The technology we’re using for CALI is a laser-ranging or ‘lidar’ device mounted on an aircraft to conduct a program of airborne laser scanning, which is capable of producing incredibly detailed maps of the ground surface even beneath dense tropical forest canopy. Variations in the ground surface (or what we might call ‘archaeological topography’) that may not look like much on the ground can be clearly identified as patterned spaces when viewed from above, enabling us to identify entire urban networks even on the jungle floor after we virtually ‘strip away’ the vegetation.
Common misperceptions about the technology include that the lidar can literally ‘penetrate’ vegetation or that it can somehow see beneath the ground. It does neither. What the instrument does is that it fires millions of laser pulses every few seconds and in forested areas, at least a small percentage of those laser pulses find their way to the forest floor and bounce back to the sensor. The time it takes for that return is a function of the distance between the sensor and whatever the laser bounced back off. Because we know the position and orientation of the sensor very precisely thanks to the on-board GPS and IMU systems, we can calculate where the return came from in three-dimensional space within an accuracy of a few cm.
We end up with billions of returns after a few hours of flying, and the challenge then is to use software to filter out the returns that came from the ground from all of the rest of the returns, most of which come from leaves and branches.