photo mosaics in shallow water environments: challenges and results

Photo mosaics in shallow water environments: challenges and results

S. Finney & J. Stephen Department of Anthropology, University of Hawaii at Manoa, Honolulu, Hawaii, USA

Abstract Photo mosaics of underwater sites are useful tools for interpreting sites and increasing the level of detail for areas too large for conventional photography. While this tool has been used for years at deep-water sites using camera platforms, such traditional devices are difficult to use in areas that are too shallow or too rough to maintain a consistent distance to the site. In addition, some sites, notably large shipwrecks, cannot be photographed at a well-maintained distance given the time constraints of many projects. Finding alternative methods to collect data and create photo mosaics is necessary. Keywords: photo mosaic, marine archaeology, shipwrecks, underwater photography

1 Introduction

Data collection and documentation of shipwreck sites involves a variety of methods and techniques depending on the location of the site, the size of the site, the time constraints of the survey and the technology available to the researcher. Data collection using various photographic techniques is a common component to most surveys. This typically involves the conventional gathering of visual images using still images or streaming video with an underwater camera or a camera equipped with underwater housing, all of which are accessible to most researchers. In addition to the traditional images collected using this method, researchers are increasingly turning to the manipulation of these images into a larger image made up of a collection of stills and covering the all or part of a site or shipwreck. Known as photo mosaics, these can be created using a number of

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means, notably the application of various software programs to mesh the images together, thus creating a more holistic visual record of a site. This paper gives a brief overview of the history of photo mosaics in underwater photography and its relevance to underwater archaeological sites, in particular ship and aircraft wreckage found in shallow water areas. Even in locations with low visibility, rough waters or where the actual survey is constrained due to time, budget or technological considerations, the development of photo mosaics can still be a valuable tool for collection and interpretation of cultural materials.

2 Photography and archaeology

The use of photographic techniques by archaeologists enjoys a long history that continues within the academy today. It is impossible to estimate the relative importance of photography in archaeology, for its application varies widely from one project to another, but the four primary purposes of photography for archaeologists have been listed elsewhere (Harp [1]) as the acquisition of data, the recording of data, analysis and interpretation, and communication. These elements of photographic practice are appropriate for the views expressed here, but it is also acknowledged that these objectives are from a single point of view and that other factors are undoubtedly considered elsewhere. It is realizing this diverse range of application and perspective, however, which ultimately attests to photography being one of the archaeologist’s most significant and frequently used tools. Therefore, discussion concerning its use in a variety of archaeological settings and for a range of purposes is warranted and necessary. We aim to review photographic mosaics and their application in marine archaeology. Marine archaeology represents a unique undertaking from a photographic standpoint because photographic conceptualizations, undertakings, and results all rely on a different set of parameters than do terrestrial photographic endeavors. Insofar as archaeology is concerned, a number of needs for visual information are exaggerated because many facets of research at submerged sites are increased in terms of logistical complexity. The use of photographic mosaics is significant because they effectively counter some of the problems inherent in conducting photography underwater, namely those of the highest concern to the archaeologist. Thus, we will review the difficulties, as well as advantages, inherent in the marine environment when conducting photography and highlight the beneficial attributes of photographic mosaics to marine archaeologists.

3 Challenges and advantages to marine photography

The logistical challenges of conducting archaeology underwater are compounded by photography-specific considerations that must be made. The marine environment presents a number of complications that can compromise the image quality of photographs. These complications, largely derivative of the properties of light transmission through water as opposed to air, are important issues as


