Dr. Susan C. Ryan, Executive Vice President of the Research Institute
“Seeing” Ancient Roads
On the last day of July, in the blazing afternoon heat on the great sage plain, myself and three colleagues from Crow Canyon visited an ancestral village intending to “ground truth” an extremely large, circular feature over 500 meters in diameter detected on a LiDAR image. The feature, a possible road, manifested as a topographic swale (a slight depression) over 10 meters across, and encompassed the entirety of a 12th century A.D. village. As we bobbled our heads back-and-forth between the LiDAR image and where we stood, we strained to identify the ancient feature on the modern ground surface. We measured how many meters the road was from the edge of the masonry architecture, pacing the distance with our steps to land in the heart of the swale. Scratching our heads, we called out to one another, “I don’t know…what do you think? Is this it?” Near the edge of the field, we finally detected a segment of the road we felt mostly confident about. To survey the landscape and not recognize ancient features below our feet—and in plain sight—is not uncommon in Southwestern archaeology; in fact, we wouldn’t have recognized the majority of these elusive features unless high-tech imaging revealed their presence.
Remote Sensing Technologies
For centuries, archaeologists have relied on traditional methods—including pedestrian survey, excavation, and artifact analyses—to reconstruct our shared human past. Every now and again a new technology emerges that drastically expands the ways we “see” the archaeological record. Remote sensing technologies allow us to study the remains of past cultures without physically contacting them. These “magical tools” include aerial photography, satellite imagery, and numerous geophysical applications such as magnetometry, electrical resistance, and ground penetrating radar are some of the best innovations since sliced bread.
Anyone with a driver’s license is familiar with a form of remote sensing called RADAR, or Radio Detection and Ranging, that allows law enforcement personnel to measure the speed of passing vehicles. By emitting radio waves that bounce off moving objects, radar calculates a vehicle’s speed based on changes in the frequency of the returned waves. Many of you may be all too acquainted with radar technology—and consequentially, speeding tickets—but what about LiDAR?
LiDAR and Archaeology
LiDAR, or Light Detection and Ranging, is a remote sensing method first developed by Theodore Maiman and his team in 1961 that projects near-infrared visible light, or ultraviolet light, to measure the distance between an object and a sensor. Using laser pulses to determine distances between the sensor and a target, LiDAR measures the time it takes for the pulses to return, providing high-resolution, three-dimensional maps of the Earth’s surface. These pulses define topography by penetrating vegetation, providing detailed information about the ground surface below. The ability to “see through” vegetation and tree canopies is extremely valuable as it reveals the underlying built environment.
Understanding topographic details helps archaeologists infer how past peoples interacted with their surrounding environments. For instance, LiDAR has allowed for the recognition of ancient irrigation systems, agricultural terraces, settlements, and road networks. These data help us reconstruct how peoples managed resources and organized social, economic, ritual, and political systems through time and across space.
Moreover, LiDAR data contributes to cultural heritage preservation objectives. Detailed mapping allows for the creation of precise digital elevation models that can be transformed into 3D reconstructions, aiding in conservation and educational outreach initiatives. LiDAR helps identify areas of potential risk, including sites threatened by erosion or development, by providing detailed information to implement conservation and land management strategies.
Before the advent of LiDAR, locating ancient roads was a formidable task. The arid Southwest environment often leaves features obscured by erosion, overgrown with vegetation, or ever-changing depositional contexts. LiDAR’s ability to illuminate subtle shifts in elevation has led to the identification of extensive and previously unknown road systems once used for travel, trade, ceremony, and communication. Insights gained by the identification of roads allows us to reconstruct social networks, community boundaries, the distance between villages and agricultural fields, and how ancient peoples manifested cosmological beliefs within the built environment from a bird’s-eye view.
How fortunate we are to apply innovative, non-destructive technologies to deepen our understanding of our shared humanity, especially when we stand in the sweltering heat scratching our heads, attempting to “see” into the past with our mere mortal eyes.
Photo caption #1: LiDAR image of an ancestral, 12th century A.D. village (center) with encompassing road. Red arrows indicate the outline of the road.
Photo caption #2: Katie Williams, Crow Canyon’s Post-Doctoral Scholar in Geospatial Research, standing in the elusive road swale.