Technique for Manipulating Satellite Photos Now Reveals Ancient Images (2025)

Originally conceived at NASA's Jet Propulsion Laboratory, 'decorrelation stretch' (Dstretch) is an image processing technique that heightens contrasts in digital imagery, making subtle differences pop out. While initially developed to extract more information from satellite photos of celestial bodies, it has become an invaluable aid for archaeologists and rock art enthusiasts.

• Technical Foundation: Dstretch goes beyond simple contrast adjustment by mapping original colors to an expanded color range. It's rooted in complex algebra, specifically the Karhunen–Loève Transform, co-invented by a former professor of Jon Harman, who later popularized its use for ancient art.
• From Space to Stone: Inspired by its effectiveness on Mars images, Jon Harman, with a background in medical imaging and mathematics, developed the Dstretch plug-in for ImageJ and later mobile apps. He realized the technique's potential for revealing faded rock art, which often suffers from low contrast.
• Archaeological Revelations: Dstretch has been instrumental in uncovering previously invisible details in various archaeological sites, including:
  • Faded paintings at Cambodia's Angkor Wat temple.
  • Undocumented figures and new details in rock art in Baja California, Mexico, and Årsand, Norway.
  • Rare animal depictions (bats, pigs) in ancient Egyptian tombs at Beni Hassan.
  • Early forms of graffiti from a Crow warrior at Writing-on-Stone Provincial Park in Canada.
  • Buried structures and foundations in ancient urban settlements like Vlochos, Greece, using aerial imagery.
  • Even enhancing imagery of tattoos preserved on mummified human remains.
• Continuous Evolution: Ronald Alley, a JPL employee, refined the original technique in the 1990s, making it faster and more accurate for multispectral imagers like ASTER, which is still used today for geological applications like tracking volcanic plumes.

This technology exemplifies a fascinating journey from the demands of space exploration to groundbreaking discoveries in humanity's ancient past, proving that the tools we build for one frontier often shed light on another, entirely different one.