How researchers created 3D models of largest Native American cave art collection

The researchers documented their findings in a research article titled, “Discovering ancient cave art using 3D photogrammetry: pre-contact Native American mud glyphs from 19th Unnamed Cave, Alabama,” published in the journal Antiquity.

“They probably depict characters from previously unknown religious narratives, likely of the Middle Woodland period. The most striking aspects of these cave art images are their size and context. Among the 19th Unnamed Cave mud glyphs are the largest cave art images known in North America. They are so large that the makers had to create the images without being able to see them in their entirety. Thus, the makers worked from their imaginations, rather than from an unimpeded visual perspective,” wrote the researchers in the article.

The 19th unnamed cave and the location of the cave art

The 19th unnamed cave comprises more than 5 kilometres of underground passageways with the entrance at 219 metres above mean sea level. This entrance is approximately 10 metres high and 15 metres wide. An intermittent stream flows out of the cave, because of which no intact archaeological materials survived at the cave’s entrance.

From the ‘main hall’ of the cave, a passage climbs up to a 25 x 20 metre chamber that is bounded by flowstone formations. The mud glyphs are inscribed on the ceiling of this “room” which is only 125 centimetres above the floor at its highest.

3D Photogrammetry: How researchers created a 3D model of the massive art

In late 2017, Stephen Alvarez of the Ancient Art Archive capitalised on the advances in 3D photogrammetry to create a high-resolution three-dimensional topographic record of the glyphs drawn in the chamber. Photogrammetry is a software-based 3D modelling technique that makes use of photographs.

You first start by taking many photographs of the target object or location, with each photograph overlapping the next by about 60 to 80 per cent. The photogrammetry software then compares the images and overlaps and calculates the camera positions used to produce the images. The software then triangulates the pixels in a 3D space to create a ‘point cloud’.

That cloud is rendered into a highly accurate mesh of the surface being modelled and the mesh is textured with the texture from the original images to make it look like a photorealistic 3D model. The resulting model from this technique is then calibrated by measuring known distance. For the 19th unnamed cave, the researchers created three interlinked models: one of the undecorated cave passages and two of the engraved ceiling.

The first ceiling model was photographed using a Canon 5D Mark IV DSLR, which has a 30.4-megapixel sensor, and a Canon L series 24-70mm f2.8 lens set to 24mm. For the first model, the researchers used angled lighting which made some glyphs more obvious while obscuring others. But this was not a problem as the first model was created to provide a ‘roadmap’ for the second, higher-resolution model.

For the second model, the researchers use a flat light and a Canon 5DS DSLR camera with a 50megapiel sensor and a Sigma 24mm Art Series lens. These photographs were shot with an 80 per cent overlap and were used to create a really high-resolution model of the cave ceiling. The third model was of the entire cave space from the entrance to the section with the drawings. This was a lower resolution model shot with the camera in the first model and way fewer photographs.