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U.S.S. Arizona surveyed with 3D photogrammetry

3D modeling, printing help preserve Pearl Harbor relics

This Sunday will mark 73 years since Japanese dive bombers launched a surprise attack on Pearl Harbor, sinking four U.S. battleships and damaging a dozen other vessels.  Three of the four battleships were later raised and returned to service. 

The U.S.S. Arizona was too badly damaged to be salvaged and, in 1962, it was designated a national shrine. Lately, the U.S. National Park Service, which manages the underwater site, has become concerned about the effects of 70-plus years of salt water on the structure. 

The Park Service's immediate need is to find out what’s going on down there, said Pete Kelsey, a strategic projects executive at Autodesk. "It's a steel battleship, mostly under salt water for over for 70 years – you can bet it's not the same ship it was even five years ago."

Kelsey is overseeing a team that is scanning the site above and below water using three 3D technologies, including LiDAR, SONAR and photogrammetry.  While LiDAR and SONAR have been in wide use for some time, photogrammetry – the analysis of photographs to create 3D models – is new.

"This is one of the first times that multiple different 3D capture technologies above and below the surface have been used together," said team member Shaan Hurley, a technologist at Autodesk.  "Each one has its own unique strengths and weaknesses."

The team is using Autodesk’ Recap software for the photogrammetry analysis, which enables 3D models to be rendered from multiple photographs of an object taken from different perspectives.

"We take a series of photos around an object, and the program calculates the camera's position in space by looking at unique little identifiers on the object surface," Hurley said.

Having precisely located the images relative to each other, the program uses that information to render the object in three dimensions.  "You end up with a textured 3D representation of the object," Hurley said. That representation can either be viewed on a monitor or printed with a 3D printer.

While the principle behind photogrammetry is relatively simple, the computing power it requires is not.  "The algorithms require an amazing amount of calculating," Hurley said.  As a result, the team chose to implement it as a cloud service using powerful servers and graphics processors on the back end.  "The computing power it requires is beyond most laptop or desktop computers," Hurley noted. 

Hurley concedes that photogrammetry doesn't offer quite the resolution that LiDAR does.  But it offers other advantages. 

"The beauty of photogrammetry is you don't have to have a $50,000 laser scanner," he said.  "Recently I spoke to a couple of hundred archaeologists and I asked, ‘How many of you have a laser scanner here?’ One person raised their hand.  I asked, ‘How many have access to a camera of some sort?’ And every hand went up. I've done some of the most amazing models just using my iPhone."

The technology has already been deployed for other purposes.  Hurley says he recently learned that the federal Bureau of Land Management was interested in being able to capture Indian ruins in the backcountry. 

"It's hard to convey the space, the shape, the size in a photograph," Hurley said.  With photogrammetry, he said, "any of those BLM agents in the field can just take a series of, say, 12 photos." Port those photos into Autodesk Recap, and 3D records of the objects can be generated.

Hurley has also used the technology to create 3D models of coral beds in Molokai, an island in the Hawaiian archipelago.  "Up until now marine biologists have done it in 2D," Hurley said.  "But coral grows in unpredicted patterns, and things happen over time and it's really tough to figure out how it's changed.”

“With this they are able to capture beautiful 3D models of coral and at a later day go back and capture the coral again and with superlative accuracy be able to say it is changed so many millimeters in volume.  That has caused a lot of excitement among marine biologists."

While the main interest of the Park Service in scanning the Arizona may be change detection, the Autodesk team realized there are other opportunities as well. 

"Almost immediately, we knew that a three-dimensional model would provide all kinds of value," Kelsey said, who added that the team is working on developing a model than can be used for virtual tourism of the site.  He expects that model to be ready some time in 2015.

Posted by Patrick Marshall on Dec 04, 2014 at 12:03 PM


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