How aerial laser mapping reveals cities in the forest

HostYou can stand in the middle of a thick forest and feel like you're at the end of the world. All you see is a wall of green vines and trees so packed together that you can't see five feet in front of your face. But it turns out, we have been walking right over the top of massive, lost cities for hundreds of years without ever knowing they were there. How does shooting lasers from a plane actually let us see through all those trees and dirt to find these old buildings?

GuestIt's a bit like having a magic flashlight that only sees the ground. We use a tool called Lidar, which is just a fancy way of saying we're using light to measure how far away things are. We strap a box to the bottom of a plane or a drone and fly it over the woods. This box shoots out hundreds of thousands of tiny laser pulses every single second. These bits of light hit the leaves, the branches, and the trunks of the trees. Most of that light just bounces off the greenery and comes back to the plane. But the trick is that a tiny bit of that light slips through the small gaps between the leaves and hits the actual dirt on the floor of the forest.

HostHold on a second. If the forest is as thick as I think it's, those gaps must be tiny. It seems like you would mostly just get a map of the top of the trees, not the ground below.

GuestWell, you do get a map of the trees, but we can fix that later. Think about it this way. If you throw a handful of sand at a screen door, most of the sand hits the wire and bounces back. But a few grains of sand are going to go through the holes and hit the floor on the other side. Because we're shooting millions of these laser pulses, enough of them make it to the ground to give us a clear picture. The box on the plane keeps track of exactly how long it takes for each pulse of light to hit something and bounce back. Since light always moves at the same speed, we can use that time to figure out the distance to the inch. We end up with a huge cloud of millions of little dots in our computer.

HostSo you just have a giant mess of dots. How do you go from a big cloud of data points to seeing a hidden house or a road?

GuestThat's where the computer work comes in. We tell the computer to look at all those dots and sort them out. We tell it to find the dots that are the highest up, which are usually the tree tops. Then we tell the computer to just delete them. We give the whole forest a digital haircut. We strip away the leaves, the branches, and the bushes until we're left with only the dots that hit the very bottom. When you look at those remaining dots, the shape of the land pops out. You start to see things that don't look like they belong in nature. You see long, perfectly straight lines that run for miles, or big square mounds that look like platforms. Trees don't grow in straight lines, and hills usually don't have ninety degree corners.

HostBut people used to build things out of wood and mud, too. If a city is a thousand years old, wouldn't the buildings have just melted back into the earth by now? I mean, how can a laser find something that's basically just a pile of dirt?

GuestEven if a building falls down and gets covered in moss and soil, it still changes the shape of the ground. It creates a bump. If you have a thousand houses in a town, they create a very specific pattern of bumps. When we look at these maps, we see where people dug out ponds to hold water or where they built up high banks to keep their farms from flooding. We can see the ramps that led up to big temples. In places like the jungles of Central America, we used to think there were only a few thousand people living in small groups. But when we used these lasers, we found tens of thousands of homes we never knew about. It turns out these places were huge, bustling cities with big highways connecting them. We were just looking at them from the ground, where the forest hides everything.

HostThis makes it sound like we have already solved the mystery. If we can just fly a plane over the whole world and find everything, do we even need to go into the woods anymore? It feels like the hard work is done once you have the map.

GuestI wouldn't say that. The map only tells us where the bumps are. It doesn't tell us who the people were, what they ate, or why they left. We still have to put on our boots, grab a machete, and hike through the mud to see what those bumps actually are. Sometimes the laser shows us what looks like a grand palace, but when we get there, it turns out to be a natural rock pile that just happened to look square from the air. We call this ground-truthing. The laser is just the starting point. It saves us years of wandering around aimlessly, but it can't dig the holes for us.

HostThere's still so much of the world that's covered in deep woods or thick vines where no one has walked in ages.

GuestMost of the ancient world is still under our feet, and we're just now getting our first real look at how big these societies really were.

HostThat thick wall of green is finally starting to look more like a map than a mystery.