A technique NASA developed to extract detail from satellite imagery is now helping archaeologists discover ancient artwork invisible to the naked eye. Known as decorrelation stretch, the method heightens contrasts in digital images to reveal subtle features, and has become particularly valuable for studying faded rock art and paintings.
The technique’s journey from space exploration to archaeology began in the 1980s when Jon Harman, then working in medical imaging, attended a rock art conference. After seeing decorrelation stretch applied to Mars Exploration Rover images around 2005, Harman recognized its potential for studying ancient, faded artworks. He located a 1996 NASA paper explaining the algorithm and, drawing on his PhD in mathematics from UC Berkeley, adapted the technique into the Dstretch plug-in for ImageJ, an open-source image processing program.
Decorelation stretch differs from simple contrast enhancement by remapping original colors to an expanded range, a process involving complex mathematical operations including matrix diagonalization. The underlying mathematical principle, the Karhunen–Loève Transform, was developed decades earlier and applied to satellite imagery by JPL researchers in the 1970s and 1980s.
Harman’s tools proved immediately useful. At Angkor Wat in Cambodia, approximately 200 paintings were discovered between 2010 and 2012 using the technique—including horseback riders and musical ensembles on chamber walls that thousands of daily visitors had never noticed. Harman now receives around 200 requests annually for Dstretch applications, fulfilling them for $50 each. He also created smartphone apps mimicking the technique, downloaded thousands of times at $20 per app.
The applications have expanded far beyond rock art. At the ancient Egyptian cemetery of Beni Hassan, researchers used Dstretch to discover rarely depicted animals like bats and pigs. In Norway, the technique uncovered 15 previously undocumented figures at the Årsand 1 site. In Greece, Dstretch applied to aerial imagery of Vlochos revealed archaeological features missed by elevation models, ground-penetrating radar, and electrical resistance surveys. In 2023, researchers published a protocol for using Dstretch to enhance tattoos preserved on mummified remains.
According to Harman, the widespread adoption beyond rock art has been surprising. “I’ve been surprised by a lot of the different applications people have found,” he said. The technique continues to demonstrate how tools developed for space exploration can uncover secrets buried in human history.
Key facts
- Decorrelation stretch was originally developed at NASA’s Jet Propulsion Laboratory for satellite and Mars rover imagery
- Jon Harman adapted the technique into Dstretch software after recognizing its potential for studying faded rock art around 2005
- Approximately 200 paintings were discovered at Cambodia’s Angkor Wat using the technique between 2010 and 2012
- The technique has been applied to archaeology in multiple countries, revealing hidden details in rock art, ancient buildings, and mummified remains
- Harman receives about 200 requests annually for Dstretch applications and developed smartphone apps downloaded thousands of times