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REPORT FOR AN HONOR FROST FOUNDATION PROJECT IN MARITIME ARCHAEOLOGY

THE ANALYSIS OF MODERN DOCUMENTATION METHODS: A CASE STUDY OF A SHIPWRECK OFF

VERUDA ISLAND, CROATIA

ROMAN SCHOLZ, LUKA BEKIĆ, MLADEN PEŠIĆ, MARCO BLOCK-BERLITZ

Fig. 1: Island of Veruda with an inscribed location of the shipwreck (Digital Orthophoto map of Croatia)

SUMMARY The island of Veruda, also known as "Monks' Island", is a favourite holiday destination for the citizens of the city of Pula, Croatia. Archaeologists from the International Centre for Underwater Archaeology in Zadar (ICUA) surveyed the Veruda area in the autumn of 2013. They discovered a mound of ballast stones which appeared to be a shipwreck. Small archaeological artefacts and the remains of the structure of a wooden vessel were discovered under the ballast stones. By 2016, the Veruda excavation project was founded by the ICUA Zadar, the German Archaeological Institute (DAI) and the Faculty of Information Technology/Mathematics at the Dresden University of Applied Sciences for Technology and Economy (HTW-Dresden). A completely new system of digital documentation was developed and used during excavation in the spring of 2016. In this manner, the complete wooden construction of the vessel was uncovered and a very precise 3D model and drawing were produced by Structure from Motion. The artefacts that were recovered during the excavation suggest that the ship's cargo consisted of copper scrap metal and some half-finished copper and bronze objects. A few small shards of post-medieval pottery and glass can be dated to the second half of the 16th and beginning of the 17th century.

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PROJECT DETAILS In recent years, a wide variety of documentation methods have been used by field archaeologists. Both the 2D photogrammetry and the so-called Structure from Motion (SFM) method have come to the fore as means of avoiding the time-consuming process of writing measurements and descriptions by hand. The details of the procedures for such methods have already been comprehensively described elsewhere (e. g. FISCHER 2015). Although both methods have been used very successfully in excavations on land, despite all due diligence, occasional defective or partially inaccurate results do occur. Naturally, this problem can be exacerbated when the methods are utilized underwater. By presenting the first results of "The Analysis of Modern Documentation Methods: A Case Study of a Shipwreck off Veruda Island, Croatia" project, the present study offers a possible solution to those very same problems.

Fig. 2: Condition of the site upon discovery (L. Bekić)

This project was organized in cooperation with three institutions: The Romano-Germanic Commission of the German Archaeological Institute (RGK), the UNESCO cat.II Centre for Underwater Archaeology in Zadar (ICUA) and the Dresden University of Technology and Economics (HTW Dresden). All three of these collaborated in the revision and testing of this new working process. Dr. Luka Bekić and Mr. Mladen Pešić (his deputy) managed the archaeological research project. Mr. Roman Scholz and Mr. Andreas Grundmann (UWA Logistics) were responsible for the methodological research. All of the above were supported

