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21 The ROV 3D Project: Deep-Sea Underwater Survey Using Photogrammetry: Applications for Underwater Archaeology PIERRE DRAP, JULIEN SEINTURIER, BILAL HIJAZI, DJAMAL MERAD, and JEAN-MARC BOI, Aix Marseille Universit´ e, CNRS, ENSAM, Universit´ e de Toulon, LSIS UMR 7296, 13397, Marseille, France BERTRAND CHEMISKY and EMMANUELLE SEGUIN, Comex, Marseille, France LUC LONG, Department of Underwater Archaeological Research DRASSM, Marseille, France In this article, we present an approach for a deep-sea survey based on photogrammetry using a remotely operated underwater vehicle (ROV). A hybrid technique gives us real-time results, sufficient for piloting the ROV from the surface vessel and ensuring a uniform coverage of the site, as well as recording high-definition images using an onboard computer that will later provide a survey with millimetric precision. The measurements are made without any contact and are noninvasive. The time required on-site is minimal and corresponds to the time needed by the ROV to cover the zone. With the photos taken at a frame rate synchronized at 10Hz, the ROV required 2 hours to perform the experiment presented in this article: the survey of the Roman shipwreck Cap B´ enat 4, at a depth of 328m. The approach presented in this work was developed in the scope of the ROV 3D project. This project, financed by the Fond Unique Interminist´ eriel (FUI) for 3 years, brings together two industrial partners and a research laboratory. Companie Maritime d’Expertise (COMEX) coordinated this project. Categories and Subject Descriptors: I. [Computing Methodologies]; I.4 [Image Processing and Computer Vision]; I.4.8 [Scene Analysis]: Motion; J. [Computer Applications]; J.2 [Physical Sciences and Engineering]: Archaeology General Terms: Human Factors Additional Key Words and Phrases: Underwater archaeology, underwater photogrammetry, visual odometry, real-time orienta- tion, high-definition 3D model ACM Reference Format: Pierre Drap, Julien Seinturier, Bilal Hijazi, Djamal Merad, JeAn-Marc Boi, Bertrand Chemisky, Emmanuelle Seguin, and Luc Long. 2015. The ROV 3D project: Deep-sea underwater survey using photogrammetry: Applications for underwater archaeology. ACM J. Comput. Cult. Herit. 8, 4, Article 21 (August 2015), 24 pages. DOI: http://dx.doi.org/10.1145/2757283 1. INTRODUCTION 1.1 Context Deep-sea shipwrecks cannot be reached by divers, who are often treasure hunters or looters, but nei- ther can they be reached by researchers. (Beyond 50m, traditional diving with standard air is prohib- ited, and diving requires the use of enriched air, as well as significant facilities on the surface.) These deep-sea wrecks are also protected by various natural physiochemical factors due to their location at Authors’ addresses: P. Drap, J. Seinturier, B. Hijazi, D. Merad, and J.-M. Boi, Aix Marseille Universit´ e, CNRS, EN- SAM, Universit´ e de Toulon, LSIS UMR 7296,13397, Marseille, France; emails: {pierre.Drap, julien.seinturier, Bilal.hijazi, Djamal.merad, Jean-Marc.boi}@univ-amu.fr; B. Chemisky and E. Seguin, Comex, 36 bd. de l’oceans, 13009, Marseille, France; emails: {b.chemisky, e.seguin}@comex.fr; L. Long, DRASSM 147 Plage de l’Estaque, 13016 Marseille, France; email: [email protected]. (c) 2015 Association for Computing Machinery. ACM acknowledges that this contribution was authored or co-authored by an employee, contractor or affiliate of a national government. As such, the Government retains a nonexclusive, royalty-free right to publish or reproduce this article, or to allow others to do so, for Government purposes only. c 2015 ACM 1556-4673/2015/08-ART21 $15.00 DOI: http://dx.doi.org/10.1145/2757283 ACM Journal on Computing and Cultural Heritage, Vol. 8, No. 4, Article 21, Publication date: August 2015.

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The ROV 3D Project: Deep-Sea Underwater Survey UsingPhotogrammetry: Applications for Underwater ArchaeologyPIERRE DRAP, JULIEN SEINTURIER, BILAL HIJAZI, DJAMAL MERAD,and JEAN-MARC BOI, Aix Marseille Universite, CNRS, ENSAM, Universite de Toulon,LSIS UMR 7296, 13397, Marseille, FranceBERTRAND CHEMISKY and EMMANUELLE SEGUIN, Comex, Marseille, FranceLUC LONG, Department of Underwater Archaeological Research DRASSM, Marseille, France

In this article, we present an approach for a deep-sea survey based on photogrammetry using a remotely operated underwatervehicle (ROV). A hybrid technique gives us real-time results, sufficient for piloting the ROV from the surface vessel and ensuringa uniform coverage of the site, as well as recording high-definition images using an onboard computer that will later providea survey with millimetric precision. The measurements are made without any contact and are noninvasive. The time requiredon-site is minimal and corresponds to the time needed by the ROV to cover the zone. With the photos taken at a frame ratesynchronized at 10Hz, the ROV required 2 hours to perform the experiment presented in this article: the survey of the Romanshipwreck Cap Benat 4, at a depth of 328m. The approach presented in this work was developed in the scope of the ROV 3Dproject. This project, financed by the Fond Unique Interministeriel (FUI) for 3 years, brings together two industrial partnersand a research laboratory. Companie Maritime d’Expertise (COMEX) coordinated this project.

Categories and Subject Descriptors: I. [Computing Methodologies]; I.4 [Image Processing and Computer Vision]; I.4.8[Scene Analysis]: Motion; J. [Computer Applications]; J.2 [Physical Sciences and Engineering]: ArchaeologyGeneral Terms: Human FactorsAdditional Key Words and Phrases: Underwater archaeology, underwater photogrammetry, visual odometry, real-time orienta-tion, high-definition 3D model

ACM Reference Format:Pierre Drap, Julien Seinturier, Bilal Hijazi, Djamal Merad, JeAn-Marc Boi, Bertrand Chemisky, Emmanuelle Seguin, and LucLong. 2015. The ROV 3D project: Deep-sea underwater survey using photogrammetry: Applications for underwater archaeology.ACM J. Comput. Cult. Herit. 8, 4, Article 21 (August 2015), 24 pages.DOI: http://dx.doi.org/10.1145/2757283

1. INTRODUCTION

1.1 Context

Deep-sea shipwrecks cannot be reached by divers, who are often treasure hunters or looters, but nei-ther can they be reached by researchers. (Beyond 50m, traditional diving with standard air is prohib-ited, and diving requires the use of enriched air, as well as significant facilities on the surface.) Thesedeep-sea wrecks are also protected by various natural physiochemical factors due to their location at

Authors’ addresses: P. Drap, J. Seinturier, B. Hijazi, D. Merad, and J.-M. Boi, Aix Marseille Universite, CNRS, EN-SAM, Universite de Toulon, LSIS UMR 7296,13397, Marseille, France; emails: {pierre.Drap, julien.seinturier, Bilal.hijazi,Djamal.merad, Jean-Marc.boi}@univ-amu.fr; B. Chemisky and E. Seguin, Comex, 36 bd. de l’oceans, 13009, Marseille,France; emails: {b.chemisky, e.seguin}@comex.fr; L. Long, DRASSM 147 Plage de l’Estaque, 13016 Marseille, France; email:[email protected].(c) 2015 Association for Computing Machinery. ACM acknowledges that this contribution was authored or co-authored by anemployee, contractor or affiliate of a national government. As such, the Government retains a nonexclusive, royalty-free right topublish or reproduce this article, or to allow others to do so, for Government purposes only.c© 2015 ACM 1556-4673/2015/08-ART21 $15.00

DOI: http://dx.doi.org/10.1145/2757283

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21:2 • P. Drap et al.

sea. The obscurity, low temperatures, and low oxygen levels help in the exceptional preservation ofthese wrecks.

