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  • 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)

So LiDAR or light detection and ranging uses lasers to measure the

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B1 US system ground pulse plane elevation height

How Does LiDAR Remote Sensing Work? Light Detection and Ranging

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    joey joey posted on 2021/05/24
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