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by a team of archaeologists, including Maja Kaleb, Roko Surić and Dominic Hosner as well as Borna Krstulović, Sandra Kamerla Buljić and Enis Buljić. Under the leadership of Prof. Dr. Marco Block-Berlitz, Benjamin Gehmlich, Dennis Wittchen and Niklaas Görsch of the Dresden University of Technology and Economics (HTW Dresden) supported work on the shipwreck in their capacity as IT experts. The project owes logistical support to "Indie" Dive Centre at Indije campsite in Banjole. The Ministry of Culture's Department for the Protection of Cultural Heritage at the Conservation Department in Pula granted permission for the research project which was funded by the German Archaeological Institute, Honor Frost Foundation, the International Centre for Underwater Archaeology in Zadar and the "Friends of Archeology in Europe" Union. The field campaign lasted from 29th of March to the 19th of April 2016. The island of Veruda is located in the northwest of Croatia on the eastern side of the Istrian Peninsula near the town of Pula. Veruda, also known as "Fratarski otok" (Monks Island), is a popular daytrip destination for the inhabitants of Pula, especially in the summer months. It is located south of the eponymous bay with numerous ports and lies to the north of Pješčana uvala Bay (see Fig. 1). With the exception of the many stray finds which have been recovered between Veruda and the nearby Seline (or Stoka) peninsula, no archaeological evidence has been documented around the island. As a whole, the finds that have been recovered by the archaeologists of the ICUA Zadar represent items which were lost overboard and date chronologically from a variety of periods spanning from the Roman period up until modern times. It is assumed that this place was a very convenient anchorage because of its depth (up to 10 m), its sheltered location and its accessibility from both the north and the south. During a recent review of the site (2013), archaeologists from the ICUA Zadar discovered a large number of ballast stones near the island (Bekić 2013, 49) which seemed to represent a shipwreck (see Fig. 2). Upon more careful examination of the site, surface archaeological finds were discovered as well as wooden remains under the ballast stones, clearly the remains of an older wooden ship. It became obvious that archaeological intervention was required for the site to be spared from further deterioration. The wreck is positioned at about 6 m depth and less than 100 m from the coast. Due to its location in the narrow area between the mainland and the island, the surrounding area is prone to currents which frequently changes direction. Depending on the direction of the flow, the visibility ranges from 3-4 m to 15-20 m. The ballast heap stretches 0.5 m above the sea floor and measures approximately 8 m long and 6 m wide. It is situated on a rocky terrace with little sand. The individual stones are up to 20 x 20 cm in size. Once removed, the wooden structure which lays below them was clearly visible. Due to the good topographic location and the special risk to the site, this wreck was particularly well-suited to this project. To date, the SFM method has been documented and tested in underwater archaeology on smaller objects (although the areas thereby investigated have only covered a few square meters) with a relatively small number of images (see Jansa 2013). For larger objects, the use of this method requires much greater technical and/or logistical effort. As underwater archaeology's initial experiments in this area have been unsatisfactory so far, we aimed to develop a new methodology which was specifically geared to meet the requirements of large-scale underwater projects. Naturally, some of the difficulties involved therein stem from the special environmental factors present under water, such as the poor visibility caused by suspended particles in the water, poor light conditions and the filtering out of the individual spectra of the visible light. Therefore, developing an alternative to hand-held photography was inevitable.

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SFM- DOCUMENTATION WITH THE MULTIFUNCTIONAL BRIDGE The use of a multifunctional bridge developed by UWA Logistics and the Landesverband für Unterwasserarchäologie Mecklenburg-Vorpommern represents a decisive step for the project, both in terms of raw technological advancement as well as in the refinement of its applications. It follows, therefore, that this paper will discuss general approaches to various common problems by means of the utilization of the multifunctional bridge for the SFM method. The excavation of the wreck off the island of Veruda had to meet several requirements. On the one hand, the endangered site was to be excavated to the greatest extent possible. In the interest of conservation, the ship's parts were to remain in place and be secured as best as possible after excavation was completed. Of course, excavation documentation was not to fall behind the quality standards on land, but should rather have been improved by the use of SFM. Since a large number of finds were expected, these were treated as single finds and were measured individually. However, only a small window (20 days) was available for the completion of the "wet" portion of the project. Documentation and recording often take up a considerable amount of time; as a result, every detail had to be prepared and carried out as efficiently as possible. Over the course of the project, a stable local survey system was established for all documentation tasks which served as the basis for every measurement and drawing. Underwater archaeology often involves the installation of fixed frames or pole systems which can stretch to enormous proportions depending on the research object and its location. In addition, those same poles and lines can hinder the work of the diver or even be disturbed by external influences (e.g. storms or misplaced anchorages). Because of this, those fixed frames must often be replaced.

Fig. 3: Excavation under the multifunctional bridge (M. Pešić)

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The project has clearly showed that many of these problems can be solved by the newly-developed multifunctional bridge. In one and a half working days, two parallel axes were laid on the site. For this purpose, two 6 m pipes were connected and anchored in the subsoil with the axes horizontal to the seafloor. Both pairs of pipes were set approximately 6.5 m from each other as a result of the space taken up by the wreck and functioned as a kind of "rail" for the subsequently-constructed multifunctional bridge. The multifunctional bridge consists of an aluminium frame of variable length and adjustable height which can bridge even large distances without sagging in the middle (by means of precise buoyancy adjustments). Since the multifunctional bridge was positioned about 1.6 m above the object, divers could work directly beneath it. As soon as finds were cleared, they were measured and recovered by a second diver. For documentation, the excavation section was divided into five 2-3 m wide strips. The zero point of the rails was at the northeast corner, a point which also did double duty as a starting point for the excavation work. The sand deposits (between 5-30 cm thick) were removed with a vacuum (counter current principle). Finds that eluded the careful eyes of the research divers could nonetheless be located by strip and quadrant in collection baskets sorted on board the research boat. The 30 to 60 individual objects found per day were immediately photographed, measured by means of the multifunctional bridge and recovered. In just eight days, the ballast stones were removed with lift bags and the entire wreck could be cleared over an area of approximately 13 x 6.5 m. However, it was only in the photo-based SFM documentation that the real advantage of the multifunctional bridge was demonstrated. Increased demands for drawing in archaeological field documentation is leading ever more directly to the use of photogrammetry and SFM documentation. The greater information content as well as the shorter amounts of time required for the implementation of the latter methods are decisive. These advantages are particularly important for underwater work, since other methods (such as tachymetry and laser scanning) are not applicable underwater. Photo-based documentation methods have long been tried-and-true standbys in this field of research; George Bass described then to the Jesuit Pére Poidebard in his investigations of the port facilities in Tire from 1935 to 1937 when he made the first underwater archaeological photographs (Bass 1967, 117 f.)

