Sea Survey and Its Impact on Maritime Archaeological Research in the Baltic Sea During the Last 30 Years

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SEA SURVEY AND ITS IMPACT ON MARITIME ARCHAEOLOGICAL RESEARCH IN THE BALTIC SEA DURING THE LAST 30 YEARS

Marcus Hjulhammar

More than 70 percent of Earth’s surface is covered by water, a surface that is gigantic to map. Through the technical development in hydrographic surveys, we have today a completely different understanding of the seabed’s topography and geology than we had just a few decades ago. This has also meant that we find more and more shipwrecks, but also sunken landscapes and other remnants of human activity. This article is a summary of this development.

The discovery of spectacular new well- The reason for the major discoveries preserved shipwrecks at the bottom of the in recent years is entirely dependent on Baltic Sea has increased significantly in the the increasingly sophisticated technolo- past ten years. We all remember the Vasa gy in sea survey and the development of and the discovery in 1956, which created a multibeam sonar, combined with the GPS sensuous and concrete image for the first (Global Positioning System) technologies, time over the cultural treasures that can be which since 1995 have become widely avai- preserved in our waters. lable and more accurate. A multibeam echo The conservation conditions in the Bal- sounder is a type of sonar that is used to tic Sea are ideal for much organic material, map the sea bottom. Like other sonar sys- including shipwrecks. The low-salinity, tems, multibeam systems emit sound waves which prevents “the shipworm” Teredo in a fan shape beneath a ship's hull. The navalis eating up the archaeological mate- amount of time it takes for sound waves rial combined with cold and darkness, in- to bounce off the seafloor and return to a creases the chances of preservation. It is receiver is used to determine water depth. therefore possible to find wrecks with to- Unlike other sonars, multibeam systems tally preserved hulls, riggings, equipment use beam-forming to extract directional in- and remains for many hundreds of years. formation from the returning soundwaves, Of course, there are more factors than that, producing a swath of depth readings from which makes the Baltic Sea unique from a a single ping. It should be mentioned in maritime archaeological perspective, but this context that the Russian research ves- the preservation conditions is one of the sels R/V Akademik Vavilov and Akademik most important things that makes it pos- Loffe used multibeam echo sounder as early sible for us to interpret whole structures as in the 1980s and became important for such as shipwrecks. the development of this technique.1

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Side-scan sonar is a category of sonar and use photography for analysis, just as system that is used to efficiently create an archaeologists do on land. In deeper wa- image of large areas of the sea floor. It may ters however, where divers can’t be used, be used to conduct surveys for maritime or when the visibility in the water is poor, archaeology; in conjunction with seafloor different technological help methods are samples it is able to provide an understan- necessary. Of course, these technological ding of the differences in material and tex- methods also have their limits, so it is im- ture type of the seabed. Side-scan sonar portant to choose right method in relation imagery is also a commonly used tool to to the unique circumstances of the area to detect debris items and other obstructions be surveyed. For example, sub bottom pro- on the seafloor that may be hazardous to filers have been developed for providing shipping or to seafloor installations by the profiles of the upper layers of the ocean oil and gas industry. In addition, the sta- bottom, but are unable to image through tus of pipelines and cables on the seafloor gas bubbles and are influenced by weather can be investigated using side-scan sonar. conditions and the depth of the seafloor. Side-scan data is frequently acquired along I hope through this article to provide the with bathymetric soundings and sub-bot- reader a background to the development tom profiler data, thus providing a glimp- and limits of these methods, and to give a se of the shallow structure of the seabed. picture of future challenges. Side-scan sonar is also used for fisheries research, dredging operations and environmental studies. It also has military applica- Sea survey and the development tions including mine detection. until the 1990s The technology has advanced so much since the mid-1990s, and is still rapidly deve- Hydrographic survey has always been of loping, which means what we see today of great importance for navigation, but very maritime archaeological finds is just the tip little is known about how it was organized of the iceberg. Furthermore these techno- and used before the 17th century. Experience logies have also become cheaper, which was probably crucial for navigation as was makes it possible for smaller companies or the use of pilots. Hand-leads and sticks individuals, without major investments in were used, of course, during voyages, and equipment, to search for shipwrecks. the results were probably recorded in some But all that is found at the bottom form. One of the oldest types of navi of the Baltic Sea by hydrographic sur- gation sources were the so-called sailing vey does not provide a complete picture descriptions, which verbally described the of the maritime archaeological landscape. fairways, water and the characteristics of Any technology has its limits and basical- islands, islets and ports. The oldest known ly requires human interpretation of all the sailing description concerning the Baltic collected data. There are of course various Sea originates from the reign of the Danish research technologies in underwater archa- king Valdemar from the 13th century and eology. One of the best methods is diving describes the sailing route from Utlängan in survey, especially on identified anomalies Sweden to Reval (Tallin) in Estonia.2 or objects. The benefit of using a diver is During the 17th century survey was con- that humans can directly process the data ducted by smaller rowing vessels and later and make interpretations of the objects. by sailing boats or ships. Positioning was A diver can also take measurements, notes done using leading lines between significant

