Summary
- The decorrelation stretch technique was developed to enhance differences in satellite and multispectral imagery
- Jon Harman used the method to create Dstretch for archaeological imagery
- More than 200 faded paintings were revealed at Angkor Wat
- Dstretch has been applied to rock art, buried buildings and mummified remains
- At Vlochos in Thessaly, Greece, it was used on aerial images to detect archaeological features
An image-processing technique developed at NASA’s Jet Propulsion Laboratory to reveal subtle details in satellite data is now being used to make ancient artwork and archaeological traces visible when they are difficult to distinguish with the naked eye.
NASA highlighted on September 8, 2026, how decorrelation stretch moved from satellite and planetary image processing to Dstretch, a tool now used at archaeological sites around the world that helped, among other things, reveal more than 200 nearly invisible paintings at Cambodia’s Angkor Wat.
The importance of the development lies in the fact that this is not simply a filter that boosts the contrast of a photograph. The method enhances very small color and spectral differences, allowing researchers to identify features that may be obscured by fading, natural deposits, vegetation or subtle variations in the ground surface. The same approach has also been used in archaeological research at Vlochos in Thessaly, Greece.
From satellite imagery to Dstretch
The decorrelation stretch technique was designed to enhance color differences in multispectral imagery by reducing correlations between individual channels. In simpler terms, it does not merely brighten a dark area or raise overall contrast: it processes the relationships between color data so that small variations appear much more prominently.
NASA traces the technique to work at the Jet Propulsion Laboratory and particularly to applications for ASTER, the Japanese multispectral instrument launched in 1999 aboard the Terra satellite. JPL’s Ronald Alley wrote a technical paper in 1996 on its use with ASTER, recognizing its value for extracting more information from satellite imagery.
The technique remains useful in remote sensing. According to NASA Spinoff, the ASTER team still applies it to infrared imagery to monitor volcanic plumes, where sulfur dioxide, ash and water vapor have different spectral characteristics.
The connection with Jon Harman
The transition from space imaging to archaeology happened largely through Jon Harman. Around 2005, Harman, who had a background in medical imaging and an interest in rock art, saw images of the Martian surface before and after decorrelation stretch had been applied. He realized that the same idea could reveal faded ancient markings.
Using NASA’s technical description as a basis, he created Dstretch as a plug-in for ImageJ, the open-source image-processing and analysis program originally developed by the National Institutes of Health. Smartphone apps were later created that approximate the same process in a more simplified form.
While developing Dstretch, Harman applied it to a photograph from the Cave of San Borjitas in Baja California, Mexico. Processing made a yellow human figure become visible where it had previously been difficult to distinguish, providing one of the early examples of how useful the technique could be for studying ancient art.
More than 200 paintings at Angkor Wat
One of the most striking examples comes from Angkor Wat in Cambodia. Between 2010 and 2012, a Singaporean archaeologist used Dstretch to reveal more than 200 faded paintings at different locations throughout the complex.
In a chamber near the top of the central tower, the images included mounted figures and a traditional musical ensemble. The artwork is so faint that thousands of visitors can pass nearby without noticing it.
The crucial point is that the technique does not “create” archaeological content that is absent from the original photograph. It changes how existing color differences are represented so patterns already present in the pixels become easier to distinguish. The result is therefore a visual investigative tool and still requires archaeological interpretation and confirmation, rather than treating every enhanced shape as an automatic discovery.
The Greek connection: Vlochos in Thessaly
The technology has already been applied in Greece. At the archaeological site of Vlochos in Karditsa, where remains from successive settlements ranging from the Archaic to the Early Byzantine period survive, Dstretch was applied to aerial imagery to enhance very subtle color differences across the landscape.
The relevant scientific study describes imagery acquired with unmanned aircraft, processed with Dstretch and compared with other archaeological prospection methods such as NDVI and geophysical surveys. The technique highlighted areas of potential archaeological interest and traces of buried structures that were not equally apparent with other approaches.
Vlochos is particularly interesting because archaeological investigation at the site relies heavily on non-invasive techniques. The Greek-Swedish Vlochos Archaeological Project used geophysics, high-precision GPS documentation and extensive aerial photography, among other methods, to map the ancient remains.
From rock art to mummies and buried buildings
Dstretch has also been used in many other settings. In Norway it helped identify around 15 previously undocumented figures and reveal new details in dozens more. At Beni Hassan in Egypt it was used to identify depictions of animals, while in Canada it was applied to Indigenous rock art.
Its use is not limited to rock art. NASA cites applications in searching for remains of buried buildings and in enhancing imagery of tattoos preserved on mummified human remains.
Harman says he receives around 200 requests per year for Dstretch, while the mobile applications have been downloaded thousands of times. The fact that the technique continues to find new uses demonstrates how easily a tool designed for one scientific field can migrate into an entirely different one.
What we think
This is a strong example of a genuine technology spin-off: a method developed to improve the interpretation of multispectral satellite data has become a useful tool for studying cultural heritage. The most important aspect is not simply the dramatic before-and-after imagery, but the ability of such techniques to guide archaeological research in a non-invasive way. Its application at Vlochos in Thessaly also shows that the connection between space technology and archaeology is not a distant example but already has a practical presence in Greece.
Frequently asked questions
What is decorrelation stretch?
It is an image-processing technique that reduces correlation between color or spectral channels and enhances differences between them, making subtle details much easier to see.
What is Dstretch?
Dstretch is a tool created by Jon Harman based on the decorrelation stretch technique. It was developed as a plug-in for ImageJ and has been particularly adapted for studying rock art and other faded imagery.
Has the technology been used in Greece?
Yes. At the archaeological site of Vlochos in Thessaly, Dstretch was applied to aerial drone imagery to highlight possible archaeological features and buried structures.
Can Dstretch reveal things that are not present in the original image?
The method enhances differences that already exist in the image data. It is a visual investigation tool and does not replace scientific confirmation or archaeological interpretation.
Where else has it been used?
It has been applied to rock art in Norway, Egypt and Canada, to buried architectural remains and even to imagery of tattoos preserved on mummified human remains.