Fig. 4 (left) Colour change as a result of light filtration in underwater photography and (right) the correction of colour values (R. Scholz)

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The quality of such photos is dependent upon a number of factors. To whit, the absorption behaviour of light in water (itself dependent on depth and the distance of the camera to the object) gradually loses individual spectra which is then replicated in the resulting image. In order to keep excavation documentation as true to reality as possible (i.e. in order to avoid falsified colour values), this must be corrected. The placement of a colour chart within the periphery of the image area allows the results to be controlled and corrected at a later date (see Fig. 4). There are some additional problems with the subsequent use of SFM. Typically, larger objects are recorded by oblique images taken from different positions (Fischer 2015, 9). This creates images with varying distances between camera and object. This has the added (and undesired) effect of creating unfavourable spatial depth on the images for the necessary subsequent processing (see Fig. 5). Since colour correction adapts individual colour spaces, those areas closer to the camera are reddened while the background remained blue (as there is no red light). Due to the poor light conditions, photographs must be taken through a small aperture. The blurred areas produced thereby make it difficult to translate the images into a good 3D model. It is advisable, therefore, that the assumption of this strategy be discouraged on land.

Fig. 5 Undesirable colour change following colour correction (R. Scholz) The use of a colour chart on each image proved to be of little use due to the time and effort required for its inclusion as well as its tedious subsequent removal from the 3D point cloud. Moreover, the transfer of the correction values to a series of images is only possible under constant recording conditions. It is, therefore, inadvisable for a series of hand-held photographs; the use of technical aids is recommended instead. In addition, the visual conditions at the site had a decisive influence on picture quality. In contrast to the Adriatic, visibility in the Baltic Sea, for example, often stretches only over a few meters. In order to produce correspondingly usable images, the distance between the camera and the object must be adapted to the viewing conditions. To achieve the recommended image overlap of 60-80%, a corresponding number of individual images are

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necessary. In the case of larger objects, this can quickly lead to a rapid increase in the number of images and, thus, to increased processing time. Although lenses with an average focal length are usually recommended in such cases (Fischer 2015, 2 f.), here is more advisable to make use of wide-angle lenses. Attempts at photographing the site from oblique angles while free-floating for SFM documentation led to less than optimal results, even under the good visibility conditions of the Adriatic. Thus, the 275 individual images could only be calculated after colour correction into a complete point cloud. Since the series of images showed undesirable colour deviations such as those described above, the resulting pictures contained gaps, imperfections and nonuniform textures (see Fig. 6). Although the results could, in fact, be used to achieve a precise transformation of the structure, they did not did meet modern documentation standards.

Fig. 6 Orthogonal view of the 3D model of the ballast pile at the site of Veruda (R. Scholz) The causes of these poor results could be the blurring of the posterior areas of the image caused by the oblique image and the turbidity of the water. On the other hand, unfavourable colour changes occur on the structures due to varying distances from the objects and the colour correction.

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Fig. 7 Differences in the image after a series of SFM photos of the ballast pile after colour correction (R. Scholz) In order to calculate a point cloud and, thus, a 3D model from a series of images, the program utilized here used the SIFT algorithm to search for so-called key points: areas that occurred in several images. In this multi-step process, the pixels of the image were compared with neighbouring pixels in the next image. In this way, the same points in two photos were recognized and used for further processing. The more blurred the images and the more differences there were between the colour from one photo to the next, the lesser the likelihood that the program would find sufficient key points for the next step in the process. When this occurred, images were not included and the calculation "tore". The greater the number of images which could not be included, the higher the likelihood that subsequent recordings would not have enough overlap with their predecessors. This, in turn, resulted in an incomplete point cloud (see Fig. 6).