However, even these deep-sea sites are now threatened by new ways of trawling that destroy thesurface layer of the sites and interfere with their readability. In fact, the protection that should beguaranteed due to their depth is outdated: trawling nets today can be deployed to depths of up to1,000m. Consequently, many of these shipwrecks are likely to be destroyed even before they can bestudied—or even observed.

Aside from its accessibility by divers, an underwater site cannot be reached physically and realisti-cally by the majority of archaeologists, marine biologists, or experts in the fields studied (geologists, en-trepreneurs). It is therefore important, even crucial, to implement easily deployed techniques that areable to accurately survey these deep-sea sites without contact. This kind of methodology is highly rec-ommended by UNESCO in the 2001 convention on the Protection of the Underwater Cultural Heritage[UNESCO 2001]. For example, the 16th rule says: “The methodology shall comply with the project ob-jectives, and the techniques employed shall be as non-intrusive as possible.” In this article, we presentan approach of pure optical survey that respects these constraints.

The technique described here is photogrammetry. Three high-resolution cameras are synchronizedand controlled by a computer, all of which are mounted on a remotely operated underwater vehicle(ROV). The use of LEDs ensures adequate lighting for the scene. The high frequency of the frame rateensures full coverage, and the large scale of these photos gives the result its extreme precision.

Deployed as such, the system is totally contactless, nondestructive, and extremely accurate. Commu-nication with the surface ship makes it possible to have a real-time 3D calculation of the zone coveredby the ROV, which ensures an optimal guidance of the vehicle over the site and ensures that the ROVpilot completely covers the survey area before the vehicle returns to the surface.

The main goal is not to automatically survey the site, but to offer the ROV pilot real-time control ofthe survey process by visualizing the surveyed part of the site in real time. At the same time, high-resolution images are taken and stored to produce a high-resolution 3D model of the site offline.

Photogrammetric surveys in an underwater context make it possible to obtain, without any contact,an exhaustive survey of all visible parts of the site in the shortest amount of time and with a highdegree of precision. This approach offers specialists, as well as the general public, a global view of asite that is normally prohibited by an undersea environment [Drap et al. 2013].

1.2 The ROV 3D Project and Its Participants

The ROV 3D project, approved by the competitiveness cluster “Mer PACA,” was partially funded for a3-year period in the scope of the Fond Unique Interministeriel (FUI). The French Ministry of Industry,the European foundation FEDER, the PACA Region of France, the General Counsel of the Bouches duRhone, and the Urban Community of Marseille Provence Metropole also participated in its financing.

The consortium consists of a university research laboratory, LSIS (Unite Mixte de Recherche CNRS7296), and two industrial partners, Compagnie Maritime d’Expertise (COMEX) and SETP. COMEXspecializes in high-tech underwater operations, and SETP in dimensional control using opticalsystems.

The project’s objective is to develop automatic procedures for 3D surveys, dedicated to the under-water environment, using optical and acoustic sensors. Acoustic sensors capture a large amount oflow-resolution data, whereas optical sensors (used on a large scale) enable the survey of smaller sur-faces but with high resolution. The combination of these two data sources enables the reproduction oflarge complex scenes without being limited to traditional terrain elevation structures such as digital

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Deep-Sea Underwater Survey Using Photogrammetry • 21:3

terrain models (DTMs). These complex surveys, which may include caves and overhangs, can be usedto study archaeological or industrial sites, or even to study the evolution of landscapes over time.

The ability to measure and model important underwater sites, in a short lapse of time, brings newperspectives to underwater archaeology, marine biology, and the underwater industry (offshore, har-bour industries, etc.) [Drap et al. 2014].

This article mainly covers automatic optical surveys, developed and integrated on the Remora 2000submarine, as well as on the Apache ROV, both of which are operated by COMEX. Using the subma-rine, it is possible to bring an expert in the field on board so that he can have a more close-up view ofthe site and be able to select the most relevant zones for study. Implementing the system on a ROVprovides a lighter and faster solution, which made it possible to complete this survey during a 2-hourdive. Both approaches are presented in this article.

2. THE STATE OF THE ART

2.1 A Brief History

The originator of underwater archaeological photogrammetry was without a doubt the archaeologistG.-F. Bass, in 1963, on the Byzantine shipwreck Yassi Ada 2, near Bodrum, Turkey [Bass 1970]. Backedby the National Geographic Society, he proceeded to take stereoscopic photographs from the submarineAshera, which was made available to him by the University of Pennsylvania Museum. Even thoughthe site was not very deep (35m), the process proved to be very efficient for the collection of a maximumamount of data in a very short period of time [Bass 1970; Bass and Rosencrantz 1973].

Beyond the simple photographic mosaic that gave a flattened view of the entire site by adjusting thepictures between themselves, the stereophotogrammetry formed the only system capable of measuringa shipwreck and creating a 3D model of the upper layer of its cargo.

A short time after Georges Bass’s first experiments on Yassi Ada 2, Joseph Pollio used photogram-metry to make an underwater topological map with the Naval Oceanographic Office in Washington.His surveys took place in Florida, between March 1967 and June 1968, successively aboard the sub-mersibles Pegasus, Star III, and Aluminaut. The main problem at this time was maintaining a constantheight when taking the photographs [Pollio 1968]. Other experiments of this type took place involvingthe basic concepts still in use today [Ciani et al. 1971; Faig 1979; Hoehle 1971; Pollio 1971].

At the same time, in the scope of autonomous diving, stereophotogrammetric surveys took place inFrance, beginning in 1968 and more generally in the 1970s, around various shipwrecks accessible todivers. In Marseille, between 1968 and 1971, Andre Tchernia developed a metal housing for takingstereoscopic photographs on the shipwreck Planier 3 at a depth of 30m [Liou 1973]. First equippedwith a bridge crane for moving the photographic plates, this tool was later perfected by Jean-ClaudeNegrel and Jean-Paul Clerc on the wreck Pointe de la Luque B at a depth of 28m [Negrel and Clerc1973]. The same tool was also used on the shipwreck Dramont A at a depth of 36m in Saint-Raphaelin 1974 by Claude Santamaria [Santamaria 1975] (Figure 1). These advancements quickly took shapewith success on the excavation of the Madrague de Giens at a depth of 20m. Having become a globalreference point, this field school organized by the CNRS (French National Centre for Scientific Re-search), under the guidance of Antoinette Hesnard, Andre Tchernia, and Patrice Pomey, acted as atesting ground for archaeological surveys for 10 years [Tchernia et al. 1978].

