How does Lidar see through trees

Surveying and mapping technologies have come a long way in the last few years, of which one is LiDAR. LiDAR stands for Light Detection and Ranging and is used to locate objects and their positions based on how far it is from the light-emitting source.

As the name suggests, a light source is used to send lasers pulses to the ground, typically from an airborne vehicle. These pulses hit the objects on the ground and return to the source.

At the source, there exists a sensor that measures the time taken for the laser pulse to come back. Based on the time, the distance between the light source and the object on the ground is calculated. This is the reason the name also includes the word “Ranging”.   

The original LiDAR technology, however, has a few limitations. While they were able to map objects correctly through minor obstructions such as fog and mist, there were frequent errors when objects were hidden beneath denser obstacles, such as foliage.  

Recent advancement in the technology solves this challenge and now LiDAR can see through dense forests as well. Read on to know more about how that is made possible. Let us also look at the detailed working of LiDAR and whether seeing through the forests has any limitations. 

How Does LiDAR Work? 

As explained before, LiDAR is a setup that is carried around in an airplane. The airplane travels over the ground and the LiDAR system begins scanning the surface from side-to-side.

During the scan, the LiDAR units start sending laser pulses to the ground. As the airplane is constantly in motion, most pulses travel at an angle, although some may travel directly below the airborne vehicle.  

A LiDAR unit can send about 16,000 pulses in a second. Every second, every 1-meter pixel on the radar receives around 15 pulses. This helps is creating millions of cloud points that help in determining precise 3D locations of objects on the ground.  

As the light proceeds to the ground, it bounces off different objects present thereof and returns to the source. If there are no objects, the light will simply hit the ground.   

On-board the aircraft, a sophisticated GPS receiver is present that helps determine the geographical location of the object from which the light has bounced back.

Sophisticated computers then take into account the different point clouds, which are nothing but data points, and create a 3D spatial mapping. Thus, the LiDAR unit maps a landscape. 

Seeing Through Foliage 

Let us now dig a little deeper and understand how LiDAR can see through foliage. The challenge with dense forests is that there is a large number of inevitable branches and leaves to take care of before the light can reach the objects beyond the trees. These branches tend to return most of the laser pulses thrown on them and a lot of them even throw the light away.  

How does the LiDAR equipment work out the position of a piece of boulder beyond thick vegetation or even an animal trapped underneath multiple branches? The newer technology ensures that it can be done.  

In this process, the LiDAR unit sends a pulse to the vegetation and sends a second one before the first one reaches the ground. Then, the unit is controlled in such a way that when the first beam returns, it collides with the second beam that has just started its journey from the scanner.  

This process continues until the pulses that arrive the soonest are filtered out. The unit waits for the pulse that takes the longest time to return. This is then considered to have been reflected from the objects beyond the vegetation.  

The cloud points collected through this method is processed using advanced computer-based holography. Expert analysts evaluate this data and chalk out a 3D model of the forest and the structures beyond. 

Limitations 

Deep forest penetration using LiDAR is still quite nascent as the earlier technologies had major limitations. With the new technology, scientists hope to be able to solve these challenges. However, there are certain limitations to this method as well.   

Firstly, this process requires a special kind of laser source. This is because it is essential to measure the exact time when an outgoing beam collides with an incoming beam.

This complete coherence needs advanced detection systems on the camera as well. Also, multiple variables need to be optimized to get accurate results, and even then, accuracy may not be 100%.   

Secondly, a high level of expertise is required to read the data. LiDAR surveys can vary based on seasons because the density of vegetation varies. These variations must be accounted for when calculating the final distance of objects.   

Lastly, depending on the application, some manual surveys might also be required to verify the data. For example, if the data was being collected purely for archaeological purposes, the LiDAR system sending data about manually created structures like modern drainage is of no use. A manual survey, in such cases, might provide the desired outcome. 

Applications 

There are several reasons an exploration team may want to use LiDAR to look beyond dense canopies. Here are a few of them. 

Archaeology: For archaeologists, digging out accurate historical evidence is of utmost importance. To do that, they need to completely understand the micro-topography of the surface before it got covered in thick vegetation. LiDAR can help tremendously with that. 

Disaster Management: A lot of times, if there is a storm or an earthquake in a remote location, it becomes difficult for rescue operations teams to locate people trapped underneath large trees and branches. LiDAR can see through these obstructions and help the stranded people.  

Animal Rescue and Rehabilitation: Endangered species all over the world are often monitored using radio frequency identification (RFID) collars. Sometimes, if there has been a natural calamity in the forest, RFID signals may be hard to read. Using LiDAR, animal rescue teams can locate potentially injured animals and provide the desired rehabilitation.  

Predicting Fires: Data Science algorithms can be used to study the pattern of forest fires using cloud points collected by a LiDAR unit. This helps the forest management department to be more prepared for future fires. 

Summary 

LiDAR technology is a boon for mapping ground data and creating 3D spatial images of objects that can be hard to reach. Earlier methods of penetrating the deep forests had several challenges including not being able to penetrate the thick branches to reach the ground.   

Novel technologies are now trying to solve this issue by creating a special LiDAR sensor that measures the coherence between an incoming and an outgoing beam and discarding the earliest returning beams.   

This method, although promising, has certain limitations and needs highly-advanced equipment. However, under optimal conditions, seeing through forests easily through LiDAR provides numerous benefits and can be used in many applications, including archaeology and disaster management. 

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