Daniel C. McCarthy, Senior Editor/Special Projects
After the American Civil War, the Confederate prison camp in Salisbury, N.C., carried the reputation as being one of the South's harshest installations. A monument on the site records that 11,700 Union prisoners are buried in mass graves there. For decades, historians have attributed the high number of deaths to malnutrition and to the lack of adequate clothing or shelter. But infrared imaging is revealing the prison's history and structure under a different light.
For instance, written descriptions of the 12-acre site's layout conflict with a hand-drawn sketch made by a Confederate quartermaster. This and other riddles drew the attention of Bob Melia, a history buff and consultant who owns Real-Time Thermal Imaging.
Using a PalmIR camera from Raytheon Commercial Infrared, Melia imaged the prison site from the air. "We found that, if we shifted everything a couple of degrees, we could find the [latrine] area by recognizing a flow pattern like a creek that flowed through the city at the time," he said. "Once we had that, we had a better idea where everything else was laid out."
God doesn't make straight lines. Structural patterns revealed by thermal imaging, and highlighted here, indicate the location of graves at the site of a Confederate prison. Infrared
imaging is causing historians to revise earlier hypotheses about the prison's conditions. Courtesy of RealTime Thermal Imaging.
The images indicated that the latrines were placed much closer to the water supply than historians had guessed, leading to speculation that dysentery, not Confederate neglect, killed most prisoners. The PalmIR's imagery also raised doubts about the number of prisoner deaths at the camp. Judging from the size of the burial sites, the death toll was closer to 4500 than 11,700.
Melia's contribution took about three hours in the air and about 30 man-hours to review the tape and perform software enhancements on the images. A team of archaeologists also used ground-penetrating radar, which took half a day to collect data, but after two weeks of processing time, the radar revealed no additional information. Thermal imaging is most effective for sensing foundations and objects that retain more heat than the surrounding soil. "It's easy to break those out between 7 and 12 µm," Melia explained. "God don't make straight lines. A lot of the things you'll run into will have some sort of dimension you can identify. If there's a pattern there, you'll find something."
Melia used the PalmIR for the task in Salisbury because it provided greater sensitivity than cameras he had been using, in terms of thermal gradients. "The PalmIR had a stronger signal, which made it easier to articulate differences in the ground," he said, adding that the PalmIR is smaller and lighter, requiring less than a third of the battery weight of other infrared cameras.