Incidentally, in the Marseille archipelago, just as on the Madrague de Giens, photogrammetric pho-tographs were always subject to strong geometric constraints, notably for stereoscopy that requiredparallel optical axes, which was always inevitably translated by an assembly of fixed and sliding metal-lic structures used to guide the photographers. These devices, which were heavy, bulky, and took a long

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Fig. 1. Dramont A shipwreck, the metal framework dedicated to photogrammetry survey. Photograph courtesy of DRASSM.

time to set up, required a team of well-trained divers and, consequently, limited the depth at which itcould be deployed. This was a major impediment in this field for many years.

The system started becoming lighter after 1984 when an excavation brought Hesnard the doliashipwreck Grand Ribaud D at a depth of 18m. After having marked the site with numerous targets,a Sapho metric chamber (COMEX) with neutral buoyancy now moved along a guide made of a simplePVC tube 12m long, placed perpendicular to two crossbars 16m long, also made of PVC. The systemwas lightweight and held in a consistent position by opposing forces created by floats and weights[Hesnard 1989].

This method was further simplified by Long (DRASSM) in 1984 and 1985 during the excavation ofa wreck carrying cut limestone Carry-le-Rouet at a depth of 6m. Two semimetric Hasselblad camerasequipped with 38mm wide-angle lenses were mounted this time on a small hydrodynamic plate, at firstmade of aluminium, then of plastic, that moved at a constant height above the stone blocks. The heightalways remained the same from one pair to another thanks to buoys pulling it toward the surface andnylon string attached between the photo plate and a metal triangle moved along the seafloor by diversand readjusted after each photograph using a reference length of piano wire [Long 1986]. In bothcases, on the Grand Ribaud D and the Carry-le-Rouet, the company SETP (from Salon-de-Provence)reproduced the photos in the form of topometric maps with measured heights and contour lines.

During the 1980s, this system was exported to several shipwrecks explored by DRASSM in shal-low waters. This was first the case in 1985 on the ancient wreck Ploumanac’h at a depth of 5m, thewreck Pointe Lequin 2 off the islands of Hyeres, and finally in 1986 on the Dutch post-medieval vesselMauritius (Gabon) at a depth of 10m [L’Hour et al. 1989].

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2.2 Photogrammetry and Deep-Sea Shipwrecks

In 1987, inspired by the stereoscopic photos taken over the previous years of wrecks in shallow wa-ters, a dive with the submarine Neree 201 allowed a manual survey of the Roman wreck Basses deCan (Saint-Tropez), resting on a slope between 70 and 200m deep. Even though the field of amphorawas calibrated using measuring tapes and elevation reference points, photogrammetry was disqual-ified from the beginning at that time for financial reasons: the cost of reproducing the photos. Thearchaeologist therefore limited his experiment to a graphical survey more or less accurate to within50cm, based on video recordings and vertical photographs [Long 1988]. At the same time, a studyof the requirements needed for the future of deep-sea archaeology resulted in a preliminary analysis[Tchernia and Pomey 1990]. But a qualitative leap was made in 1993 during the photogrammetricsurvey of the deep-sea wreck Plage d’Arles 4, at a depth of 700m, supervised by Long (DRASSM) andBlaustein (SETP) with the support of IFREMER. The stereophotogrammetric coverage was performedusing semimetric, medium-format cameras [Long 1995]. Whereas the photos of the Roman wreck Plaged’Arles 4 (20m × 10m) were taken over 4 days, only 1 day was needed to photograph the wreck of thevessel La Lune, sunk in 1664 east of Toulon, at a depth of 90m [Long and Illouze 2002].

Later in 1996, the method was further perfected in the Bay of Marseille, on the Roman wreck Sud-Caveaux 1 at a depth of 64m, using the ROV Super-Achille and the submersible Remora 2000, equippedwith a metric chamber using standard photographic film (DRASSM, COMEX, SETP). The survey sup-plied by SETP even takes into account the zones cleared using the Blaster blower, reattached duringthe 3D reproduction of the surface layer [Long and Delauze 1996].

Deep-sea photogrammetry again progressed between 2000 and 2002 on the Etruscan wreck GrandRibaud F at a depth of 61m, notably due to the sudden entrance in this field of researchers from theCNRS, namely Drap (MAP-GAMSAU and LSIS), who worked together with SETP since the beginning.Moreover, the development since the 1990s of microcomputing and photogrammetry software, whichwere freed from the constraints of stereoscopy, in parallel with the emergence of digital cameras, wouldconsiderably popularize photogrammetry in archaeology and increase its degree of precision. The studyof the wreck Grand Ribaud F benefited on one hand from simplified photography systems that nolonger functioned based on the principle of independent stereoscopic pairs but by bundle adjustment,and on the other by the use of digital photogrammetry tools and expert systems for the generation of3D data used later in a second stage. Previously, the photos of the Etruscan wreck were taken both bydivers and submersibles because the moderate depths, not far from the surface, allowed both types ofintervention [Drap and Long 2001; Long et al. 2001].

It was this improved system that prevailed while working on the European Virtual Exploration ofUnderwater Sites (VENUS) project off the coast of the Calanques of Marseille on the wreck Port-MiouC at a depth of 105m. Previously, during the same project, Drap tested the method by scuba diving onthe Pianosa site (Italy) at a depth of 35m, then on that of Sesimbra (Portugal) at 60m [Chapman et al.2008; Long and Drap 2010].

Today, a well-trained team in a single day can take highly accurate optical measurements, withmillimeter precision, up to a depth that can reach 2,000m (current limit of the camera housings for theconfiguration used in the scope of the experiments presented in this work) and no longer requires anyphysical intervention on the site, neither for scaling nor for the absolute orientation along the verticalaxis.

The proposed method is intended to be as light as possible, requiring the use of the least amount ofsophisticated devices. Neither inertial devices nor acoustic sensors are used. The results are wholly ob-tained by photogrammetry and the use of a small ROV, such as the Super-Achille. This approach differsdrastically, for example, from the method developed by Foley and Mindell [2002], where “sonar beam

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on vehicle scans across wreck site and records precise altitude information” and where, additionally,transponders have to be placed around the site. On the other hand, we can cite the methods employedby Singh et al. [2007] to obtain high-resolution imaging, as well as Pizarro et al. [2009], which alsocombines acoustic sensors, SFM, and photogrammetry. However, this approach needs the combinationof both optical and acoustic sensors to obtain a model at a correct scale. The high-resolution modelobtained by photogrammetry is merged with navigation and sonar data. Another example of a methodusing both acoustic and optical data is the recent operation carried out at the initiative of the DRASSMon the wreck of the vessel La Lune, whose measurements were mainly of the acoustic type. The imageswere only used afterward to drape the terrain model [Gracias et al. 2013].

At present in the field of high-definition deep-sea surveys, automatic industrial processing does notexist. However, a few private companies offer services for underwater metrology based on now tra-ditional techniques of close-range photogrammetry. These offers use bundle adjustment and an auto-matic recognition system to identify coded targets and require an on-site intervention at least whenthe targets are placed on the objects to be measured, which in an underwater context may be a majorhandicap [Johannessen and Prytz 2005] (http://www.parkermaritime.no/services-and-products/subsea-metrology).

