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‘The Rock’ revealed by radar, lidar

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CHERYL L. HULSEAPPLE cheryl.hulseapple@photonics.com

In 1934, one of America’s most notorious underworld gangsters, Al Capone, was sent to Alcatraz, also known as “the Rock.” He was just 35 years old. According to a little-known personal letter written to his son, he was once allowed to cash in “good time” to start an inmate band, the Rock Islanders. Now, thanks to the power of 3D mapping and remote sensing, the Rock has revealed some of its own little-known secrets.

A group of researchers from Binghamton University, N.Y., have used a combination of ground-penetrating radar, terrestrial lidar, and georectifications to develop a 3D model of the above-ground and subterranean structure and grounds. The model revealed a series of secret tunnels — possible escape routes — just centimeters beneath what was once the prison recreation yard.

Alcatraz Map Courtesy of NPS.


Courtesy of NPS.

To peer noninvasively beneath the old concrete veneer, the researchers used 200- to 500-MHz radar antennas. Then they used a lidar system that could collect “up to 11,000 points per second with a range of 400 m and accuracy of 2 mm, yielding 0.002° angular resolution,” according to a January paper they published in Near Surface Geophysics.

The lidar system’s camera provided an RGB color value to each scanned point, and the scans of the recreation yard represented a global point cloud of more than 20 million points, the researchers reported.

Between 1934 and 1963, Alcatraz operated as a federal penitentiary and inmates lived on top of what used to be a Civil War-era Army military fortification. In 1910, the government added the first cell block to house military prisoners. When the fortification was razed and the federal prison expanded, some of the old tunnels and munitions buildings were simply paved over to save money.

Binghamton archaeologist Timothy de Smet and colleagues created the 3D reconstruction of the island, and de Smet said he was “surprised” at the structural integrity of the underground remains.

“These remains are so well preserved, and so close to the surface,” de Smet told PBS. “They weren’t erased from the island — they’re right beneath your feet.” At present, the tunnels remain hidden to the more than 1 million tourists who visit the location yearly, but the researchers have said they hope the Rock’s military past will soon be brought out into the open.

As far as anyone knows, neither Capone nor any of the nearly 1600 rock island inmates ever successfully escaped from Alcatraz and lived. The tunnels add another fascinating layer to the history of one of America’s most notorious structures.

Photonics Spectra
Jul 2019
GLOSSARY
lidar
An acronym of light detection and ranging, describing systems that use a light beam in place of conventional microwave beams for atmospheric monitoring, tracking and detection functions. Ladar, an acronym of laser detection and ranging, uses laser light for detection of speed, altitude, direction and range; it is often called laser radar.
lidarAlcatrazCheryl HulseappleLighter Side

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