Researchers discover ancient drawings in a cave thanks to photogrammetry

Researchers used 3D photogrammetry to uncover ancient mud glyphs, or designs, inscribed on the ceiling of a cave in Alabama.

Using photogrammetry, a software technique that creates 3D models, the researchers produced a high-resolution topographic 3D record of the very faint drawings, including images that were invisible during their in-person visits to the site.

Stephen Alvarez, photographer and founder of the Ancient Art Archive (Sewanee, TN; www.ancientartarchive.org); board member Jan Simek, professor and archaeologist at the University of Tennessee; and Alan Cressler, a photographer, undertook the project and reported their findings in the journal Antiquity (https://bit.ly/3NlyAkm).

“We knew the glyphs existed, but they were so faint that it was impossible to distinguish the complete images,” they wrote in a press release describing the project. Using photogrammetry, they uncovered “thousands of additional glyphs and images, most outside the canon of Native American iconography previously detected in the southeastern United States.” Several of them appear to be human forms in ceremonial dress while another represents a serpentine.

The mud glyphs, found in a dark 25 × 20 m chamber in the cave, are very large, including a design measuring 1.81 m in length. The researchers also noted that the space in which the ancient artists would have applied their craft was cramped with a very low ceiling, which meant that they would not have been able to see the entire design as they created it. In fact, without 3D modeling techniques, the researchers couldn’t have done it either.

The photogrammetry process

“What photogrammetry and 3D modeling allow us to do is to abstract that ceiling and remove things that distract us, like the color of the cave, the water droplets, things like that, and to shine virtual lights through the mesh to really look at the carvings there and see them without the distraction of the rest of the cave,” says Alvarez.

In the first step of the photogrammetry process, researchers took 16,000 photographs, each overlapping neighboring images by 80%, to use as the basis for two 3D models of the etched ceiling and a third 3D model covering the cave entrance. through the room. with the drawings. The 3D models, which are linked together, can be manipulated virtually to examine the designs in detail.

They used a Canon (Tokyo, Japan; https://global.canon/en/) 5D MK IV camera with a 30 MPixel sensor and a Canon L-series 24-70mm f2.8 lens set to 24mm and a Canon 5DS DSLR camera with a 50 MPixel sensor and a Sigma 24mm Art Series lens. The photos were taken by hand and with a tripod. For photos from the ceiling, they also mounted the camera on a 12-foot movable rail, allowing the camera sensor to be pointed upwards.

To illuminate the area, they used an in-camera flash. For some of the images, they also used four LED lights mounted on tripods, which were strategically positioned to illuminate the part of the ceiling they were photographing.

Once back in the office, Alvarez used photogrammetry software, Agisoft Metashape (St. Petersburg, Russia; www.agisoft.com), to create the 3D models. The software calculates the camera positions used to take the images by examining the photos and the overlaps. It then triangulates the pixels to create a 3D point cloud. From the cloud of points it has created, the software produces a mesh of the surface to be modelled; the mesh is then textured with the original images, creating the 3D model.

To interpret the models, Alvarez downloaded them into software from Autodesk (San Francisco, CA, USA; www.autodesk.com) called Maya. “It allows you to see things in different ways. You can texture things. You can remove the textures. You can just look at the mesh. You can light it from any direction,” says Alvarez.

Alvarez expects the team’s work to lead to more widespread use of photogrammetry and 3D modeling in archaeology. “This means that by using relatively inexpensive tools, namely a digital camera and a flash, you can resolve incredibly fine etchings that would otherwise be invisible. You don’t have to throw a $250,000 lidar unit at a site, you can with these inexpensive tools. This opens a new way not only to discover the new engravings but also to be able to quantify and interpret them.