As Pomey [1985] predicted more than 30 years ago, deep-sea wrecks have every chance of beingbetter preserved than the others. At that time, the wreck Cap Benat 4, discovered east of the islands ofHyeres by IFREMER at a depth of 330m, inaugurated a long list of ancient deep-sea shipwrecks thatall escaped destructive reefs when they sank and reached the seafloor without any heavy damage. Itis on this symbolic wreck that we focused in November 2014 and for which we will present our firstresults here.

3. PROPOSED APPROACH

In the scope of the experiments presented in this article, no acoustic measurements were performedon-site, as the targeted zone (approximately 20m × 15m) was small enough to be easily processedusing photogrammetry from the ROV. Moreover, millimetric precision was required for a detailed ar-chaeological study. Multibeam surveys would be inappropriate, as they do not provide enough detailsor precision.

The adopted solution is purely photogrammetric and built around two calculators: one mounted onthe ROV with cameras and lights, and the other on the surface dedicated to performing the photogram-metric calculations as well as tracking and piloting the ROV.

The tasks were divided as follows: the underwater computer was in charge of managing the cameras,storing the images locally, extracting homologous points on at least two consecutive pairs (by combiningSIFT, SURF, and Harris detectors) and then sending to the surface computer the 2D points matchedand identified on at least four images.

The surface computer receives these 2D points; theoretically knowing the calibration of the under-water cameras, as well as their related intrinsic and extrinsic parameters, it calculates the movementsof the ROV and displays the position of the vehicle and the calculated 3D points in real time.

From a photogrammetric point of view, three issues had to be addressed:

� The orientation in real-time of the photo pairs taken by the ROV at a frame rate of 10Hz to displaythe trajectory and zone surveyed by the ROV in real time: To do this, we developed a dedicated bundleadjustment framework [Sunderhauf et al. 2005; Xue and Su 2012] and an orientation method usingbestFit3D software on the 3D points calculated in real time.

� The generation of a dense point cloud using the underwater photos taken with nonmetric cameraswith wide-angle lenses and through a window less than perfect: The calculation of this dense point

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Fig. 2. The synoptic schematic of triplet camera acquisition and ROV/surface communication.

cloud, based on the works of Furukawa and Ponce [2010], takes into account the strong distortionof the images, and our developments directly use the images taken by the cameras without anyrectification or correction processes for the distortion.

� For high-resolution results, the creation of a software bridge between our data structures and com-mercial software: For calibration and bundle adjustment, the software PhotoScan from Agisoft andBingo were used.

3.1 ROV 3D Architecture

The ROV 3D system was designed to be mounted on a lightweight remotely controlled vehicle, implyingan optimized size and weight for the assembly (Figures 2 and 3).

A first step in the functional validation on the submarine Remora 2000 enabled it to be liberatedfrom previous communication constraints. In the ROV 3D configuration, the operator has direct ac-cess to the onboard acquisition unit (UAE) via a high-speed Ethernet link between the inside of theinhabited vehicle and the system on the surface. This preliminary step also enabled us to validate thelighting specifications required for an adequate exposure for the photos. The following step consistedof grouping together all components of the ROV 3D system in a removable assembly that could beattached under the remotely controlled vehicle.

The main ROV 3D approach is built on synchronized acquisition of high- and low-resolution imagesby video cameras forming a trifocal system. The three cameras are independently mounted in a sep-arate waterproof housing, and two separate calibration phases are performed. The first one is carriedout on each set camera/housing to compute intrinsic parameters; the second one is done to determinethe relative position of the three cameras, which are securely mounted on a rigid platform. The secondcalibration can be done easily before each mission and is responsible for the final 3D model scale. Thetrifocal system is composed of one high-resolution, full-frame camera synchronized at 2Hz and twolow-resolution cameras synchronized at 10Hz.

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Fig. 3. The ROV Apache surveying the Cap Benat 4 wreck. Photograph taken from the Super-Achille ROV. Photograph:COMEX.

The lighting—a crucial part in photogrammetry—must meet two criteria: the homogeneity of expo-sure for each image and its consistency between images. To meet these requirements, a system basedon LED technology and controlled by the onboard acquisition unit was used.

The trifocal system has two different goals. The first one is the real-time computation of the systempose and of the 3D zone of the seabed visible from the cameras. The operator can pilot the systemfrom the surface computer with a dedicated application that displays the position of the vehicle in realtime and can send commands. A remote video connection also enables the operator to see the imagesproduced by the cameras in real time. Using the available data, the operator can assist the ROV’s pilotto ensure complete coverage of the zone to be surveyed. This is done by estimating the movement ofthe ROV between two consecutive shots. Here, homologous points are found on two successive pairs ofimages in low resolution (four images) and sent to the surface computer. The computation is describedin detail in Section 3.4. The second goal is the offline realization of a 3D metric model in high resolution.This process involves the use of high-resolution images for the production of a dense model and theuse of distances between cameras for computing the scale.

To achieve these goals, the ROV 3D architecture (Figure 2) must be able to record a large quantity ofdata in a synchronous way and perform real-time computations. Within the submarine context and itsresulting constraints, we have developed a modular architecture on which the different features aredistributed. The first module is dedicated to image acquisition. It consists of three numeric cameras(two low-resolution cameras and one high-resolution camera) that are coupled with two LED strobes.An electronic synchronization mechanism ensures that all shots are lit correctly. This synchroniza-tion mechanism is also used to tag images with homogeneous timestamps and to make it possibleto retrieve pairs (for real-time processing) and triple (for metric high-resolution processing) shots. Thethree cameras are linked by an Ethernet/IP link to an onboard computer that archives all produced im-ages. The cameras, the synchronization, and the archiving can be parameterized by remotely sendingUDP commands to the onboard computer.ACM Journal on Computing and Cultural Heritage, Vol. 8, No. 4, Article 21, Publication date: August 2015.

Deep-Sea Underwater Survey Using Photogrammetry • 21:9

The visual odometry computation is divided in two modules. The first step of the processing (detec-tion of the homologous points) is made within the onboard computer when low-resolution images pairsare archived. The set of 2D homologous points attached to two successive couples are then sent by anEthernet–TCP/IP link to the surface computer. An application on this computer is dedicated to thevisual odometry calculation and visualization.

The system presented here is patented by COMEX and the CNRS.

3.2 Photogrammetry

To ensure the complete coverage of the studied zone by the ROV, knowing its position in real time is cru-cial. Since a rigid transformation links the ROV and the photogrammetry system, tracking the latteris sufficient for being able to deduce that of the ROV. The movements of the photogrammetry system,consisting of three high-resolution cameras whose internal and external orientations are theoreticallyknown, must therefore be evaluated. In addition, the assembly must be calibrated before starting themission. Its movements are calculated by the orientation of the photos taken of homologous points ontwo consecutive photos identified and paired on the fly by the ROV’s onboard computer.

In the literature, the bundle adjustment methods used for orienting the photos have proved theireffectiveness [Lourakis and Argyros 2009; Sunderhauf et al. 2005; Xue and Su 2012]. Nonetheless, acalculation phase is required to determine the approximate values required for bundle adjustment.