Fig. 8 (left): SFM point clouds of the ballast pile without colour correction (right) Demolition of the SFM calculation of the same data with colour-corrected images; the red frame shows position in the overall geometry

(R. Scholz) In the example shown in Figure 6, 193 images were used for an aSPECT3D calculation. For the non-colour corrected images, a total of 134 single-point clouds were generated. Only 13 single-point clouds could be determined during the colour-corrected image run. However, neither result could be put to meaningful use. One of the possible solutions to this would be

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the use of the multi-functional bridge. The multi-functional bridge was equipped with a measuring slide. This was intended for the capturing of single points (e.g. finds or pass points) as well as for the installation of a camera. Thus, it is possible to produce perpendicular photos with only small changes in distance and good depth sharpness. This also limits the possibility of areas being forgotten due to human error and then later being missing from the final 3D model. At the same time, the increasingly time-consuming act of focussing is also thereby omitted.

Fig. 9 The 6 x 2 m version of the multifunctional bridge (M. Kaleb) Due to the special working conditions at underwater sites, it is almost impossible to completely uncover and document a structure at the same time. Within a short time, a thin layer of sand covers the object under study and makes photo recording even more difficult. Therefore, it is recommended that one always uncover an object in sections followed immediately by the taking of pictures. Since the multifunctional bridge was positioned about 1.6 m above the object, divers were able to work directly beneath it. For the SFM documentation, the excavation trench was divided into five 2-3 m strips. The documentation of each strip generated between 104-241 images. Depending on the computing power of the PC used, each individual strip (or even the entire digging area) could be offset with the SFM software.

Fig. 10 (right): Laying the rails for the multifunctional bridge (M. Pešić) Fig. 11 (left): Exposure of the wooden structures under the multifunctional bridge (M. Pešić)

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Since the SFM software generates an average of 100 million individual points per strip, it is advisable to calculate the object piece by piece and to geo-reference the points over the markers after a targeted session. After the completion of this step, the point clouds can be merged. Thus, even with an average computer, it is possible to process such a quantity of data. Due to the high image quality and the constant recording conditions, it should be noted that the scatter of the dots in the available density is relatively low. If the percentage of measuring points that deviate at least 1.9 cm from the computational object plane would normally have a low two-digit value, in the instance of the wreck they were at a maximum of 0.2% (see Fig. 10). Since all the colour values of the images had previously been corrected, the texture is very natural (see Fig. 11). The resulting 3D model enables a highly accurate orthogonal view which, in turn, offers a suitable basis for the conversion.

Fig. 12 (left): Representation of the point scatter in the aSPECT3D program (R. Scholz) Fig. 13 (right): Orthogonal SFM view of a section of the wreck (R. Scholz)

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Fig. 14: Orthogonal SFM view of the entire wrecks (R. Scholz)

In a second step, an image can be georeferenced, converted and interpreted in a GIS or CAD program (see Fig. 19). The underlying 3D model can also be used for further evaluation or for reconstructions and queries. However, this part of the process is still underway; our understanding of its finer points will be deepened by further study. SFM- DOCUMENTATION WITH A UAV The projects Archaeocopter and Archaeonautic (HTW Dresden and Freie Universität Berlin) were initiated in cooperation with the German Archaeological Institute (DAI) and the Archaeological Heritage Office in Saxony. The philosophy of both projects is not to ask for the maximum possible, the latest in documentation techniques, it is more advised to ask, what do we really need. And with this motivation behind, we try to find cost-effective solutions as alternatives to established documentation techniques in archaeology and to make them practical for everyone. Videogrammetry in Underwater Archaeology In the last decades, archaeologists highly profit from developments in UAV (Unmanned Aerial Vehicle) and enormous progress in camera technique. Complete easy-to-use solutions are available off the shelf. Videogrammetry versus Photogrammetry Beside proprietary solutions, with VisualSFM , MicMac (DESEILLIGNY 2011), OpenMVG (MOULON 2013) or Bundler (SNAVLY 2006) also free software packages are available. At the same time, experiences and methods of aerial archaeology and close-range photogrammetry meet each other (LUHMANN 2007). The biggest percentage of publications uses the proprietary software AgiSoft PhotoScan indeed, but to reduce some costs while producing stable quality, the percentage of using free software is increasing (RENDE 2015). In contrast to related projects, we prefer to use videogrammetry (GREENWOOD 1999, PAPPA 2003, NISTÈR 2004. POLLEFEYS 2004) instead of the usual photogrammetric approach (HARTLEY 2004). The main challenge in videogrammetry is to find an answer of a two contrary constraints: On one hand, we want minimize the distance between correlating images to maximize the intersection set and produce more 3d points (Fig. 14).