196 Maritime Heritage and Modern Ports

most photographic media is designed for non-marine application. Understanding the unique properties of the marine environment and how they relate to image quality leads directly to the ability to deliberately choose photographic techniques that maximize image quality. In sum, the three major issues concerning image quality for marine environments are contrast, light attenuation, and exposure. Contrast in the marine environment is comparatively less than that typically encountered terrestrially. Subjects often exhibit a low contrast range, or limited tonal variation between light and dark elements within a composition. The contrast transmittance of a scene is reduced by back-scatter (the superimposition of light scattered from particles in the transmission path on the desired image), forward-scatter (the scattering of direct light rays from the scene), refractive deterioration of the image, and the selective absorption of light (Pitcher [2]). In many cases, all of these factors decrease the effective contrast of marine subjects. Second, the light transmission path in water is drastically reduced from that of air since light being transmitted through water travels a shorter distance before losing intensity. Thus, ambient light levels decrease relatively rapidly as water depth increases. The photographic implications of light attenuation dictate that the distance between camera and subject matter (operating altitude) is more limited and that effective area coverage is reduced. Finally, correct film exposure becomes more of a challenge in marine environments than on land. Due to fluctuations in the intensity of light reaching the film, exposing film or digital media correctly may be more difficult. Shifts in the intensity of light reaching the film are the product of variations in camera/strobe altitude, target reflectivity, and the topographic relief of the bottom. On the other hand, plan view photography is more easily achieved in a marine environment where buoyancy can be utilized to maintain elevation above a site without the use of extraneous equipment such as platforms. In addition, camera gear that can be cumbersome on land become effortless to carry and maneuver in the water. Divers can multi-task and carry multiple pieces of equipment to more quickly facilitate the recording of information. This is especially pertinent within the time sensitive context of compressed air diving.

4 The advantages of constructing photographic mosaics

Photo mosaics counter a number of the problems inherent in conducting photography underwater. Most significantly to marine archaeologists, effective area coverage and perspective control can be increased through the use of mosaic techniques. This technique is a useful tool for large areas underwater where conventional photography is impractical due to low visibility. This means that photographs can be constructed of sites that would otherwise only be viewable in a segmental fashion, where the ability to see the total site may be beyond the visual range of the archaeologist. Perspective, while still a potential problem in mosaic images, can be controlled more effectively. The temptation to use lenses with short focal lengths



underwater to compensate for light attenuation and to have wide area coverage produces single photographs that suffer from distortion. A mosaic done of the same scene will produce a more useful photograph by using a longer focal length lens to take multiple images that are then “stitched” together to form a composite. Area coverage is increased when creating mosaics because an unlimited number of images can be taken from an unlimited number of positions. Assuming that they can be stitched together and that considerations are made for perspective, the area coverage of a mosaic is theoretically unlimited as long as the subject matter remains a fixed distance from the camera along a straight plane (a common configuration for both marine and aerial photography).

5 Limitations

Mosaics are employed most effectively in situations where a photographic subject presents a horizontal or vertical perspective or dimension. Adjustments can be made for variation in distance and perspective, but the quality of the image degrades accordingly. Computing and processing power of personal computers are another limitation for the construction of large photographic mosaics in the manner described below. File sizes can exceed 300 megabytes depending on the complexity and size of the mosaics. Many computers cannot efficiently manage such a large file; processor speed and memory are the two main considerations to make when considering the use of a computer for this purpose.

6 Photo mosaics in marine archaeology

As early as 1970 National Geographic magazine employed photo mosaics to help illustrate the details of a classical Greek ship (Katsev [3]). In 1975 researchers investigating the newly located USS Monitor off the North Carolina coast used over 2,000 separate photographs to create a photo mosaic of the site (Newton [4]). The use of photo mosaics grew in popularity in the 1980s: surveys of the wrecks of the HMS Pandora, RMS Titanic and the DKM Bismarck all included this technique as part of the data collection (Marden [5], Ballard [6], Ballard [7]). The majority of these site surveys involved the use of sophisticated equipment and a lengthy photography process involving hours of data collection and hundreds, often thousands of photographs to make one mosaic. In several of these early surveys the site is too deep to reach using conventional diving gear, requiring the use of deep-water submersibles and remote operated vehicles (ROVs). Despite the time constraints imposed, several of these projects took no more than two days to complete the photography, the use of submersibles and ROVs allows researchers to spend more hours per day underwater than if the sites had been in shallower depths and the researchers used SCUBA. Photo mosaics are especially useful for highly detailed material such as shipwrecks. In these cases the size of many wrecks and wreck sites is too large