3.3 Orientation of Photos

Let us consider that the camera assembly to be oriented, where Mj is the set of projection matrices,observes a set of points in space Xi and that xij is the 2D projected point of the ith 3D point on the jthimage. Thus, the orientation of the photos depends on finding values Mj and Xi that solve the followingequation:

Mj Xi − xij = 0. (1)

The orientation of the photos by bundle adjustment is determined by the expression of the equation(Equation (1)) as a minimization problem:

minMj Xi

i, j

d(Mj Xi, xij), (2)

where d(x, y) is the Euclidean distance between two image points, x and y.The minimization (2) can be resolved using the method of least squares that gives a solution by

iteratively solving a normal equation in the following form:

JT Jδ = JT ε, (3)

where J is the Jacobian of the reprojection function. (For more information, refer to Lourakis andArgyros [2009] and Triggs et al. [2000].)

The method of least squares is very costly is terms of calculation time and memory. Moreover, its costincreases considerably as the number of parameters (i.e., extrinsic parameters of the cameras and the3D positions of the points) grows. Thus, to reduce the calculation time, we have to reduce the numberof parameters.

In our application, the photogrammetry system used is a stereo system whose relative orientationis fixed and previously determined through a calibration phase. This characteristic lets us reduce thenumber of parameters linked to the cameras by a factor of 2 [Xue and Su 2012]. In fact, for a stereopair, the extrinsic parameters of the right camera can be determined using those of the left camera.By taking this characteristic into account, Xue and Su proposed a method for bundle adjustment that

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Fig. 4. Real-time visual odometry display as seen on board the ship. This figure shows the 3D point cloud calculated over thesurveyed zone and the position of the vehicle for each photo pair. The density of the points measured on each photo is color coded(yellow, green, and black), which gives an overview of the quality of the orientation (yellow less than 30 points, green between100 and 200 points, dark green greater than 300 points).

reduces the number of parameters while keeping the information from the observations of the left andright photos of the stereo pairs. Since this reduces the calculation time in relation to the bundle adjust-ment methods carried out on all of the photos, this method proves to be sufficient for our application.An illustration of the orientation of several images is shown in Figures 4 and 5.

3.4 Visual Odometry: Rapid Calculation of the Orientation of Photographs

In the context of the estimation of the movement of robots, Sunderhauf et al. [2005] proposed a methodthat simplified and accelerated the bundle adjustment. This method consists of working with a win-dow containing a subset of consecutive photos instead of all of the photos. The photos in this windoware oriented, and the estimated parameters are used for the initialization of the next orientation,whose window is shifted one image. Moreover, to compensate for the lack of precision due to a tinyvariation between the exposures of the photos taken in the window, only photos whose exposures aresufficiently separated (i.e., over a preset threshold distance from the previous one in the window)are retained. This corresponds to the minimal distance of displacement of the vehicle between twophotos.

By combining the works of Sunderhauf and those of Xue and Su, we implemented a new algorithmto orient the stereo images taken by the ROV:

� Starting with image Ii, we look for the closest image Ii+k so that the displacement between Ii andIi+k exceeds a certain threshold. This is determined using odometry.

� Add image Ii+k to the window.� i becomes i + k, and we repeat the first step until we have three stereo image pairs in the window.

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Fig. 5. Four consecutive stereo pairs, oriented using the bundle approach described earlier, with a local densification of theoriginal images.

� Adjust the bundles using the method proposed by Xue and Su using the exposures obtained in thefirst step for the initialization.

� Then, shift the window over one image.

3.5 Calculation of the Approximate Orientation of Photographs for Bundle Adjustment

To calculate the bundle adjustment as described in the previous paragraph, it is necessary to calculatethe approximate position of the cameras.

We have a series of 2D points at our disposal, homologous over four images (two consecutive pairs).The points in the field have been identified on the stereo camera pair at time t, and the same pointswere identified at time t-1. At each step, a set of 2D points, shown on pairs t and t-1, are sent to thesurface computer to calculate the new position of the vehicle. Calculation of the orientation of pair tover pair t-1: The vehicle moves slowly, and we consider that the distance traveled between times t andt-1 is slight, as well as the change in orientation (t, t-1). Under these conditions, we assign the cameraexposures at time t to the results of camera exposures at time t-1. (We consider that the vehicle hardlymoved in relation to frequency of photographs.)

Knowing the relative orientations of the left and right cameras and knowing that these values areconsidered fixed in time, we can calculate the 3D points using the 2D points from pairs t and t-1. Wetherefore have two homologous point clouds calculated at times t and at time t-1 but with the cameraexposures for times t and t-1:

[RT]rightt = [RT]rightt-1 (4)

[RT]leftt = [RT]leftt-1. (5)

If the vehicle was effectively motionless, the two point clouds would be mixed together. In fact, thevehicle’s movement causes a displacement of the images of the points of the terrain on pairs t-1 andt and the calculated points representing the two different point clouds. The rigid transformation [RT]

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required for expressing the cameras t in the reference pair t-1 is the rigid transformation required tomove from the 3D point cloud at time t-1 to the one obtained at time t.

The problem with calculating the orientation of the cameras at time t in relation to time t-1 leadsback to the calculation of the transformation used to move from one point cloud to another. This ispossible under our configuration, with small rotation.

Next, the calculation of the transformation for passing from the point cloud calculated at time t,marked P, to the one calculated at time t-1, marked P′ is shown. We have two sets of n homologouspoints P = {Pi} and P ′ = {P ′

i } where 1 ≤ i ≤ n.We have

P ′i = R × Pi + T , (6)

where R is the rotation matrix and T the translation.The best transformation minimizes err, the sum of the squares of the adaptation residues:

err =n∑

i=1

RPi + T − P ′2i . (7)

To solve this problem, we can use the singular value decomposition (SVD) of the covariance matrix C,which corresponds to a robust algorithm that does not cost much in time. We note COMP and COMP,the center of mass of the set of 3D points P and P′:

C =n∑

i=1

(Pi − COMP

) × (P ′

i − COMP ′)T (8)

[U, S, V

] = SV D (C)

R = VU T

T = −R × COMP + COMP ′ . (9)

Once the exposures of pair t are expressed in the reference system of pair t-1, the 3D points can berecalculated using the four observations that we have for each point. A set of verifications is put inplace to eliminate pairing errors (verification of the epipolar line, the consistency of the y-parallax,and reprojection residues). Once validated, the camera exposures at time t are used as approximatevalues for the bundle adjustment as described next.

3.6 Dense 3D Reconstruction

Following the orientation stage—in our case, the orientation of two consecutive stereo pairs—the 3Dpoint cloud that we obtained is of low density. In the processing chain implemented here, the orienta-tion of these two pairs is performed in a real-time loop at 10Hz. It may be useful for operation managersto have a partial 3D model (to scale) while the ROV performs its survey. To do this, we developed apoint cloud densification model based on the original photos of the photo sequence, and which is possi-ble as soon as the images are transferred on board. (In the future, this densification could be performedusing the computer in the ROV, but the current lack of resources for this machine make it difficult. Infact, the onboard computer is subject to size, power consumption, and temperature restrictions, whichaffect its performance.)

However, this densification is necessary to reconstruct a realistic 3D model. This is done using multi-view stereo (MVS) methods that produce a dense point cloud using photos and the camera parameters.ACM Journal on Computing and Cultural Heritage, Vol. 8, No. 4, Article 21, Publication date: August 2015.

