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LiDAR: Light Detection and Ranging

20 Jun
LiDAR

LiDAR: Light Detection and Ranging

LiDAR Point Cloud

What is Light Detection and Ranging (LiDAR)?

How would you like to wave your magic wand and all of a sudden find out how far everything is away from you? No magic wands necessary. This is how LiDAR (Light Detection and Ranging) works – minus the magic wand.

LiDAR is fundamentally a distance technology. An airborne LiDAR system actively sends light energy to the ground. This light emitted is known as a pulse.

The LiDAR measures reflected light back to the sensor. This reflected light is known as a return.

So pulses of light travel to the ground. They return and are detected by the sensor giving the range (a variable distance) to the Earth. This is how LiDAR earned its name – Light Detection and Ranging

That was easy.

But let’s dissect LiDAR a little more. What are LiDAR applications in GIS? What are LiDAR outputs? What are the components in a LiDAR system?

Today, we will demystify light detection and ranging. You will go from zero, to a LiDAR hero with this LiDAR guide.

Sculpt laser-accurate outputs using LiDAR

Light detection and ranging is active remote sensing. This means the LiDAR itself sends a pulse of near infrared light and it waits for the pulse to return. This is different than passive sensors which collects reflected energy from the sun. Active sensors are very accurate because it’s being controlled in the platform.

LiDAR is a sampling tool. It has the brute force to send 160,000 pulses per second. It creates millions of points. Point density is usually less than one meter with accuracy of about 15 cm vertically and 40 cm horizontally.

Airborne Light Detection and Ranging (LiDAR)
Airborne Light Detection and Ranging (LiDAR)

A LiDAR unit scans the ground from side to side as the plane flies because this covers a larger area. Some pulses will be directly at nadir. But most pulses travel at an angle (off-nadir). So a LiDAR system accounts for angle when it calculates elevation.

Light detection and ranging is an exciting technology product with a variety of applications. What are some of the outputs from LiDAR?

1. Number of Returns

Imagine you’re hiking in a forest. You look up.

Forest LiDAR
Sunlight through Forest Canopy

If you can see light, this means that LiDAR pulses can go through too. This means that LiDAR can also hit the bare Earth or short vegetation. A significant amount of the LIDAR energy can penetrate the forest canopy just like sunlight.

But LiDAR won’t necessarily only hit the bare ground. In a forested area, it can reflect off different parts of the forest until the pulse finally hits the ground:

Using a LiDAR to get bare ground points, you’re not x-raying through vegetation. You’re really peering through the gaps in the leaves. LiDAR collects a massive number of points.

These multiple hits of the branches is the number of returns.

Number of Returns

In a forest, the laser pulse goes down. We get reflections from different parts of the forest – 1st, 2nd, 3rd returns until it finally hits the bare ground. If there’s no forest in the way, it will just hit the surface.

Sometimes a pulse of light doesn’t reflect off one thing. As with the case of trees, one light pulse could have multiple returns. LiDAR systems can record information starting from the top of the canopy through the canopy all the way to the ground. This makes LiDAR highly valuable for understanding forest structure and shape of the trees.

2. Digital Elevation Models

How do you build a Digital Elevation Model from LiDAR?

Digital Elevation Models are bare earth (topology) models of the Earth’s surface. You can derive Digital Elevation Models (or Digital Terrain Models) by using the ground hits from LiDAR. Ground hits are the last return of the LiDAR.

Digital Elevation Model

Sometimes the last return may not even make it to the bare ground. But for LiDAR, this is more rare than you think.

Which points are ground hits? There are ways to filter the LiDAR points. Take the ground hits (topology only) meaning the last returns from LiDAR.

Filter last return points. Interpolate. Build your DEM.

With a DEM, you can generate products like slope (rise or fall expressed in degrees or percent), aspect (slope direction) and hillshade (shaded relief considering illumination angle) maps.

LiDAR Digital Elevation Model
LiDAR Digital Elevation Model

3. Canopy Height Model (CHM)

Light detection and ranging attains very accurate information about the ground surface. We can also get very accurate information about what’s on top of the ground with a Digital Surface Model (DSM).

A Canopy Height Models (Normalized Digital Surface Model (nDSM)) gives you true height of topological features on the ground.

So how do you get true height of features on the Earth?

Take the first return including topology (tree, building). Subtract the last return which are the ground hits (bare Earth).

Canopy Height Model

For example:

The top of the tree height minus the ground height. Interpolate the result. You get a surface of features real height on the ground.

LiDAR Canopy Height Model
LiDAR Canopy Height Model

4. Light Intensity

The strength of LiDAR returns varies with the composition of the surface object reflecting the return. The reflective percentages are referred to as LiDAR intensity.

Light Intensity

But a number of factors affect light intensity. Range, incident angle, beam, receiver and surface composition (especially) influences light intensity. When the pulse is tilted further away, the return energy decreases.

Light intensity is particularly useful in distinguishing features in land use/cover. For example, impervious surfaces stand out in light intensity images. Object-based image classification segmentation can separate these features using light intensity values.

LiDAR Light Intensity

5. Point Classification

LiDAR data sets may already be classified by the vendor with a point classification. The codes are generated by the reflected laser pulse in a semi-automatic way.

Not all vendors add this LAS classification field. (It is usually agreed in the contract beforehand).

LiDAR Point Classification

The American Society for Photogrammetry and Remote Sensing (ASPRS) has defined a list of classification codes for LiDAR. Classes include ground, vegetation (low, medium and high), building, water, unassigned, etc.

Point classification may fall into more than one category. These points are usually flagged and have secondary classes.