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Fig. 14 – Left: A high intersection set of two images A and B1, results in a short baseline. The same 3d point X is represented as an 2d point x in image A and x’ in image B. With small variance in both 2d points leads to a big area of uncertainty. Right: As longer the distance as smaller the area of uncertainty. The correctness of the estimated position of X increases. But, on the other hand, we need to maximize the distance between correlating images to reduce the uncertainty of the resulting 3d points, because only good points will survive in the 3d model. From our perspective in recording data while moving, videogrammetry is the more fault-tolerant, more cost-effective and easier-to-use approach. The software JKeyFramer, an automatic key frame selection tool, was one of the important outcomes of the project Archaeocopter. This tool was at that time an important step towards fast 3d reconstruction. Meanwhile, it allows us to render fast preview models on site. Reconstruction pipeline Archaeo3D Within the scope of the Archaeocopter project, the semi-automatic software Archaeo3D was developed to optimize and fulfil the complete reconstruction process. Videos and photos were automatically imported and processed. The software is able to reorder or change the pipeline modules and adjust the parameters, according to the current hardware and the real recording situation and complexity. VisualSFM and CMPMVS are mainly used. Additional software components like JUndistortion, for automatic camera calibration, and JKeyFramer, for automatic key frame selection, were developed and integrated. The reconstruction pipeline of Archaeo3D includes the following processing steps and software packages: 1) Data recording GoPro Hero 3/4 or photo sets 2) Keyframe extraction VLC, MPlayer , ffmpeg , JKeyframer 3) Image undistortion OpenCV, JUndistortion 4) Image improvement Resizing, JEnhancer 5) Feature extraction SiftGPU, JFeatureManagement 6) Sparse reconstruction VisualSFM 7) Dense reconstruction CMVS+PMVS (FURUKAWA 2010) 8) Compare or reduce point clouds CloudCompare 9) SGM, Surface fitting Poisson recon. (KAZHDAN 2006), CMPMVS 10) Producing orthoimage CMPMVS (JANCOSEK 2011) 11) Georeferencing, cleaning MeshLab (CIGNONI 2008) 12) Integrate data into GIS gvSIG CE, Quantum GIS

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The georeferencing step, after the 3d reconstruction process, is an important step due to the fact that 3d models of areas without local identification or 3d models of artefacts without scale often are scientifically meaningless. In the Archaeo3D workflow the free software package QuantumGIS is integrated and fulfils this task. As an alternative, the point cloud can be georeferenced also in VisualSFM. In Archaeo3D point clouds can be produced very fast and already be examined on-site with the benefit to validate the results as soon as possible. The final reconstruction with Archaeo3D at home will produce more detailed results. This technique was first used during the campaign in Tamtoc/Mexico 2013 (BLOCK 2013). At first, different pointclouds of an Huastec temple site were produced, computed and validated on-site and in the aftermath the complete 3d model was produced in Dresden. Aspects of Underwater Archaeology. Today, the used documentation methodology in underwater archaeology is still complex and expensive, even when sonar and laser scanning be deployed (MOISAN 2015). The significance of the photogrammetric approach analogous to aerial documentation is increasing (BALETTI 2015, PRUNO 2015). The results of land- and water-based photogrammetry are quite comparable. Depending on the used setup, only small details are lost in underwater scenes (TROISI 2015). Today, scientific divers use cameras with high resolution, plan sets of photos and needs a special qualification to work underwater (PAPADIMITRIOU 2015). The dive time is limited to the diving depth. However, the careful and systematic excavation under water is still a human domain. In comparison to aerial documentation with UAVs, the georeferencing underwater is still a crucial challenge. Natural or additional artificial markers on the ground (Ground control points, GCP) need to be good detectable and recognizable. But the measuring of these points is still a problem, because no GPS signal is directly available underwater. To localize the markers accurate, often indirect solutions be deployed. In areas with shallow water, the distance between the water surface to localize relative to the satellites and the marker position can be bridged by a perpendicular stick (BALETTI 2015). OpenROV-based UUV Eckbert II Beside the documentation of archaeological sites, the exploration of potential sites gets more and more interesting. Small diving robots, so-called UUVs, can be used to both cases. In respect to our philosophy, three available cheap UUVs were needed to be compared. The Neptune SB-1 has a wireless control, but is only able to work 45 minutes with a maximum distance of ten meters in a maximum depth of five meters. Without changing the hardware drastically, this UUV is impractical for our purposes. The technical datasheets of both solutions BlueROV and OpenROV in terms of distance and depth are more or less equal. We decided to work with the OpenROV due to the large community and the open-source software approach. To know things behind or to have the ability to change things behind are always advantages. Later, its theoretical possible to integrate the software developed for OpenROV into the BlueROV. To pursuit the aims of documenting archaeological sites and exploring unknown areas, the small submarine Eckbert-II based on the OpenROV was developed in our project Archaeonautic and is still under modification. As two useful extensions, cable management and a balance setup for underwater taring to salt and fresh water was designed. Promising 3d reconstruction results using GoPro cameras with structure-from-motion (RENDE 2015) and stereo vision (REPOLA 2015) underwater were shown successfully. Therefore, on both sides we added two diving torches to be able to get good light conditions if necessary and also one GoPro Hero 4 BE (Fig. 15, left).

