So LiDAR or light detection and
ranging uses lasers to measure the elevation of things like the ground forests and even buildings. It's a lot like sonar which uses sound waves to map things, or radar which uses radio waves to map things, but a LiDAR system uses light sent out
from a laser. But aren't lasers used for stuff like, I don't know, scanning barcodes, making your favorite band look awesome, removing hair? Eww! Seriously how does a LiDAR system use lasers to measure stuff? Now just for the record there are
different ways to collect LiDAR data from the ground, from an airplane, or even from space. Airborne LiDAR data are the most commonly available LiDAR data and airborne LiDAR data will also be freely available through the National Ecological Observatory Network or NEON. So we'll focus on that in this video. So to get how lasers are used to calculate height in airborne LiDAR, we need to understand the four parts in the system. First, the airplane contains the LiDAR
unit itself which uses a laser to scan the earth
from side to side as the plane flies. By the way, for your remote sensing geeks out there, the laser system uses either green or
near infrared light because these wavelengths or types of light reflect strongly off of vegetation. The next component of a LiDAR system is
a GPS receiver that tracks the altitude and XY location of
the airplane The GPS allows us to figure out where LiDAR reflections are on the ground. The third component of the LiDAR system is what's called an inertial measurement unit or IMU not to be confused emu -- an IMU. The IMU tracks the tilt of the plane in the sky as it flies. which is important for accurate elevation calculations. Finally, the LiDAR system includes a computer. The computer records all that important height information that the LiDAR collects as it scans the earth's surface. No computer, no data. It's as simple as that. Now you may be wondering how the laser, the GPS, the IMU and the computer all work together to get us these fantastically useful later dataset? Well, the laser in the LiDAR system scans
the earth actively emitting light energy towards
the ground. Now before we go any farther, let us get two key LiDAR terms associated with this emitted light energy out of the way. You know, so we can speak LiDAR and all. First, let's define the word "pulse". A pulse simply refers to a burst of light
energy that is admitted by the LiDAR system. And second, lets define the word "return". Return the first reflected light energy
that has been recorded by the LiDAR sensor. So pulses of light energy travel to the
ground and return back to the LiDAR sensor. Now we're speaking LiDAR. Now all of this pulse and return business is great but speaking a language doesn't give us height. To get height the LiDAR system records the time that it takes for the light energy to travel to the ground and back. The system then uses the speed of lightto calculate the distance between the top but that object and the plane. So let's break down with this distance calculation looks like: you take travel time multiplied by the speed of light, divided by 2 since the light traveled to
the ground and back. This calculation gives us how far the
light actually traveled to the ground. So using this math, we know the distance between the plane in the ground. But we're not done just yet. How do we figure out the actual elevation of the ground? To figure ground elevation, we take the plane's altitude, calculated using that GPS receiver and then we subtract the distance that the light travel to the ground. And that covers the basics of how a LiDAR system uses lasers to measure height. So now we've got the basics down. But there are two more things in a
LiDAR system has to consider when calculating height. First, the plane rocks a bit in the sky as it flies due to turbulence in the air. These movements are recorded by the
inertial measurement unit or IMU. No, no, no, not emu. IMU. So that they can be accounted for when
height values are calculated for each LiDAR return. and also an airborne system scans the earth from side to side to cover a larger area on the ground when flying. So while some light pulses travel vertically from the plane to the ground or directly at nadir, if we're still talking LiDAR lingo here, most pulses leave the plane angle or off nadir. so the system also needs to account for pulse angle when it calculates elevation. So let's put this all together one last time the LiDAR system emits pulses of light energy towards the ground using a laser it then records the time it takes for the
pulse to travel to the ground and return back to the sensor. It converts this time to distance using
the speed of light. The system then uses the plane's
altitude, tilt, and the angle of the pulse to
calculate elevation. It also uses a GPS receiver calculate
the object's location on the ground. All this information is recorded on that
handy dandy computer also mounted on the airplane. And that, my friends, is how a LiDAR system works. Now, there is one component of a LiDAR
system that makes it so useful. A pulse light doesn't just reflect off one thing like the top of a tree and yield one return. Sometimes a LiDAR pulse travels through things like the gaps between tree branches and leaves. Think about standing on the forest floor and watching the sunlight filter through the top to the trees making the leaves and branches glow. So light energy that passes through
the canopy reflects of the branches and leaves within the canopy. This ability of a LiDAR system to
travel through and record information starting from the
top of the canopy, through the canopy, and all the way to the
ground makes LiDAR systems unique and highly
valuable to scientists studying trees. These returns from within the canopy
tell us more about what's happening inside the forest, or the forest structure. For example, they can tell us about the shape of the trees or the density of the leaves on the trees. They can sometimes even be used to
estimate whether they're shrubs below the trees on the forest floor but we'll save that topic of using LiDAR to estimate forest structure for another video. And, now my friends, you know how a LiDAR system generally works and also, you can consider yourself fluent in the language of LiDAR. (music)
