Thermal Imaging a Hot New Archeology Tool

Time has erased almost all traces of one of the Confederacy’s most notorious prison camps, save for a fenced cemetery on a hill where a monument notes that 11,700 unknown Union prisoners were buried in mass graves.

For decades after the Civil War, the U.S. government and historians believed the Union prisoners at Salisbury died from malnutrition, unsuitable clothing and lack of shelter.

With the help of infrared thermal imaging cameras, history could be rewritten.

History buffs using the cameras, which can scan objects as deep as 25 feet underground or find moisture seeping into walls, took images of the underground features left by the camp. They drew two important conclusions: that roughly 4,500 prisoners died at Salisbury and that dysentery--not harsh treatment by the Confederate overseers--took many of the lives.

Thermal archeology is one of the space-age technologies increasingly applied to archeological sites from U.S. battlefields to Roman-era Egypt.

In addition to thermal imaging, archeologists are using ground-penetrating radar, which emits sound waves and records the echo created by obstructions. Instruments that measure variations in the earth’s magnetic field or what happens to electric current when it is sent between two underground poles have also been applied.

The technology used depends on the type of soil and the condition of the environment, said Boston University professor Farouk El-Baz, director of the school’s Center for Remote Sensing, which promotes the different methods for researchers in archeology, geology and geography.

“In desert areas, the imprint of the disturbance can remain for a very long time. We have picked up differences in terrain that were used in agriculture over 2,000 years ago by Roman-age people in Egypt,” he said.

The subterranean limit for most radar is about 30 feet, El-Baz said. Infrared imaging, which detects surface-level heat differences above buried objects, is most effective to a depth of about five feet, he said, though it, too, has been used to track objects 25 feet down.

At Shiloh National Military Park, site of a bloody 1862 battle in western Tennessee, bluffs at the center of the battlefield are being examined with radar for clues to a mound-building culture that thrived about AD 1200.

At Salisbury, thermal cameras called into question the long-held belief that an estimated 11,700 Union POWs were buried in 18 parallel trenches at the prison about 40 miles north of Charlotte.

“We just found these pit areas that pretty well represented about 4,500 people,” said Bob Melia, a history buff who owns Realtime Thermal Imaging of Kenner, La., and is one of thermal archeology’s pioneers.

More than 600,000 soldiers lost their lives during the war, which began in 1861 as the South broke away from the United States to protect its use of slavery and the North fought to preserve the Union. An estimated 30,218 of those soldiers were Union prisoners.

At Salisbury, thousands of imprisoned Union soldiers crowded the grounds as the war dragged on and prisoner exchanges all but ended. Prisoners’ diaries describe daily rations of a half-loaf of bread and a pint of rice soup; they record prisoners being forced to dig burrows for shelter from the elements, covering the holes with anything at hand.

At one point, the prison’s population exploded from about 1,500 POWs to about 10,000 inside a six-acre compound. George H. Nichols of the 118th Regiment of the New York State Volunteers was captured about three miles outside Richmond, Va., and shipped to the prison by train. A transcription of his diary is held by the county library.

“Took the cars for Sallsburrough. Got there after dark. Rained almost all night. No grub and no shelter,” Nichols wrote of his arrival on Nov. 7, 1864.

The next day, he wrote: “Quite warm today. Drew 3 spoonsful of soup and half a pt. of flour.”

Melia, familiar with infrared technology from serving as a U.S. Coast Guard intelligence officer, flew over the camp with a thermal imaging camera in early 1998. He recorded areas that retained the sun’s heat at different rates than the surrounding dirt. They bore the various signatures of graves, underground water wells, a covered-over latrine and the foundations of the prison’s fence.

Tossing aside a Confederate quartermaster’s sketch that had been the standard reference map for the 12-acre site, Melia came up with a picture that realigned the camp to conform to diary entries.

In the process, he learned that the camp’s latrines were about 20 yards away from the well water supply, suggesting that many of those buried here died from dysentery.

“It would potentially indicate that the death rate was much less than had been believed” at a Confederate prison thought to have one of the highest mortality rates, said William Blair, director of the Civil War Era Center at Pennsylvania State University.

Two other possibilities are that thousands of corpses were buried elsewhere or, as was common after the war ended in 1865, Northern families trekked south to recover and repatriate bodies of loved ones, he said.

“I don’t know if that went on there. I don’t know of 7,000 bodies being exhumed,” Blair said.

M. Sue Yarborough, director of the national cemetery established around the POW grave trenches, said there is no record of a mass exhumation or of large numbers of bodies buried elsewhere.

The public likely got its first glimpse of the capabilities of thermal imaging cameras from grainy images captured at night by U.S. troops during the Gulf War. The heat-sensitive cameras help scientists in Alaska find polar bear dens beneath the snow. Firefighters in Durham, N.C., and Fort Lauderdale, Fla., now use the cameras to navigate smoky buildings. General Motors Corp. began offering infrared night vision aids on its 2000 model Cadillac DeVilles.

At Salisbury and other Civil War sites in Ohio, Virginia, Mississippi, Alabama, Tennessee and Louisiana, history buffs are deploying the high-tech cameras with growing success.

“It doesn’t replace the shovel and brush, but it narrows down the area where you have to use a shovel and a brush,” Melia said.

Melia’s camera weighs about 3 1/2 pounds and is smaller than a standard home video camera. It translates differences in heat as little as .05 degree Centigrade into 256 shades of gray on a two-dimensional videotape image. Interpreting the meaning of the recorded temperature differences takes experience and skill, he said.

Whether it’s radar or infrared cameras, the new technology offers advantages over old-fashioned spadework, said Woody Harrell, superintendent of Shiloh.

“There’s a wealth of information, but in uncovering that information [with a shovel and brush], you’re destroying the ability to do anything more with that,” Harrell said. “We can find this information and yet the site is still there; the history book is still there to read again.”

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On the Net: Archaeological Remote Sensing Geophysics Image Library at University of Arkansas, Fayetteville:

https://www.cast.uark.edu/~kkvamme/geop/geop.htm

National Park Service, Heritage Preservation Services, American Battlefield Protection Program:

https://www2.cr.nps.gov/abpp/Hidden/Hidden.htm

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Thermal Imaging at Work

Thermal imaging cameras, which read infrared, rather than visible, waves of light, are being used with success at various historical sites nationwide:

* A thermal infrared camera found a century-old burial ground for blacks outside a cemetery for whites in Salisbury, N.C. The more than 100 unmarked graves were in a portion of the Freedman’s Cemetery where slaves and freed black residents were buried from about 1840 to 1910. Thermal maps of the graves across from the parsonage of Soldier’s Memorial AME church--named for the Union troops who liberated the city’s slaves--fueled an effort to find and record the names of the slaves buried there. Funds are being raised for a memorial and an arch for the burial ground entrance.

* In the Nevada desert, a researcher is testing equipment that senses reflected light or retained heat from aircraft to detect stone tools that might indicate Native American settlements.

* Researchers are using infrared thermal images to help resolve a dispute over how many black Union cavalrymen wounded in a Civil War battle at Saltville, Va., were executed by Confederate troops. As few as a half-dozen or as many as 150 wounded soldiers from the 5th U.S. Colored Cavalry were killed there in 1864.

* Thermal imaging equipment is also being used to root out damaging moisture within the fortress walls of Texas’ Alamo.