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Fig. 6. Photogrammetric stereo photos taken during odometric navigation while surveying the wreck Cap Benat 4. In the imageon the right, we can see a representation of the epipolar curve clearly showing the strong distortion present in photos takenusing underwater cameras. This modeling using an arc of circle is essential in calculating the visual odometry in real time aswell as in the point cloud densification step. Photographs: ROV 3D system, COMEX.

Furukawa and Ponce [2010] proposed a method based on patch-based multiview stereo (PMVS), a“patch” type of reconstruction. They made a model of the surface S, with a random 3D point p fromsomewhere in the scene, and modeled the patch using a square section of a plane tangent to S at p.The 3D position of the patch is determined by minimizing the variance between these projections onthe photographs. The algorithm functions in three steps:� Initialization of a set of patches by interest points.� Expansion, which consists of reconstructing new patches around those already identified.� Filtering to strengthen consistency and remove any erroneous patches.

We have inserted this method for integration in our processing chain (see Figure 5). On the otherhand, contrary to PMVS, our developments directly use the images produced by the cameras, withoutany rectification or distortion correction, with an adapted algorithm, especially for the calculation ofepipolar lines.

In fact, the distortion transforms made it possible to control the final adjustments and the precisionof the resulting model. To simplify the calculation, this curve is modeled by an arc of circle based onthe fact that the radial distortion is much greater and mainly disturbs the projection of the scene onthe image (Figure 6). The following algorithm is used to calculate the equation of the epipolar curvefor a point M:

(a) The M coordinates are corrected for distortion and eccentricity.(b) Using the new M coordinates, the equation for the ideal epipolar line (dr) is determined as de-

scribed previously.(c) The two intersection points of (dr) with the window of the image are calculated.(d) The center of mass of these two points is calculated.(e) The distortion and eccentricity of camera 2 are applied to the three points (the two intersection

points and their center of mass).(f) The three new points thus obtained are the points that define the arc of the circle.

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3.7 Precision and Control

The orientation in real time performed during the odometry, based on the fact that the vehicle movesslowly and that the images are very close together from one shot to the next, cannot give excellentresults for the whole set of photographs. In fact, the main problem is limiting common points to onlyfour or six photographs (i.e., two or three consecutive stereo pairs).

If the odometry presented here is sufficient to ensure the complete coverage of the site in real time, itis not necessarily true for the precision of the final model, because our goal is the automatic recognitionof each artifact and to evaluate its variance with the theoretical model. We therefore implementeda second step where the homologous points are extracted and paired on all the photographs, and abundle adjustment was performed to ensure the best orientation possible while taking into accountthe constraints related to the set of three fixed and calibrated cameras. Two software programs werethen used to interface with this system: Agisoft’s PhotoScan and Bingo. The use of both programstogether made it possible to control the final adjustments and the precision of the resulting model.

When comparing the results of the two models, the one obtained using odometry and traditionalbundle adjustment, which take into account all observations possible of the 3D points (the pairing ofthe 2D points is done for all photos), reveals the presence of residues of approximately 5mm on theplane (X,Y) and in the 1cm range depthwise on sequences with more than 1,000 photographs. Thesedata should in fact be modulated in function of the quality of the surveyed terrain. Where the seaflooris sandy and without much contrast or easily distinguishable points, the identified points are fewerand of lesser quality. In the areas where the amphora were found, the number of points and theirquality is much better and the residues between models is less pronounced.

The overall model, obtained through bundle adjustment of all high-resolution images, is scaled downby introducing a stereo base of 0.297m (value obtained after the triplet calibration, done before themission in shallow water) as a constraint in the bundle adjustment. In the end, more than 1,000 stereopairs are affected by this constraint, and the residues on their base are all less than 1mm.

4. ARCHEOLOGICAL EXPERIMENTATION

4.1 The Shipwreck

An immense mound of amphora, the wreck Cap Benat 4 is located in a zone where three other ship-wrecks, less deep, have been catalogued during the inventory done by Parker, based on the DRASSMarchives [L’Hour and Long 1983, 1985; Long 1989; Parker 1992; Rieth 1979]. It was discovered in 1977at a depth 328m, approximately 1km off the coast of the semaphore of Bormes-les-Mimosas (Figures 7and 8) by the submarine Griffon (of the French Navy) while wandering the abyss. The wreck receivedtwo short inspections: the first one by Griffon, the same submarine that first detected it, and a secondtime by the submersible Cyana (IFREMER) in 1981. These two dives, during which a dozen containerswere collected, were enough to evaluate the approximate size of the amphora burial ground, which wasabout 15m long and 5 or 6 m wide, as well as its cargo of several hundred Dressel 1A amphoras. In1981, to complete our reconnaissance of the find, three urn goblets were recovered at the same time assome amphora using the robotic arm of the submersible Cyana.

4.2 Stakes and Benefits of the Cap Benat 4 Mission

Following the European VENUS project, which was initiated in French waters in 2008 on the deep-seawreck Port-Miou C at a depth of 105m, this new mission reunited three of the original partners fromMarseille at the origin of the VENUS project in 2014. It was in fact under the supervision of COMEX,DRASSM, and the LSIS laboratory (CNRS) that a new incursion of a scientific nature was able tobe carried out this time, in the mesopelagic zone, on one of the deepest ancient wrecks identified inACM Journal on Computing and Cultural Heritage, Vol. 8, No. 4, Article 21, Publication date: August 2015.

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Fig. 7. General location of the Cap Benat 4 wreck. The red frame is detailed in Figure 8. Map chart from Konmap software,1983.

Fig. 8. A detailed map of the Cap Benat 4 shipwreck. Map chart from Konmap software, 1983.

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Fig. 9. Handmade drawing, by Luc Long, made in 1981 from Cyana.

Frane,the shipwreck Cap Benat 4. Note that these three partners were implicated in the study of deep-sea wrecks since the 1990s, a highly sensitive field of operations that DRASSM initiated in the 1980sand which concerns the study and protection of a very rich yet very fragile underwater heritage.

For the wreck Cap Benat 4, the goal was to obtain, using the Apache wire-guided ROV, a high-resolution and highly accurate (millimetric) 3D model of the deposit with a colorimetric definition ofthe site. But the difference with other similar operations was that the survey of the wreck Cap Benat4 did not include any intrusive actions on the deposit, as it was based only on vertical photos taken ata distance of 1m above the wreck. Therefore, there was not any physical contact with the seafloor, norwere any markers or measuring and calibrating equipment used. In the same spirit, taking sampleswas strictly prohibited because the goal of this mission was also to set ourselves apart from otherprestigious operations, heavily funded and publicized, where the archaeologists, subject to “dumpstersyndrome,” play the role of the sorcerer’s apprentice on sites that they mutilate and for which onlyan erroneous representation is kept. Thus, in 1981, the dive on the wreck Cap Benat 4 using thesubmersible Cyana, and logistics provided by IFREMER took place (Figure 9). Even though the teamACM Journal on Computing and Cultural Heritage, Vol. 8, No. 4, Article 21, Publication date: August 2015.

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Fig. 10. Orthophoto of the site, projection on a horizontal plane. Image produced by CNRS-LSIS.