LiDAR data is a rare, precious GIS resource

If you gave me a 5 second countdown to choose one GIS data type to work with for the rest of my life…. I’d probably start screaming LIDAAARRRRR!

Yes, I’d be all dramatic because you gave me a 5 second countdown to respond.

Light detection and ranging is accurate, large-scale and covers the most ground. You can understand bare ground elevation, canopy heights, light intensity and more. Anyone who is serious about understanding landscape topology should use LiDAR.

But LiDAR is a beast of a data set to work with. LiDAR is stored in LAS file format as a point cloud. This file format is maintained by ASPRS. The LAS format facilitates exchange between vendors and customers with no information being lost.

So where IS the LiDAR data? Where can you find sample LiDAR data?

Here is a list of the top 6 free LiDAR data sources for you to get a jump start on your search.

open topography webmap

Nothing is better than free.

But in most cases, you will have to purchase LiDAR data. LiDAR is generally flown commercially by helicopter, airplane and drone.

From ground to air, explore the types of LiDAR systems

1. Profiling LiDAR was the first type of Light Detection and Ranging used in the 1980s for single line features such as power lines. Profiling LiDAR sends out an individual pulse in one line. It measures height along a single transect with a fixed Nadir angle.

2. Small Footprint LiDAR is what we use today. Small-footprint LiDAR scans at about 20 degrees moving backwards and forwards (scan angle). If it goes beyond 20 degrees, the LiDAR instrument may start seeing the sides of trees instead of straight down.

Two types of LIDAR are topographic and bathymetric:
i. Topographic LIDAR maps the land typically using near-infrared light.
ii. Bathymetric LiDAR uses water-penetrating green light to measure seafloor and riverbed elevations.

3. Large Footprint LiDAR uses full waveforms and averages LiDAR returns in 20m footprints. But it’s very difficult to get terrain from large footprint LiDAR because you get a pulse return based on a larger area which could be sloping. There are generally less applications for large footprint LiDAR. Only SLICER (Scanning Lidar Imager of Canopies by Echo Recovery) and LVIS (Laser Vegetation Imaging Sensor), both built by NASA and are experimental.

4. Ground-based LiDAR sits on a tripod and scans the hemisphere. Ground-based LiDAR is good for scanning buildings. It’s used in geology, forestry, heritage preservation and construction applications.

You have x-ray vision using these LiDAR applications

LiDAR elevation points

Light detection and ranging is being used every day in surveying, forestry, urban planning and more. Here are a couple of LiDAR applications that stand out:

Riparian ecologists use LiDAR to delineate stream orders. With a LiDAR-derived DEM, tributaries become clear. It’s easier to see where they go far superior than standard aerial photography.

Foresters use LiDAR to better understand forest structure and shape of the trees because one light pulse could have multiple returns. As with the case of trees, LiDAR systems can record information starting from the top of the canopy through the canopy all the way to the ground.

If Google’s self-driving car got pulled over by the cops, how would it react? Self-driving cars use Light Detection and Ranging?. The first secret behind Google’s self-driving car is LiDAR scanner. It detects pedestrians, cyclists stop signs and other obstacles.

Archaeologists have used LiDAR to find subtle variations in elevation on the ground. It was a bit of a surprise when archaeologists found square patterns on the ground over vegetation. These square patterns were ancient buildings and pyramids by ancient Mayan and Egyptian civilizations.

LiDAR system components: breaking it down

How does a light detection and ranging system work? There are 4 parts of an airborne LiDAR. These 4 parts of a LiDAR system work together to produce highly accurate, usable results:

  • LiDAR sensors scan the ground from side to side as the plane flies. The sensor is commonly in green or near-infrared bands.
  • GPS receivers track the altitude and location of the airplane. These variables are important in attaining accurate terrain elevation values.
  • Inertial measurement units tracks the tilt of the airplane as it flies. Elevation calculations use tilt to accurately measure incident angle of the pulse.
  • Computers (Data Recorders) record all of the height information as the LiDAR scans the surface.

These LiDAR components cohesively make up a Light Detection and Ranging system.

Storage of the return: full waveform vs discrete LiDAR

Light detection and ranging return pulses are stored in two ways:

  • Full waveform
  • Discrete LiDAR

What are the differences between full waveform and discrete LiDAR systems?

Imagine that in the forest that LiDAR pulse is being hit by branches multiple times. Pulses are coming back as 1st, 2nd, 3rd returns. Then you get a large pulse by the bare ground return.

When you record the data as separate returns, this is called Discrete Return LiDAR. Discrete takes each peak and separates each return.

Discrete LiDAR

Light Detection and Ranging is moving towards a full waveform system:

When you record the WHOLE RETURN as one continuous wave, this would be called full-waveform LiDAR. Full waveform data is more complicated. You can simply count the peaks and that makes it discrete.

Full Waveform LiDAR

Conclusion

Light Detection and Ranging uses lasers to measure the elevation of features like forests, buildings and the bare earth.

It’s similar to sonar (sound waves) or radar (radio waves) because it sends a pulse and measures the time it takes to return. But LiDAR is different than sonar and radar because it uses light. This means LiDAR is an active remote sensing system.

The applications for LiDAR is stunning. It’s definitely growing in GIS.

Forest structure, archaeology, land use mapping, flood modelling, transportation planning, architecture, oil and gas exploration, public safety, automated vehicles, military and conservation. If we had a nickel for everywhere LiDAR is being integrated, we’d be Bruce Wayne rich.

We’ve broke down light detection and ranging with this LiDAR guide. You can now consider yourself a LiDAR guru.

 

Source

Gisgeography

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