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points. Compass was also used with calcu- technique resulted from a problem with lations of the estimated distance. During sounding lines being heavily burdened by winter mapping was conducted from the water currents when using conventional ice. One tried to add the sounding lines survey vessels thus affecting accuracy. In parallel to one another to systematically find the article the author describes how a tube the depths. The lake Hjälmaren in Sweden could be vertically attached from the deck was sounded in this way in 1690.3 of the ship down to the bottom of the In the end of the 17th and begin- hull, where in its funnel opening a bell was ning of the 18th centuries the sea charts is placed to sound in all directions. When the increasingly filled with depth figures and bell tolled, a receiving phone then picked deep curves. Sounding was mainly conduct- up the reflecting sounds and ocean depths ed in ports, at anchorage places and in nar- could thus be determined. The method row waterways. Fairway straits were usually was considered reliable, especially when only dashed without depth information. measuring deeper than 120 meters, but by In the 18th century, sea surveyors tried installing a resistance roll, a fully accurate to get deep curves symmetrically distributed measurement could be received regardless over the map by sounding in parallel lines the depth. The article also explains how this – just like today. The mid-18th century is acoustic equipment could be attached on known as the "great chart improvement" the ship’s side with the advantage of more in the Baltic Sea. The scale was set to 1: frequent cleaning of the clock.6 20 000. Vertical lines were put up between Photogrammetry made its entry in 1931 capes and well-defined objects in a pattern, and came over several decades to provide a which has been termed "Star-sounding". convenient and relatively safe base for the This system was used until about 1860. coastline and the choice of angle points to In the early 19th century deep curves sea survey. and depth figures were introduced and they Positioning was carried out in the were also coloured on the sea charts. From archipelago areas right up to the 1950s the 1860s surveyors reverted to sounding mainly by means of horizontal bias in parallel lines and the scale of the sea against known objects. At sea a system of charts was enlarged. In 1879 steam boats triangulation seamarks was used for map- were used for the first time in shallower wa- ping and survey. In the 1930s, a system of ters, but deep-sea measurement was done radio and acoustic means was developed, with larger survey vessels in scales from 1: but it was not until after the Second World 50,000 to 1: 200 000. War, that navigational systems, such as the Later on at the end of the 19th centu- British Decca Navigator, were introduced ry the surveys accuracy was improved by and tested for hydrographic survey. introduction of mechanical sounding. Also, The big breakthrough for positioning during the First World War the quality of came during the 1980s when the GPS was surveys was increased.4 introduced. New sounding techniques like In 1930 the first sonar system was laser bathymetry were also developed. This introduced.5 However, the idea to carry technology could be used to survey from out hydrographic survey with sound waves airplanes or helicopters in shallow waters is older than that. In the German techni- with great accuracy. During this time the cal journal "Mitteilungen aus dem Gebiete first multibeam sonars were introduced for des Seewesens" the method was presented sea surveys. On the Baltic Sea, the first mul- as early as 1908. The development of this tibeam sonar was installed on HMS Jacob

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Hägg in 1993 (ELAC, Honeywell). It had 56 at Oxdjupet North of Stockholm, the sea- individual rays directed from the sounding fortress Tre Kronor outside Copenhagen or line vertical 60 degrees to each side.7 at Suomenlinna in Finland. The best known early hydrographic map of Stockholm's water was performed Hydrographic survey and de- in two versions by Carl Friedrich von Haus- tecting of shipwrecks until the wolff in the 1730s (1736 and 1739). On 1990s Hauswolffs map from 1736 43 wrecks are marked, which may indicate that a number Among older maritime surveys, where of these ships were sunk during this period, shipwrecks have been identified, the con- and were under the supervision of Haus- cern has almost exclusively been on harbor wolff. We know the names of three of the areas, where wrecks have been dotted out vessels and also exactly where they were and in some cases even been named. The sunk, but whether the wrecks are still there reason these early maritime surveys were on the seabed is not known. The first is the performed was usually because the wrecks frigate Ruschenfelt, the only wreck named on would be destroyed or that they would be Hauswolffs map. The other ship is named used as a basis for various types of marine Förlovade landet (=The Promised Land). It was structures such as bridges or piers. In some launched in 1723 and the wreck is marked cases, however, wrecks are also marked on on a detail map conducted by the well- maps of sounds or straits.8 The straits were known Swedish sea surveyor, captain Jonas often of a military strategic nature, such as Hahn in 1750. Hahn printed an engraved

