LiDAR Mapping Through Vegetation
January 11, 2023
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The purpose of LiDAR
LiDAR, or light detection and ranging, is a tool used to quickly, accurately and densely measure 3D features from the world around us.
LiDAR is a form of “remote sensing” - meaning we aren’t physically measuring things with our hands, but rather, using sensors. To measure topography, vegetation or other data across large areas, we need remote sensing methods that can take many measurements quickly, easily, accurately. LiDAR sensors can take hundreds of thousands of measurements each second and are represented in the form of a point cloud.
The point cloud can be used in various analytical processes to take distance measurements, compute volumes, and model objects in three dimensions.
So how does LiDAR work?
1.) LiDAR is an ACTIVE remote sensing system meaning that the lidar system itself actively generates laser pulses used to take the measurements.
2.) These laser pulses are pulses of light that travel from the sensor to the ground and reflect off of things like buildings and tree branches.
3.) The reflected signal then returns to the LiDAR sensor where it is recorded and stored as distance.
A LiDAR system can be used for a variety of applications
General Land Survey
Corridor Mapping
Utilities/Powerline Mapping
Archeology
Open Pit Mining
Development Planning
Education Research
Hydro/Bathy Mapping
Land Erosion
Precision Agriculture
How does LiDAR measure distance?
In addition to emitting a laser pulse, a LiDAR system measures the time between when each pulse is sent from the system to the time it is reflected back to the sensor.
Since the speed of light is known, we can calculate distance using a simple mathematical formula (distance = speed x time). With the help of positioning and orientation sensors within the lidar system, distance is then represented as an elevation.
Position Measurements are typically collected once every second, orientation measurements are typically collected between 125 to 400 times per second. These positioning and orientation sensors are critical components and are integral for every aerial lidar system to precisely calculate where and when each point is collected.
In summary, a LiDAR system is an active remote sensing system, composed of components that emit the laser, receive a returned signal, and calculate the system's position and orientation, to produce a geospatially accurate depiction of the world around us in the form of a 3D point cloud.
Specifically, a GPS (or GNSS) identifies the X,Y,Z location of the lidar system and its position in the physical world, and an Inertial Measurement Unit (IMU) provides the orientation (roll, pitch, and heading) of the lidar system as it collects data.
How Can LiDAR Measure the Ground Through Trees?
A laser pulse emits light energy as a bundle of photons. As these photons move towards the ground, they hit objects, such as buildings, trees, and ground vegetation. In the case of trees and vegetation, some of the photons reflect off the branch and return immediately to the sensor.
However, some of the photons pass through gaps in the tree or vegetation, striking other features, like lower branches, or even the ground, before returning to the lidar sensor.
In this way, multiple reflections may be recorded from one pulse of light. In the lidar industry, this is known as multiple returns.
Some LiDAR Systems at Phoenix LiDAR Systems return two returns, some three, some four, and some even return up to 15 returns to the sensor.
This depends on the power of the laser used in the system and a few other factors. The result is a point cloud with a true 3D representation of both the vegetation and ground data.