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Fig. 15 – Left: The flexible Camera-Lighting-Setup can be composed, depending on present documentation and environmental conditions. To document the palafittes in Mondsee, three GoPro Hero 4 BE were deployed. Subsequently, available weights and buoyancy bodies were combined to balance the UUV. Right: The images show Eckbert-II in Mondsee. The documentation can be fulfilled in complete darkness, due to the used diving torch setup. Under these conditions 3d models can be also produced successfully. The Pro LED Scuuba 860 with 860 lumens and a potential maximum depth of about 100 meters was chosen as diving torch. It outreaches the GoPro standard case with about only 40 meters. One torch is placed 15 cm ahead and the other 15 cm after the central placed GoPro. Both torches produce a homogeneous lighted area around the recording field (Fig. 15, right). Recording strategies The strategy of recording the video data in underwater scenarios (Fig. 16) is quite similar to the grid-based version in aerial recording (GEHMLICH 2015). Between an UAV and Eckbert-II there exists a difference in control. The UUV is limited in one degree of freedom, it is not able to move sideways.

Fig. 16 – Left: To each side of the OpenROV, two diving torches and one GoPro Hero 4 BE were extended. Middle: To improve the control of the UUV, a human supporter takes the cable. With supporting the cable, he decreases the pulling force from the basis and increases significantly the mobility of the UUV. It’s like walking the dog. Right: The recording strategy with parallel and crossing stripes is similar to the aerial documentation. Team communication A stand-up meeting is always on the agenda before the documentation starts. The upcoming diving session will be discussed in the team. During the diving session, the communication between copilot 1 and copilot 2 is realized by walkie-talkies including headsets (DeTeWe Outdoor 8500 PMR). The pilot represents the basis and controls the UUV via laptop. The control channel and video stream between the UUV and the pilot is realized via web service. Close to the pilot, the first co-pilot is placed. He manages the cable and communicates via gesture between the pilot and the supporter.

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Some gesture commands are equivalent to the international diving signs, but additional communication codes are necessary, when controlling the UUV (Tab. 1).

Basis Supporter

Sign Description/Meaning Sign Description/Meaning

Is everything ok?

Yes, everything is ok.

Check the UUV status, the cable, the cameras, the lightings and so on.

Something goes wrong, co-pilot 1 and 2 need to notice the current position and orientation. Back to the basis.

How about the UUV orientation?

Showing current UUV orientation.

The recording process is finished. Back to the basis.

Recording stripe is finished. Maybe repositioning.

