How a NASA Satellite Technique Is Revealing Faded Ancient Art

A digital-imaging technique first developed at NASA's Jet Propulsion Laboratory (JPL) to study satellite pictures is now helping archaeologists recover ancient artwork too faded for the human eye to see. In the central tower of Cambodia's Angkor Wat temple, paintings of horseback riders and a musical ensemble had faded almost to invisibility. Between 2010 and 2012, an archaeologist used the technique to reveal them, along with about 200 other paintings across the temple complex.

The method, called decorrelation stretch, heightens contrasts in a digital image to make faint features easier to see. It was first described in a 1996 paper by Ronald Alley, a JPL employee developing applications for the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), a Japanese instrument aboard NASA's Terra satellite. One of Alley's former JPL supervisors had co-invented decorrelation stretch, and Alley saw its potential for extracting information from ASTER images.

The technique reached rock art research through Jon Harman, a rock art enthusiast with a background in medical imaging, now retired in Pacifica, California. Around 2005 he saw NASA images of the Martian surface shown with and without decorrelation stretch and realised what it could do for faded ancient images. "I Googled it and found a NASA paper that explained how to do the algorithm," he said. "I knew from my medical imaging experience that I could do it, so I did."

Harman built a plug-in for ImageJ, an open-source imaging program developed by the National Institutes of Health, and says he still fulfils about 200 requests a year for the tool, known as Dstretch. Around 2010 he also created smartphone apps that mimic decorrelation stretch; they have been downloaded thousands of times, and researchers have published papers on the tool's use in archaeology.

Dstretch has since been used to spot and clarify imagery at an ancient site beneath a cliff in Norway, inside an Egyptian tomb and at a park in Canada. It has also helped archaeologists find buried remains of ancient Greek buildings and examine tattoos on mummified human remains.