The archaeologists used a technique called photogrammetry, which involves taking hundreds of overlapping photographs of a site and then feeding them into a computer program that can stitch them together. The application is able to establish the spatial relationships between photos, which allows it to create a so-called 3D point cloud that maps each image in 3D space.
"Once you have a point cloud, you can turn it into a solid surface," McCarthy said. "Then you have a 3D model of the site that's not subjective or an artist's impression, but entirely objective."
The benefits of photogrammetry are that it produces very high-resolution images and it can capture the true color of the site, McCarthy said. The method is easily thwarted, however, by excess marine growth or poor visibility, and it is not well-suited to covering large areas.
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Sonar, on the other hand, can see through the murk and can cover much larger areas, McCarthy said. For the 329-foot-long (100 meters) HMHS Anglia, another team from Wessex Archaeology used multibeam sonar - which operates in a similar way to a laser scanner - to do a much larger survey of the shipwreck site.
While multibeam sonar can't match the subcentimeter resolution of photogrammetry, using higher-end equipment and doing many passes can boost accuracy, McCarthy said. The Anglia survey was a particularly high-resolution one, he added, which was part of the reason it was selected for the 3D printing project.
McCarthy pointed out that the Wessex Archaeology team is not the first to create 3D-printed models from underwater imaging data. He said that the field has been booming in recent years, with big advances in both sonar and photographic techniques, and even some novel laser-scanning approaches are beginning to come through.
"All maritime archaeologists are engaging heavily with these techniques now," McCarthy said. "Advances in hardware and software in the last five years has allowed us to do very rapid and cheap surveys, and it has added to the tools we use underwater."
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