Tab. 1 – A list of some important gesture codes between the supporter and the two copilots, to get an idea about the communication. Due to the head protection, the snorkel and the distance of the supporter, no verbal communication is possible. The basis controls the time and fills out the protocols. To improve the workflow and identify bottlenecks, it is always advised to document every step seriously. Especially in a so early project stage, it is important to clock, to discuss the task order, etc. Diving sessions and recorded data All in all we had about ten diving sessions to record useful documentation data. The OpenRov is able to work three hours in a row, but the GoPros are the energy bottleneck. With the energy-saving video mode 1080p and 30 fps we were only able to work 45 to 60 minutes. Therefore, we decided to limit the diving sessions to 45 minutes. As recording strategy we used the orthographic double-grid-based approach (GEHMLICH 2015) with a stripe distance of about one meter. Flexible taring setup The basis OpenROV is well balanced for salt and fresh water. But, if attachements like cameras and torches are added, a new taring setup is needed. To be more flexible, we designed a lightly oversized buoyancy body for each side. To balance the UUV more sensitive depending on the current setup, additional plumb will be added. The positions of theses plumb elements are important to balance the alignment while moving. The post evaluation of the recorded sensor data like IMU or the depth sensor given by the UUV are very important. They tell us about the underwater behaviour. In one recording scenario we tested our flexible taring system. The UUV needed to hold the depth while moving (Fig. 17).

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Fig. 17 – While documenting the shipwreck in Veruda between 12:51 and 13:21, the UUV automatically holds the depth of 300 cm for about 30 minutes.

Fig. 18 – To reconstruct excavation stage III, we used 2574 images out of 15 minutes video data (1080p). The left image shows the resulted orthoimage of the 3d model using the original image data taken by a GoPro Hero 4 in a depth of six meters at excavation stage III. The right image shows the result, using the improved images with our new modul JEnhancer (HETTMANCZYK 2016).

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SHIP CONSTRUCTION Even upon its discovery, it was possible to recognize fragments of the former wooden components of the shipwreck. It was assumed that wooden structural elements were also present beneath the ballast stones. Beyond the area covered by the ballast stones, the area was dusted with a thin layer of sand. It was for this reason that the underlying wood was not visible until the ballast has been removed. This was made even more visually disconcerting by the presence of visible rock as well as isolated natural stones around the site. As a rule, the wooden elements were considerably damaged by shipworm (mainly because the sand layer with which they were covered was very thin). Nevertheless, the remains are relatively stable, compact and solid. The majority of the wooden elements were kept in their original position so that their characteristics could be analyzed as a whole. In this way, we discovered that both sides as well as part of the planking and the keel are present (see Fig. 19).

Fig. 19: Map of the ship construction (R. Scholz)

The rectangular and massive frames suggest that the vessel in question was a large sailboat. A total of 26 fragments were obtained. The thickness of the ribs varies between 13 and 17 cm, while the height measures about 13 cm. In some places, the edges were doubled. Unfortunately, due to the incomplete preservation, it is impossible to determine to what extent this was the case. Isolated timbers extend over the keel, while others are aligned with it and stretch up laterally. In some places, round and rectangular holes are visible into which iron bolts and nails were placed. These last were used to join together the frames and the planking. The timbers run northwest to southeast and their density is most pronounced in the centre and on the eastern side. On the western side of the site, however, only impressions on the sides of the frame give any hint to the presence of the former planking. They are visible in the form of a thin yellowish-brown print which has the same width of the edges. At these points, rectangular openings are visible once again which betray the presence of nails long since corroded. In addition, their distribution can be seen on the planking (attached to each individual beam by two nails). It should also be noted that the nails were each hammered against the edges of the plank. The planking consists of timbers of irregular size. On the northern side of the site, they are better preserved and include six rows. On the southern side, only three rows are preserved.

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In several places, one can recognize long-running sections in which the planks which line up with one another and, thus, form a uniform whole. We discovered that it was the outer surface of the ship which was connected to the ribs by means of iron bolts. The thickness of the planking is 5-7 cm. In addition to the lower planking, wooden elements on the south side of the site were also found. It is assumed that these were parts of the upper cladding or deck. The wood of the planking is arranged in from northeast to southwest. A wooden element was also found which was thought to represent the remains of the keel. Although its thickness varies, on average it is about 10 cm high. It runs from northeast to southwest and is quite damaged (it is split in two sections in the southwestern part). The very strongly corroded iron bolts and nails were documented and their exact shape and size were further examined following radiographs. The ship was also loaded with a ballast of larger, round stones which obviously came from a river. During the excavation, comminuted construction waste, plaster, brick and roof tile pieces as well as crushed ceramic, glass and animal bones were found. The presence of this material is proof that boats used waste as ballast. Although they are still being analyzed, samples were taken from all components of the wooden elements of the ship in order to determine the species of wood used in construction. An AMS 14 C analysis of a wood sample from the ship's construction was carried out in Poznan (Poland). The result of sample Poz-57874 is 320 ± 25 BP. This corresponds to calAD 1566 ± 51 and refers to the approximate construction time of the boat. Unfortunately, there was not enough information for the cause of the shipwreck to be reconstructed. However, further analysis and comparison with other contemporary finds will allow for further information about the original appearance and size of the vessel. At present, estimates place it at approximately 20 m long and suggest that it was a trading ship.

