Drone lidar is one of the most commonly used methods of land surveying and mapping. The emergence of drone lidar has reduced the traditional surveying and mapping method, greatly saved the cost of surveying and mapping, and greatly improved the efficiency of surveying and mapping.
LiDAR, short for Laser Detection and Ranging System, is a method that uses lasers to measure the height of objects on the ground. UAV LiDAR mainly consists of four parts.
LiDAR, the device used to scan the ground line by line. LiDAR systems emit green or near-infrared light, because vegetation on the ground will reflect both types of light strongly.
GPS receiver, which is used to record the altitude and X/Y coordinates of the aircraft.
Inertial Measurement Unit (IMU), which is used to record the attitude data of the aircraft during flight, which is very important for the calculation accuracy of the height of the objects behind.
The computer is used to record various information collected by the aircraft during the flight of the ground scanning. It can be said that without a computer, there is no data.
So, these four parts are combined together to carry out the data set? Here are two key concepts of laser emission explained first:
what is a “pulse”?
The “pulse” is actually the light energy generated and emitted by the LiDAR’s laser.
what is “echo”?
“Echo” refers to the light energy that is reflected and recorded by the sensor.
Both of these concepts seem to have little to do with height.
To obtain altitude, the LiDAR system records the time it takes for a pulse to be emitted and reflected back to the sensor. Then, the system calculates the distance from the surface of the ground object to the laser emitter based on the recorded time combined with the speed of light.
The detailed calculation process is to multiply the recorded time by the speed of light and divide by 2. The reason for dividing by 2 is because the recorded time includes the two-way time to reach the ground and return, so the calculated result is the travel distance of the laser from the aircraft to the ground.
But it is not enough to just get the travel distance of the plane to the ground, you also need to know the elevation of the ground. The flight altitude of the aircraft can be obtained from the GPS receiver on the aircraft, and then the distance from the aircraft to the ground calculated just now is subtracted, and the result is the ground elevation. This is the basic principle of measuring ground elevation with a laser.
After the basic concepts are understood, two more factors need to be considered.
The first is that the plane will be turbulent in the air by the influence of air flow, and this fluctuation will be recorded by the inertial measurement unit (IMU) on the plane. This factor needs to be taken into account when calculating the height.
In addition, since the airborne LiDAR scans the ground line by line during the flight, it can cover a large area. Some of these pulses are fired perpendicular to the ground and are called NADIR. But most of the pulses are emitted obliquely, called OFF-NADIR. When calculating the height, the angle of emission of the pulse must also be taken into account.
Summary: The LiDAR system emits the laser to the ground, and then records the time for the laser pulse to be emitted from the ground to the ground, and then reflected back to the system from the ground; the distance can be calculated based on this time combined with the speed of light, and the system can then calculate the distance based on the aircraft altitude, attitude and pulse angle. Calculate the height of the surface object; at the same time obtain the space coordinates of the ground object according to the information of the GPS receiver. All this information is recorded in the computer, which is how the drone LiDAR basically works.