Fig. 11. Orthophoto of the site, projection on a vertical plane passing through the longitudinal axis of the wreck. Image producedby CNRS-LSIS.

was technically authorized to remove samples of the amphora, resulting in only a small amount ofdamage, in no way was the archaeologist able to record the slightest map of the deposit.

However, the results of the 2014 mission were completely different. All information was collected inonly 2 days on the site and all objectives were met without the least technical glitch, except for a smallproblem with the focus mechanism of Apache’s main camera that was quickly solved.

Although the data are still being processed at the LSIS laboratory, it is already clear that the orien-tations and evaluations performed during the two previous campaigns in 1977 and 1981 have becomecompletely obsolete and have been correctly updated in 2014. It is important to remember the interestfor archaeologists in studying a deep-sea site that has not been trawled and which, consequently, maydeliver capital information about its structural organization and the disposition of its cargo.

In light of this new mission, we can now state with certainty that the wreck lays in a west-eastdirection and is 16.5m long and 7m wide with a maximum height of 2m. The number of containers,originally thought to be a few hundred when first discovered, immediately reached a thousand individ-ual items after revision; an exact count to the very last amphora will be published shortly. Moreover,we now know the exact location of the urn goblets that were part of an additional cargo and whichin all likelihood were in the stern of the boat in the west. However, the most enlightening discoveryis the new assessment of the wreck’s morphology, which clearly shows two different cargo areas. Onthe first third of the ship, to the east, only two layers of amphora, visibly lying toward the east, weredetected. In the second third, corresponding to the middle of the ship, straight amphora are this timepiled much higher (2m), or at least three layers deep (Figures 10 and 11). In the last third, to thewest, the amphora are again covered in sand and seem to take up less space. Although it still maybe premature to make any conclusions about these new observations, as an ancient ship may havesubject to many fluctuations after sinking and collapsing under the weight of the amphora as its hullsprogressively absorb water, we can nonetheless put forward a preliminary hypothesis. It may in fact

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be due to the prior existence of bridged sections, on the prow and stern, which limited the number ofamphora, and the parts that were open, in the middle, where as a result the cargo had an extra layer ofamphoras.

4.3 Mobile Archaeological Material

4.3.1 Amphoras. From a historical viewpoint, this cargo of amphora is part of the vast trade routeof Italian wine and tableware or storage containers flowing toward Gaul in the second half of the 2ndcentury BC. The first assessments of the cargo in 1981 led to the belief that it could have held 300amphora, as stated in the Lex Claudia in 218 BC, which, according to Tite Live, limited the loadingcapacity of ships belonging to senators or their sons. But the number of containers counted in theship, thanks to the first reproductions that were performed in the scope of our mission, shows a muchgreater number of amphora—approximately 1,000 individual items.

From a typological viewpoint, these amphora from the cargo correspond to Dressel 1A type contain-ers used to transport Italian wine, relatively standardized and without any markings. In view of themodels that were removed in 1977 and 1981 by the submarine Griffon and the submersible Cyana,the amphora are rather small with an overall height less than 1m (99cm) and a capacity that variesbetween 19 and 19.5 liters. They are therefore quite far from the 26-liter model that prevailed for mostDressel 1B type amphora from the 1st century BC, which corresponds to the amphora, or amphoraquadrantal (the Roman measuring unit equal to a cubic foot). From their morphology, they correctlybelong to the Dressel 1A family, even if they seem to correspond to a hybrid model between Greco-Italic and Dressel 1. The Dressel 1A shape was defined by Lamboglia in 1955, based on the very firsttypological table started in 1872 by Dressel and published in 1899 (Corpus Inscriptionum Latinarum,volume XV). In addition, this shape corresponds to the Lyding-Will Type E class. The Dressel 1A refer-ence models are therefore a little different from the amphora from the Cap Benat 4 wreck, which havecertain details that remind us of late Greco-Italic amphora, with a ogive-shaped body, from the secondhalf of the 2nd century BC. In fact, from a morphological viewpoint, the Cap Benat 4 amphora arenotably recognized by the shape of their handles attached close to the rim, which itself is a truncatedcone flared toward the bottom, with sloping shoulders, a tapered belly, and a short and narrow foot.They can be classed in a group of amphora whose immersed cargos were inventoried on the followingshipwrecks: Mont Rose in Marseille; Canonnier du Sud in La Ciotat; Filicudi A and Punta Scaletta inItaly; and Pointe du Brouil, Cavaliere, and Roche Fouras in the Riviera. Consequently, and althoughstill under study, the chronological period seems to best correspond to amphora dating from 125 to 100BC. On the models that were recovered, the colored clay included a degreasing agent made of blackparticles and fine shiny inclusions, closely resembling Vesuvian clay.

4.3.2 Urn Goblets with Almond-Shaped Rims. During the brief observational campaign of the wreckusing the Apache and Super-Achille ROVs, a total of eight urn goblets with almond-shaped rims werelocated at one end of the site, which makes a total of 11 vases of this type confirmed on the wreck, if wetake into account the three examples recovered in 1981. From this moment, we can consider that theseollae made of common ceramic, of which some are still found inside each other, were in the middle ofbeing loaded and completed the cargo of amphora. These vases, whose almond-shaped rim was directlyattached to the top of the body, thus have a quite rounded belly and a variety of sizes and shapes thatlet them be stacked inside each other during shipping.

This type of vase, used for cooking and storing products, is well documented in the northern andcentral southern areas of the Tyrrhenian Basin in the 2nd and 1st centuries BC. It is also foundin southern Etruria and Latium in the late Republican era. Its very common shape is similar toACM Journal on Computing and Cultural Heritage, Vol. 8, No. 4, Article 21, Publication date: August 2015.

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traditional Etruscan bucchero urns from the end of the 5th century BC, of which samples were foundon the shipwreck Grand Ribaud F.

In the current typology, we can add this shape to the following types: COM-IT 1 olla from Lattes; 2from Vegas; 1, 2, 3, 4, and 6 from Albintimilium; and three of the common ceramic typology producedin Rome and Latium.

In Italy, these vases were confirmed in Albintimilium, Luni, Cosa, Gabii, Sutri, Ostie, Roma, wherethey belong in a context of the Domus Publica dated between 15 and 10 BC, in the ager Veientanus,where the oldest date from the 5th century BC, and on numerous sites in the Tiber River Valley. Theycan also be found in Pompeii and Stabies; in Gaul in Olbia and in Marseille, in a Hellenistic context(Baou de St Marcel); on the Iberian Peninsula in Pollentia (Majorca), Ampurias, Burriac, L’Argilera,Valence and Siviglia in the lower Guadalquivir Territory. Elsewhere, urns with Tarraconensis almond-shaped rims were recently attributed to a production area located in south-central Italy based onarchaeometric analyses.

On the shipwrecks, excluding Cap Benat 4, a small number of ollae with almond-shaped rims, mostlymedium size, were found on ships from the late Republican era, along with Dressel 1 amphora andsometimes with tableware from the Campania Region having black glaze. This is the case with wrecksfrom Albenga (circa 90 to 80 BC), Spargi (circa 120 to 100 BC), the Secca dei Mattoni (Ponza) (end ofthe 2nd to beginning of the 1st century BC) on the Tyrrhenian coast; in France on La Ciotat 3 (circa80 BC), Fourmigue C (circa 70 to 60 BC), Cavaliere (circa 100 to 75 BC), Grand Congloue 2 (circa 110 to70 BC), and Sant Jordi (circa 100 to 75 BC) in Majorca. These fragile creations, most probably loadedin crates on top of the amphora, were associated with long-distance maritime trade routes most oftenfrom the end of the Republican era.