Hydrographic map of Stockholm's water performed by Carl Friedrich von Hauswolff in 1739. Swedish War Archives.

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sea chart over the Baltic Sea and Skagerrak scan sonar, a magnetometer and a remote the same year.9 The third ship, Halland, also operated vehicle (ROV). marked on Hahn's detail map, was built by During this time in the early 1980s the Robert Turner in Karlskrona 1682 and was side-scan sonars were very expensive equip- nearly 40 meters long and 10 meters wide. ment and were mainly used by navies for It was built for a crew of 290 people and submarine chase, but increasingly tested had an armament of 56 guns. The ship was more and more for research purposes. scuttled in 1737. After the rediscovery of the Vasa by Anders Franzén in 1956, maritime From the 1990s until today archaeological excavations started in other part of the Baltic Sea as a spin-off effect.10 The big breakthrough for maritime In Finland salvaging attempts were made on archaeological surveys came in the mid- the then newly located well-preserved 17th- 1990s when side-scan sonars started to be century merchant ship St Michel and in Den- used together with a better positioning sys- mark salvaging was made of five Viking age tem than before and with the introduction or early medieval clinker built ships in the of multibeam and computers. Roskilde fjord in 1962. Also on St. Nikolai, Onboard bigger survey vessels today found 1948 outside of Ruotsinsalmi in Fin- the main instruments are different types land, excavations started in the 1960s. of sonars and differential GPS (DGPS). Consequently, the interest for mari- The sonars are usually side-scan sonars time historical research increased in the and multibeam. As a complement to this Baltic Sea, but old survey results were not sea bottom penetrating echo sounders are used as a background source for maritime also used. For this great computing power archaeological discoveries. There were ol- is also needed. The different techniques der sea charts available at this point; it is save large amounts of data in the form of quite interesting to note that in some cases depth and position numbers and the com- they could have been helpful in maritime puter must constantly calculate the position archaeological research. An example of of all depth registrations, which can involve this is a sea chart from the 1830s where the several hundred positions per second. This wreck site of Vasa is marked. The marking is because sound pulses go out in many of the wreck on the map was made around different angles and hit bottom in many the same time as the Swedish diving pio- different positions. neer Anton Ludvig Fahnehjelm applied for To get the most accurate depth pos- diving permit at the Vasa.11 sible, it is necessary to determine the speed Anders Franzén, however unawa- of sound through the water exactly at the re of this older information, developed current depth. Sound speed also depends new technology for further research af- on the water's salinity, temperature and den- ter famous naval vessels, the Kronan (=the sity. The exact audio speed is therefore de- Crown), sunk in 1676, not the least among termined by using a probe. This is lowered them. Kronan was discovered in 1980, six down to the maximum depth in the area. It kilometers outside of the southeastern then sends a light pulse up to the ship's hull Öland by Anders Franzén, Sten Ahlberg, to a sensor that records how long it takes Bengt Börjesson and Bengt Grisell. The for the sound to come up. When you know ship was found thanks to the help of side- the distance that the sound has gone and

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ROV equipped with multibeam sonars and HD cameras during the study of the Ghost ship. Photo: Marin Mätteknik AB/ the Ghost ship project. Reproduced with the permission of Johan Rönnby, MARIS.

