overhead photo of a dense forest

How Does LiDAR Mapping Find Ground in Dense Vegetation?

Paul TiceDrones Leave a Comment

LiDAR (Light Detection and Ranging) is a remote sensing method that uses light in the form of a pulsed laser to measure variable distances to the Earth. These light pulses—combined with other data recorded by the airborne system—generate precise, three-dimensional information about the shape of the Earth and its surface characteristics.

How can you leverage LiDAR mapping though dense vegetation and find ground? Read on to learn about Multi-return LiDAR and how it works.

Multi-Return LiDAR from an Aerial Platform

Multi-return LiDAR systems are capable of capturing multiple reflections from a single laser pulse that is emitted towards the ground. When a laser pulse is emitted, it can encounter various obstacles before reaching the ground, such as tree branches, leaves, buildings, or other structures. A multi-return LiDAR system is able to record several of these encounters:

  • First return: The first thing the laser pulse hits, which could be a tree canopy or the top of a structure.
  • Intermediate returns: May be recorded in systems capable of multiple returns, representing mid-level vegetation or features.
  • Last return: The last reflection detected by the system, often from the ground.

This capability is particularly useful in forestry, urban planning, and even mapping roads that traverse through forests; any environment where it’s important to understand both the ground surface and the vegetation or structures above it.

LiDAR from an Aerial Platform

When mounted on an aerial platform, such as a helicopter, drone, or fixed-wing aircraft, LiDAR systems can cover large areas rapidly, making them ideal for surveying and mapping landscapes, forests, cities, and more. The aerial platform moves over the target area, and the LiDAR system emits thousands of laser pulses per second. The sensor records the time it takes for each pulse to return to the sensor, which is used to calculate distances.

Imagine an aircraft flying over a forest. The LiDAR system on the aircraft sends out a laser pulse that travels to the ground. This pulse may first hit the top of a tree canopy (first return), then maybe a branch lower down (intermediate return), and finally the ground (last return).

Each of these encounters generates data points that, when combined, provide a detailed 3D map of both the vegetation structure and the ground surface.

an illustration that shows a drone using LiDAR over dense vegetation

The illustration above shows how an aerial LiDAR system operates over a forested landscape. A drone emits laser pulses that penetrate the tree canopy, reaching the ground. It captures multiple returns from these pulses, highlighting the first return from the tree tops, intermediate returns from branches, and the last return from the ground.

This process effectively maps the terrain and the vegetation structure in three-dimensional detail, demonstrating the capabilities of multi-return LiDAR from an aerial platform.

The Technology Details: In The Weeds

How does a single laser pulse get through a leaf, for example, and end up at ground with a multi return capability, as described above?

The multi-return capability of LiDAR systems is a fascinating aspect of how they collect data, allowing a single laser pulse to provide multiple pieces of information about the path it travels.

This capability stems from the physical properties of the laser pulse and the way the LiDAR system processes the reflected signals. Here’s a simplified explanation:

  1. Pulse Behavior and Interaction with Surfaces: When a laser pulse is emitted towards the ground, it can encounter various types of surfaces and materials, like leaves, branches, and the ground itself. Each of these materials has different reflectivity properties. A leaf, for instance, does not reflect 100% of the laser light. Instead, it partially reflects and partially transmits the light through it. This partial transmission is key to how a single pulse can generate multiple returns.
  2. Partial Reflection and Transmission: As the laser pulse hits a leaf, a portion of the light is reflected back towards the LiDAR sensor (recorded as the first return). The rest of the pulse continues downwards, potentially hitting more foliage or objects, causing further reflections (intermediate returns), until eventually reaching the ground, where the final portion of the pulse is reflected back (last return).
  3. Sensor Sensitivity: LiDAR sensors are designed to detect these multiple reflections from a single pulse. The sensor records the time for each return to arrive, distinguishing them based on their time of flight. Since the speed of light is constant, the distance of each reflecting surface from the sensor can be calculated. This process allows the system to map both the canopy and the ground from a single pulse.
  4. Pulse Width and Timing: The technology behind multi-return LiDAR involves sending out pulses that are broad enough to allow for parts of the pulse to be reflected by different surfaces at different times. The timing and duration of the pulse are crucial in ensuring that the system can distinguish between the multiple returns.
  5. Data Processing: Advanced signal processing algorithms interpret the returning signals, separating them into individual returns based on differences in return time and intensity. This allows for the detailed 3D representation of the area being scanned, including both the vegetation and the terrain beneath.

In essence, the multi-return capability relies on the natural behavior of light as it interacts with different materials, combined with sophisticated sensor technology and signal processing, to extract as much information as possible from each laser pulse.

To learn more about how you can implement this incredible powerful technology on your project, contact ToPa 3D for a consultation.

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