4.4 Analysis of Results

One of the advantages of deep-sea shipwrecks is their straightforward readability, which makes it eas-ier to understand a ship’s layout. Located far from shore, well away from the vegetation and sedimentthat is always heavy near the coastline, the remains of the wreck Cap Benat 4 offer themselves toscientific study without difficulty.

On this point, the information obtained in a very short time on the site during the 2014 missionmakes it possible to completely reconsider our knowledge of the deposit (see Figures 14 and 15). Al-though the analysis only covers the upper part of the pile of amphora, above the sand, it nonethelessprovides additional information concerning the orientation, size, and shape of the site. Indeed, new hy-potheses help to nourish possible explanations about this deposit, and more precisely concerning theship’s structure and layout, the different parts and size of its cargo, the number of amphora visible,and the morphological variations between the containers. The archaeologist’s degree of knowledge isbased on a more overall perception of the cargo, as well as his now deeper knowledge of the extra cargo,consisting of a series of urn goblets stacked inside each other at one end of the ship, probably the stern(Figures 12 and 13).

This reconstruction of the wreck, made using quick, noninvasive photogrammetrical techniques,enables us to continue to study the site once back on land. The measuring of objects with millimetricprecision lets us refine our observations and come up with new sets of issues.

By working with collected data, our analysis will be based on, for example, reproducing the am-phora almost completely hidden by the sand to reevaluate the successive decks of cargo and virtuallyrebuilding the collapsed parts, thus putting the ship back into its original shape.

Today it is physically possible to study in depth an ancient deep-sea deposit through its virtualrepresentation. This creates a certain perception that is easier to understand than the reality, withouthaving to dive back into its natural environment.

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Fig. 12. An urn goblet visible on the high-resolution survey. Orthopho detail. Image produced by CNRS-LSIS.

Fig. 13. Two of the three urn goblets taken from Cyana in 1981. Photograph courtesy of Ph. Foliot, CCJ/CNRS.

Paradoxically, the recent data acquired on the wreck Cap Benat 4 rival the scientific contents of theDRASSM archives of many identical shipwrecks, excavated by divers on easily accessible seafloors, butoh how much more destructive.

5. CONCLUSIONS AND PERSPECTIVES

The approach presented in this article was developed in the scope of the ROV 3D project. It involvesthe implementation of a 3D survey of a shipwreck, or more generally of any deep-sea site, located at adepth greater than 100m. The method developed here is completely contactless and nonintrusive. It isdeployed using a small ROV; only the time required for the vehicle to maneuver over the entire site isneeded for the survey.

The photogrammetric approach consists of two aspects corresponding to the two requirements neededfor this type of survey: first, an estimation in real time of the zone covered with the possibility of usingmeasurements by sounding locally; second, a high-definition survey, with a time delay, with millimetricprecision and a practically complete coverage of the zone. These two aspects are simultaneously man-aged using the instrumentation mounted on the ROV. A real-time calculation of the odometry, withan onboard display, enables the approximate orientation of all photos taken by the ROV and in realACM Journal on Computing and Cultural Heritage, Vol. 8, No. 4, Article 21, Publication date: August 2015.

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Fig. 14. 3D model of the wreck Cap Benat 4: 328m deep, 2-hour dive, 12,000 photographs, 250 million points.

Fig. 15. 3D model of the wreck Cap Benat 4. Detail.

time. At the same time, the high-resolution images are stored in the onboard computer and are usedto calculate a high-resolution 3D model of the zone covered.

Note that the chosen approach, which is purely photogrammetric, enables the survey of complexstructures, such as caves and overhangs, and is no longer limited to traditional bathymetric surveyswhere the final product is nothing more than a digital elevation model. A preliminary experiment ofthe system, presented in this article, was performed with a single dive off the coast of Bormes LesMimosas on the wreck Cap Benat 4 at a depth of 328m.

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5.1 Beyond This Approach

As archaeological excavations often result in irreversible damage, it is important to ensure that theyare accompanied by relevant documentation that takes into account the accumulated knowledge gainedfrom the site. This documentation is generally iconographic and textual. Graphic representations of ar-chaeological sites such as drawings, sketches, watercolors, photographs, topographies, and photogram-metries are an essential phase when talking about surveys. However, as highlighted by Buchsenschutz[2007] in his introduction to the images and archaeological surveys, from proof to demonstration con-ference, in Arles in 2007, “Even when very accurate, drawings only retain certain observations tosupport a demonstration, just as a speech only retains certain arguments, but this selection is notgenerally explicit” (p. 5). In a certain way, this sets the foundation for the further development of thiswork: surveys are both a metric representation of the site and an interpretation of the same site by thearchaeologist.

Surveys are very important components of this documentation, and their importance is mostly dueto the fact that the concepts handled by archaeologists during an excavation are strongly related tospace. The very structure of an excavation is based around the notion of a stratigraphic unit. Inheritedfrom a geological approach, then formalized for archaeology by Harris [1979], the stratigraphic unitsare connected to each other through geometrical, topological, and temporal relationships and givestructure to the reading of the excavation.

Two families of objects must be distinguished: parts of terrain, or more generally, areas of space,organized into stratigraphic units, and the artifacts that we try to position in that space and poten-tially try to accurately represent after laboratory study. The work presented here builds on these twoaspects: surveys of both areas and of artifacts, using two different approaches—one based solely ongeometry and the second using theoretical knowledge during the survey. Terrain surveys, for exam-ple, are based on photogrammetry tools and are only represented by a set of points linked together.The second approach, based mainly on one’s knowledge of surveyed artifacts, uses this knowledge todimension and position the object in space based on the existing point cloud. Research into knowledgerepresentation by ontology [Monroy 2010] and both 2D and 3D shape recognition has already started[Mahiddine et al. 2014] and should rapidly allow the extraction of a point cloud of relevant objects,recognize their typology, and highlight any deviations from the theoretical model. This aspect of theresearch is being developed in the Generalisation du Releve, avec Ontologies et Photogrammetrie, pourl’Archeologie Navale et Sous-marine (GROPLAN) project (http://www.groplan.eu) funded by the FrenchNational Agency for Research.

REFERENCES

G. F. Bass. 1970. Archaeology Under Water. Penguin Books, Harmondsworth, England.

G. F. Bass and D. Rosencrantz. 1973. L’utilisation des submersibles pour les recherches et la cartographie photogrammetriquesous-marine. Paper presented at the L’archeologie subaquatique, une discipline naissante, Paris.

O. Buchsenschutz. 2007. Images et releves archeologiques, de la preuve a la demonstration. Paper presented at the 132e congresnational des societes historiques et scientifiques, Arles.

P. Chapman, K. Bale, and P. Drap. 2008. Exploring underwater sites: Virtual submarine allows access to Europe’s shipwrecks.Journal of Ocean Technology 3, 4, 36–43.

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Received December 2014; revised March 2015; accepted April 2015

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