will continue as new data becomes available. From an archaeological point of view there are efforts being made to put national wreck registers into a pan-European database of underwater cultural monuments. This would the time it took, a computer can calculate enable all interested parties to find infor- the audio speed. If there are significant sea mation quickly and easily from one place.12 conditions there is another computer that However, the information in national da- receives signals from the sensors that re- tabases is at the moment not accessible gister movements, and then compensates in English with the exception of Estonia. these so that hydrographic survey can be This tightens the circle of data users, but conducted despite these conditions. hopefully these databases will be translated In the Baltic Sea there are formations on in the coming years. This is an important the seabed, from huge mountains to small task, because sailing is international and a gaps that now are quite easy to explore. The shipwreck might only be understood by Baltic Sea Bathymetry Database (BSBD) international research cooperation. is developed in cooperation with the Bal- tic Sea countries, with the purpose of vi- sualizing and providing depth data for the Major exploitation of the sea- whole of the Baltic Sea. The BSBD has bed been developed as part of the TEN-T- funded EU project MONALISA, within Major exploitations on the seabed have the framework of the Baltic Sea Hydro- been of great importance for maritime graphic Commission, which is an integrant archaeological research. One of the best part of the International Hydrographic Or- examples of this is The Nord Stream ganization. The Baltic Sea Hydrographic AG, which was founded in 2005 to plan, Commission promotes the technical co- construct and operate a pipeline system operation in the domain of hydrographic through the Baltic Sea from Vyborg in Rus- surveying, marine cartography and nautical sia to Lubmin near Greifswald in Germany. information among the Baltic Sea area The pipeline passes through the economic countries. zones of Russia, Finland, Sweden, Den- The BSD provides a consolidated pic- mark and Germany and the territorial wa- ture of the seabed’s topography across na- ters of Denmark and Germany. tional borders and will provide researchers From 2005 to 2009 several large-scale and stakeholders in the marine environment geophysical investigations using different with the knowledge of the entire Baltic Sea. methods were made as part of the plan- Work on the improvement of the model ning process for the natural gas pipeline.

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A multibeam view over the Ghost ship. Photo: Marin Mätteknik AB/the Ghost ship project. Reproduced with the permission of Johan Rönnby, MARIS.

The 140m wide pipeline corridor and the 1.6 km wide anchoring zone were investigated separately. In all, 74,000 hectares of bottom were surveyed. One of the most interesting aspects in this One of their main objects, which are project from an archaeological point of also of interest because it is located at 125 view was that the pipeline was supposed to meters, is a flute from the 17th century with lie partly on deep waters, were multibeam the working title the Ghost ship, discovered in surveys had not usually been done before. 2003. One of the purposes of the study of Following interpretation of the survey the Ghost ship was to evaluate new methods materials, target reports were made and tar- for measurement without using divers but gets were selected for inspection by a ROV. instead using ROVs equipped with multi- During the different surveys close to 20,000 beam sonars and HD cameras. The results targets were inspected by a ROV. The bot- were above all expectations and the archa- tom survey materials and the inspected tar- eologist managed to make a detailed dra- gets were evaluated from a culture heritage wing of the ship's structure thanks to these point of view and an impact assessment for survey experiments. The technique did not each target considered to be cultural heri- only give great opportunities to bring out tage was carried out. As a result of the in- the shape of the hull, the archaeologist also vestigations circa 40 wrecks and some other received data from inside the wreck with in- targets were evaluated from a cultural heri- dications of different deck levels. tage point of view.13 In recent years the MMT have, among This large-scale survey also became the other objects, tested and developed its starting point for several new archaeologi- technology at the wreck sites of Svärdet cal studies of well-preserved shipwrecks, (=the Sword), sunk in 1676 and Mars, sunk even if they did not lie in the path of the in 1564. planned pipeline. There are several private At the wreck site of Mars a so-called 3D operators, who have successfully combined Mechanical Scanning Sonar (Blueview BV their survey skills with collaboration with 5000) was used. The combination of using researchers in maritime archaeology, such the ROV with a multibeam from above and as the German Innomar, Finnish Subzone a Blueview allows one to move around the and Swedish Marin Mätteknik (MMT). One wreck and provides a great opportunity to interesting example is the SASMAP –pro- produce an accurate picture of the wreck ject with its geological modeling and sea site. The 3-dimensional measurement data floor survey in Stone Age dwellings. The obtained can then be "dressed" in pictures MMT has started several new projects to- by photographing the wreck site. In this gether with MARIS (Maritime Archaeologi- way a photogrammetric model that is so cal Research Institute of Södertörn Univer- precise that it would not have been pos- sity) and a production company (Deep Sea sible to get similar information using diving Productions). archaeologists is obtained.

