979,L'archeologia sottomarina a grande Profondita: gli scavi di Capistello.Sicilia Archeologica,
12.39. (Trapani)
Kapitan G., 1978, Exploration at Cape Graziano, Filicudi, Aeolian Islands, 1977. IJNA, 7.4: 269-277. Picozzi S., 1979, La nave di Capistello. Il Subacqueo, 7.70.
Sebastiano Tusa

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