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FINDS Even before research had begun, single surface archaeological finds were recovered at the site. For the most part, these included differently-shaped metal pieces which had obviously been deliberately melted, smashed or flattened. It is probable that these were the by-products of the smelting of copper, brass, bronze, tin, etc. The presence of an intentionally-flattened vessel, the half of a round bronze bar (see Fig. 20.11) and a strip of bronze testify to this assumption.

Fig. 20: Some metal finds (L. Bekić)

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Interestingly, among the damaged metal objects of the cargo lay a luxurious cauldron (Fig. 20,1) which bears the Arabic inscription "La-ilahe-illallah-Muhammeden Resulullah" alongside geometric decorations. The inscription is in the specific graphic style of the 16th / 17th century. One other such special find is a lamp handle with decorative dragon heads which could (according to some analogies) be the product of south German workshops. In addition, there were numerous damaged unidentified metal objects that could be interpreted as handles, brackets, human figures, tin bowls, plates, etc. found at the site. The highly-corroded iron nails and bolts probably belonged to the ship itself. There were numerous rectangular bronze plates with a central bore in various sizes which were also recovered. Such things were commonly used to connect long ship bolts (Figure 20,9). At present, we cannot say whether they belonged to the ship itself or whether they were also part of the metal cargo. However, the two tin bowls (fig. 20,10) were probably part of the ship's equipment. The majority of the ceramic finds from the site were completely crushed and were found in the context of waste ballast material. Therefore, we assume that only a fraction of the ceramic fragments were the property of the ship's crew, while the rest were probably from one of the ports where such ballast was loaded. These ceramic fragments can be typologically dated to the second half of the 16th century or to the beginning of the 17th century. They come mainly from northern Italian workshops and are common in the northern Adriatic. The glass finds were also crushed and seem to belong to the ship's ballast. In addition, two larger fragments of a glass chalice which supposedly was part of the ship's equipment were found. These pieces form parts of a goblet which presumably dates from 16th century Venice (see Fig. 21).

Fig. 21: Part of a goblet with a lion head relief (R. Surić) In addition to the wooden construction, other organic remains from the site included a small part of a coarse and rotted rope and many small, partially-burned animal bones. The latter were presumably part of the ballast. Initial analysis of the find material and the results of the 14 C studies suggest that the ship and its equipment can be dated to the second half of the 16th and the beginning of the 17th century.

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SITE PROTECTION As mentioned at the outset, the favorable location of the site means that it is threatened by the predations of tourism and, most especially, by recreational divers. Therefore, it was all the more important to document the structure of the ship as well as the finds as best as possible. In order to prevent robbery, all small finds were recovered and sent to conservation. The remains of the boat were carefully secured after the investigation. For this purpose, the ship's construction was first covered with sand and then sealed with overlapping webs of geotextile (see Fig. 22). This geotextile layer was covered with sand once again before the ballast stones were placed on top. While this was, on the one hand, intended to restore the original situation at the site, it also served to support the security of the material and to avoid any loss or movement of the archaeology which remains. Annual visits are intended to help monitor and evaluate the success of these measures.

Fig. 22: Securing the wreck with geotextile (R. Scholz) RESULTS The results of this excavation helped us to better understand the location and causes of the ship's destruction. Large and small burnt and charred pieces of wood were found in all layers present at the site, suggesting that the boat fell prey to fire before it sank. At the time of its sinking, the bulk of its cargo of half-finished metal products were probably intended for further processing. A small portion of the finds which can be counted as a cargo or even parts of the ship point to possible salvage or plundering efforts after the wreckage. According to the analysis of the finds, most of the cargo, ship's inventory and the ceramic fragments from the ballast suggest that the ship was built in the middle of the 16th century and that it sank at the beginning of the 17th century. Following conservation of the finds, further analyses will contribute to a better understanding of these events.

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In addition, the project has shown that it is possible to implement 3D documentation using SFM methods. On the basis of the results presented here, the practicality of the newly-developed multifunctional bridge was also demonstrated. The data show that the use of photo-based documentation technology / methodology is possible with a prevailing visibility of approx. 3.5 m. The next step will be to explore the possibilities of employing the same method in areas in which visibility is less than 2 m. The first field tests are currently being planned.

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