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The photogrammetric documentation in the form of tracks from maritime activi- means that digitally documented details – ties. Multibeam mapping creates very large or the whole wreck site – can be printed datasets, which can be difficult for human out via a 3D-printer, which is a tremendo- to interpret. New archaeological knowledge us breakthrough in maritime archaeological may be found by processing data from a de- research, not least from the perspective of signated area. By making use of a so-called preservation. In principle, valuable shipw- backscatter analysis it might for instance be recks can in the future be allowed to remain possible to detect flat wrecks that are not on the seabed while all relevant information visible as increases in the height model. By can be retained on land digitally for further looking at the signal strength from a multi- research. One of the major benefits of this beam survey anomalies in the sea bottom is that it has now suddenly become possible hardness can be detected. These can be ex- to visualize a wreck site on land. plained by natural geological formations or One of Sweden's ongoing maritime as a result of human activity.15 archaeological projects is entitled "Land scapes Lost" and is aiming to study sunken landscape that has been populated since the Barriers to accessibility? Stone Age. In connection with this, a high resolution seabed penetrating sonar was Hydrographic survey is something that eve- used to try to identify potential landscapes ry country takes care on its own, primari- covered by sediments. Known locations of ly for the production of nautical charts. Stone Age finds can be related to the data Through varied national legislations hyd- sets to find a method that can streamline rographic surveys are largely licensable, their identification. Several other methods primarily for reasons of defense. Conse- have been used in the project to docu- quently this knowledge is not widespread ment mud embankments with remnants among all surveying for wrecks. In additi- of ancient trees, photogrammetry among on to the licensing issue, laws may demand them. that data must be kept in a safe way without As with the investigations of Mars the spreading it. Legislation on the protection combination of photogrammetry and mul- of geographic information, including hyd- tibeam has been very successful to get a rographic information, has a long tradition. 3D model of the landscape. This sunken Several of the 200-hundred-year-old charts landscape extends far out from the borders that we today can study in various archives of Sweden towards Poland and in the long were once highly secret information.16 run, we will most likely get a very different Despite the fact that there is an on- picture of geological development of the going debate that the information on the area.14 bottom of the Baltic Sea should be widely available, legislation does not need to be a major obstacle to maritime archaeological Amounts of data and problems research. Through applications, one can of interpretation very well be allowed to survey and also to get the excerpt of the survey data from An inventory of already conducted marine various government agencies.17 surveys shows that national databases hide archaeological information both in the form of hitherto unknown shipwrecks but also

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The inland waters – the next challenge?

Within the next ten years, the bottom of the Baltic Sea is likely to be that well surveyed with modern technology that it will be possible to see any major sunken ship. How many there will be found in total is impossible to say, but we can certainly say that we are facing a completely new cultural landscape. Maritime Archaeology, however, is so much more than sunken ships. It also means sunken landscapes, fis- hing traps, harbors, maritime fortifi- cations and cultural layers. The technology will likely never be able to replace the human eye, or the human analysis, because much of the maritime archaeological material is so fragmented or covered by sediments or other layers that it cannot be detected by survey vessels alone. Maritime archaeologist Minna Leino But with help of more advanced sur- working in lake Saimaa in the 1990s. Our vey technology we will be able to de- inland water systems present a future tect so much more than was possible challenge in archaeological knowledge. just a few years ago. Photo: Reino Hemmilä. Reproduced with Our inland water systems, the the permission of Minna Leino, Finnish National Board of Antiquities. lakes and rivers – areas that may not be intended to be surveyed – present a future challenge in archaeological knowledge. As a pioneer project in this reached its maximum extent, nearly 9000 context the University of Helsinki has just km2 in size culminating in the outburst of started a project in the Saimaa Lake comp- the Vuoksi River in the south c 4000 cal BC. lex called "Lost Inland Landscapes”. Because of this, the Mesolithic and The Saimaa Lake complex in the eas- Early Neolithic sites predating the trans- tern part of Finland has constituted one of gression are to be found under water and the major inland water systems throughout the wetlands surrounding the lake. An ex- our prehistory. In the earlier stages, the wa- perimental project was launched at the turn ter level of the ancient lake was relatively of the millennium with the aim of finding low. Due to the post-glacial lake land up- submerged settlement sites by means of lift and tilting, the Saimaa basin continued underwater archaeology.18 A few new ob- to transgress into a southeasterly direction servations were made under shallow water and thus inundated earlier lakeshores. Cir- and mud into the lake in the municipality ca 6000 years ago, the Ancient Lake Saimaa of Taipalsaari and the results make it most