for one photograph and the use of a mosaic of many photographs enhances the elements. For example, a single photo of the hull of a ship may not give you enough information to pick out details such as portholes, the riveting configuration of the hull plates or information about hull integrity. Because a photo mosaic is comprised of smaller photos taken from a closer vantage point, the archaeologist can pay more attention to these details and still assemble a clear, holistic view of the site. The question then is not whether photo mosaics should be included as a component when planning survey work. We see how this device contributes to the corpus of information collected as part of many project over the last three decades. The question we want to examine is: is the application of photo mosaics valuable in circumstances where, through time, budget and project constraints, researchers may have a limited amount of data from which to prepare a photo mosaic? If so, what are the benefits to this process? Careful collection of images can lead to a detailed and accurate visual record of underwater wreck sites. In some cases this accuracy gives the researchers a 2-dimensional scale representation of the site. This type of accuracy usually takes time as each photograph has to be carefully staged and the distance between the photo and the site needs to be maintained throughout the process. In cases where researchers cannot achieve that level of precision, photo mosaics can still be used as a reconnaissance and survey tool. There are three major benefits to be realized from the application of photo mosaics to a site survey: 1. It provides a quick reference to a site. A single photo mosaic helps to quickly identify features during post-processing when researchers need help with interpretation. Streaming video images may also be used but this requires more time and still imaging does not give you a good overview of the site. 2. It provides a visual inventory that can be used as a baseline for future survey work. In isolated areas where the use of more sophisticated equipment is impossible due to a lack of infrastructure –such as areas where electricity is not available- a photo mosaic can literally be easily taken into the field and used as a reference tool. 3. It can be used as a monitoring tool for both researchers and preservationists. This is a more accessible medium than features recorded using video or written descriptions. This is especially valuable where there may be a language barrier or a lack of technology to view video or computer generated images. Simply put, a photo mosaic can be printed out and preserved to use as a site inventory record. This can even be waterproofed and used while diving to provide researchers with a quick reference during subsequent survey at a recorded site.

7 Test site: Maloelap Atoll

The Republic of the Marshall Islands (RMI) is composed of atoll islands situated for the most part between 5 and 10 degrees above the equator. Maloelap Atoll encloses a lagoon of roughly 973 square kilometers. The RMI Historic Preservation Office has been engaged in conducting an inventory of the atolls



used by the Japanese during World War II. The survey at Maloelap was the last in the series and was contracted out to the Department of Anthropology at the University of Hawaii at Manoa. RMI does not have an extensive budget for this work and that, coupled with the remote location, guided our survey. There is no electricity or running water, no phone service and one weekly flight that must accommodate all the passengers and cargo for the entire atoll. We were able to bring a small number of SCUBA tanks with us but did not have an option to refill them once they were used. Our survey was completed in one week and involved a search of the atoll based on predetermined priority areas. Investigation of these areas yielded ten recorded sites. Eight out of ten of these sites were from the World War II-era. Eight of ten sites were also partially or completely submerged. Three sites were Japanese merchant ships between 55-60 meters in length. The submerged sites were accessible for one dive only, approximately 20-30 minutes of dive total dive time per site. Maximum depth of these sites was 21 meters. Because of the time constraints we were able to complete only one sweep of these ships, collecting port, starboard and plan views. The goal was to record as much of the site as possible. We used a Sony TRV-900 digital video camera with Amphibico electronic underwater housing and supplemented the digital video data with still photography using a Nikonos-V underwater camera with Kodak ASA 200 Elite chrome film.

8 Constructing the mosaics

For these mosaics, images were obtained from the streaming video taken at the site through a process that produced digital images files in .jpg format. Images were sorted into port, starboard, and plan views, respectively. Using a Macintosh G5 workstation running Adobe Photoshop 7.0, a large canvas of high resolution (20 x 10 inches, 300 dpi) was created for each view, into which the appropriate images were pasted as separate layers. As images were identified in relation to their position within the overall site, folders were created within the Layers palette to further organize and efficiently manage their movement. Once a rough arrangement of the images was established in terms of their position, adjustments were made for variation in distance. This was facilitated through the use of the Transform function. While maintaining proportions, images were adjusted so that dimensions of congruent objects within them became more consistent throughout the composite. The opacity of layers was manipulated which allowed for the simultaneous viewing of overlapping layers and the exact comparison of the subject matter from between layers. Finally, each layer was adjusted for levels and contrast to gain the most detail possible from the information. In one case (Site 5 Port) portions of layers were removed (using the Eraser tool on various opacity settings) to “blend” the segments of the composite together, providing a more uniform final product and utilizing the best portions of every individual image to construct the highest quality mosaic possible.



