9.2.3. Working Principle

Laser pulses are emitted towards the ground.

In the working principle of Airborne Laser Scanning (ALS), the system first emits laser pulses. These pulses are sent straight downwards toward the Earth's surface. This initial action is crucial because it marks the beginning of data collection, allowing the scanner to capture information about the physical characteristics of the terrain and objects below.

Imagine playing fetch with a ball. When you throw the ball (the laser pulse), it travels straight forward towards a target (the ground). The moment it hits something and bounces back is essential to knowing how far away it landed, similar to how the laser pulse works.

Reflected signals are captured by the receiver.

After the laser pulses are emitted, they hit various surfaces—like the ground, trees, or buildings—and reflect back to the scanner. The receiver's job is to capture these reflected signals and measure the time it takes for each signal to return. This time measurement is essential because it helps determine how far away the target is based on the speed of light.

Think of it like echoing sounds in a canyon. When you shout, the sound waves travel to the canyon walls and bounce back. By listening to how long it takes for the echo to return, you can estimate how far away the walls are.

GNSS and IMU data are integrated to calculate the precise 3D coordinates.

To convert the information from the laser pulses into meaningful data, the airborne laser system uses Global Navigation Satellite System (GNSS) and Inertial Measurement Unit (IMU) data. The GNSS provides the geographic location (latitude, longitude), while the IMU tracks the scanner's orientation (pitch, roll, and yaw). By combining this data with the time-of-flight from the reflection detection, the system can accurately determine the precise three-dimensional position of each point in the scanned area.

You can think of this process like navigating a city using a map on your phone. The map (GNSS data) gives you your location, and your phone's orientation sensor (IMU) tells you which way you're facing. By knowing both where you are and which direction you’re looking, you can pinpoint the exact location of landmarks around you.

The result is a geo-referenced point cloud dataset.

Once the laser pulses have been emitted and the received signals processed with the GNSS and IMU data, the final output is a geo-referenced point cloud dataset. This dataset consists of numerous points in three-dimensional space, each representing the exact location and elevation of features detected by the laser scanner. This point cloud data can then be used for various applications, such as mapping or modeling terrains and structures.

Imagine a large-scale puzzle made of tiny 3D blocks, where each block represents a different piece of terrain or a building. As you gather all the blocks and assemble them based on their positions and heights, you create a complete picture of your environment, just as a point cloud gives a comprehensive view of the scanned area.

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