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17 Karlsson-Ottosson 2014:4-5. likely that new, submerged and paludified 18 sites in the region. Koivikko 2000. The new project is both ambitious and innovative. New techniques and experien- BIBLIOGRAPHY: ces in wetland and underwater archaeology make it possible to find hitherto unknown CEDERLUND, Carl Olof. 1983. The Old Wrecks of the Baltic Sea. Archaeological recording of the prehistoric sites on wetlands as well as on wrecks of carvel-built ships. BAR International the lakebed in the SEE Saimaa area. The Series 186. Stockholm. Diss. experiences from the southern shore of the CEDERLUND, Carl Olof. 2012. Ett oskrivet kapitel i Lake Vättern in Sweden, for example, are skeppet Vasas historia. In: Forum Navale 68. Eds: Jerker Widén och Stefan Lundblad. encouraging and it is highly possible that EHRENSVÄRD, Ulla & Frithz, Hugo. 1993. Sveriges something similar has survived on the lake- sjökarteväsende 1643-1993. Norrköping. bed and shallow shores of the Lake Saimaa. EHRENSVÄRD, Ulla. 1998. Farlederna berättar. On dry land conditions, practically all the Gamla sjökartor över Finlands skärgårdar. Före- ningen Konstsamfundets publikationsserie XVII. organic materials from the Mesolithic and Helsingfors. Early Neolithic period have deteriorated, EHRENSVÄRD, Ulla. 2006. Nordiska kartans historia. but in wet environments quite much of Från myter till verklighet. Helsingfors. these might have survived. If successful the FLOOD, Roger & Törnqvist, Oscar. 2010. Vad väntar project may produce scientifically highly i de digitala arkiven? Marinarkeologisk tidskrift important new information on the Meso- 3/2010. Pp 13-16. HJULHAMMAR, Marcus. 2010. Stockholm från lithic and Early Neolithic period in Finland. sjösidan. Marinarkeologiska fynd och miljöer. Stockholmsmonografier 211. Värnamo. Diss. Marcus Hjulhammar (PhD) is the assistant professor for Baltic maritime archaeology at the HOLMLUND, Joakim. 2014. Ny verktygslåda för University of Helsinki. He is currently leading the marinarkeologer. Marinarkeologisk tidskrift Lost Inland Landscapes –project on Southern 2/2014. Pp 4-9. Saimaa underwater archaeology. JÄNTTI, T-P. 1989. Trials and experimental results of the ECHOS XD multibeam echo sounder. The This article has been peer-reviewed. Tekniikan IEEE Journal of Oceanic Engineering (ISSN 0364- Waiheita would like to than the reviewers for their 9059) Volume: 14 , issue 4, 1989. Pp 306-313. valuable comments. KARLSSON-OTTOSSON, Ulla. 2014. Hemligstämpel stryper forskning om Östersjön: krock mellan försvaret och forskare. Ny teknik 19/2014. Pp 4-5. 1 Jäntti 1989:306-313. KOIVIKKO, Minna. 2000. Hukkunut kivikausi: veden- 2 Ehrensvärd 2006:261. alaiset asuinpaikat, uusi muinaisjäännöstyyppi 3 (Submerged Stone Age: underwater settlement Ehrensvärd & Frithz 1993:30. sites, a new type of prehistoric monument). 4 Ehrensvärd & Frithz 1993:25-41. Muinaistutkija 1: 4-11. 5 Söderman 1995:38. RUSSOV, Urve. 2013. Managing Sunken Data. In: 6 Tidskrift i Sjöväsendet 1909. Shipwreck Heritage. Digitizing and Opening 7 Access to Maritime History Sources. Muinasaja Ehrensvärd & Frithz 1993:50-51. teadus 23. Ed. Maili Roio. Pp 311-318. Tallin. 8 Cederlund 1983:23. SÖDERMAN, Inga. 1995. Från lertavla till satellitbild. 9 Ehrensvärd 1998:69. Stockholm. 10 Hjulhammar 2010:22-23, 33. Tidskrift i Sjöväsendet. 1909. Kungliga Örlogsman- 11 Cederlund 2012:14-18. nasällskapet. Pp 541-544. Karlskrona. 12 Russow 2013:314-315. WESSMAN, Stefan. 2011. Environmental impact 13 assessment and archaeological heritage. Unesco Wessman 2011. Scientific Colloquium on Factors Impacting the 14 Holmlund 2014:4-9. Underwater Cultural Heritage. 10th Anniversa- 15 Flood & Törnqvist 2010:13-16. ry of the Convention on the Protection of the Underwater Cultural Heritage. Royal Library of 16 Ehrensvärd 2006:287. Belgium, Brussels, Belgium. 13th & 14th Decem-

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