Ideally the creation of a photo mosaic should involve the interlocking of individual photos into a larger, seamless whole. However such seamless photos are not essential to record important data such as the relative position of features on the deck or key diagnostic elements. In the case of this survey many of the photos used for the mosaics were captured from more of an oblique view than recommended. This creates a more uneven perspective. Figure 1 shows the port side of Site 5 from the survey. This is a Japanese merchant ship sunk in the lagoon toward the end of World War II. Site 5 measures 55 meters in length with a beam of 18 meters and was situated upright in a sandy area at a downward slope and at a depth of between 18 and 21 meters. This was one of the three Japanese merchant ships documented during the survey.

Figure 1: Site 5 port side photo mosaic.

Despite the distortion the photo mosaic clearly shows the details of the hull sections and the lack of explosion damage on this side of the ship. Greater detail can be seen in Figure 2, the plan view of the same ship. In this mosaic the details of the remaining elements of the deck, including the intact skylight and a swivel gun still in place –a rarity on World War II-era vessels due to looting and souvenir hunting- are clearly visible. In addition, the incorporation of the diver into the mosaic was an intentional device to add another layer of perspective for the viewer. These two examples show complete views of the port side and plan view of a sunken vessel. However, a complete photo mosaic is not necessary to be useful. Even a partial image can provide significant information about features that are useful for future interpretation. In particular, focus on important diagnostic elements can help marine archaeologists identify ship types and a focus on specific construction details can narrow down identification. This may be even more useful with aircraft where there are many versions of the same aircraft. Only by identifying slight variations can an expert distinguish between them.

bow



Figure 2: Site 5 plan view photo mosaic with key features identified.

9 Conclusion

Even in less than ideal situations where the researcher does not have the time or the resources to compile enough photos to create a complete view of the site there are still advantages to collecting as much photographic data as possible and creating the partial photo mosaic. The collection of information is vital to any

bow

swivel gun

stern

cargo area

skylight



project involving the conservation, preservation or interpretation of shipwreck sites. Photo mosaics provide the marine archaeologist with an excellent tool for recording information that is also accessible by non-professionals and can be used as an effective educational tool, providing the public with a clear reference to a site most would otherwise not be able to access. Photo mosaics, as they have been reviewed here, offer the marine archaeologist a useful technique for the survey and documentation of sites in shallow depths, or where visibility is impaired due to turbidity. Additionally, the amount of resources and time required are minimal as compared to traditional mosaic techniques, further increasing the value of the method. Visual records, long a valued component of archaeological research, are enhanced in the case of marine archaeology through the production of mosaic images.

References

[1] Harp, E., Photography in Archaeological Research, University of New Mexico Press: Albuquerque, p. 4, 1975.

[2] Pitcher, D.C., Maximizing Image Quality in Underwater Photography. Underwater Photography: Scientific and Engineering Applications, ed. P.F. Smith, Van Nostrand Reinhold Company: New York, p. 386, 1984.

[3] Katsev, M.L., Resurrecting the Oldest Known Greek Ship. National Geographic, pp. 841-857, June 1970.

[4] Newton, J., How We Found the Monitor. National Geographic, pp. 48-61, January 1975.

[5] Marden, L., Wreck of the H.M.S. Pandora. National Geographic, pp. 423-451, October 1985.

[6] Ballard, R., Epilogue for Titanic. National Geographic, pp. 454-463, October 1987.

[7] Ballard, R., The Bismarck Found. National Geographic, pp. 622-637, November 1989.



photo mosaics in shallow water environments: challenges and results

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