What is LiDAR?

Light Detection and Ranging in more detail.

LiDAR, or 3D laser scanning, was conceived in the 1960s for submarine detection from aircraft and early models were used successfully in the early 1970's in the US, Canada and Australia. Over the past ten years there has been a proliferation in the use of LiDAR sensors in the United Kingdom, with several regularly used in both airborne and ground surveying. This has been accompanied by an increase in the awareness and understanding of LiDAR in previously unrelated industries as the application of LiDAR has been adopted.

Airborne LiDAR

Most airborne LIDAR systems are made up of the LIDAR sensor, a GPS receiver, an inertial measurement unit (IMU), an onboard computer and data storage devices.

The LIDAR system pulses a laser beam onto a mirror and projects it downward from an airborne platform, usually a fixed-wing airplane or a helicopter. The beam is scanned from side to side as the aircraft flies over the survey area, measuring between 20,000 to 150,000 points per second. When the laser beam hits an object it is reflected back to the mirror. The time interval between the pulse leaving the airborne platform and its return to the LIDAR sensor is measured. Following the LiDAR mission, the data is post-processed and the LIDAR time-interval measurements from the pulse being sent to the return pulse being received are converted to distance and corrected to the aircraft's onboard GPS receiver, IMU, and ground-based GPS stations. The GPS accurately determines the aircraft's position in terms of latitude, longitude and altitude which are also know as the x, y and z coordinates. The LiDAR sensor collects a huge amount of data and a single survey can easily generate billions of points totalling several terabytes.

An IMU is used to determine the attitude of the aircraft as the sensor is taking measurements. These are recorded in degrees to an extremely high accuracy in all three dimensions as roll, pitch and yaw - the vertical and horizontal movements of the aircraft in flight. From these two datasets the laser beam's exit geometry is calculated relative to the Earth's surface coordinates to a very high accuracy.

The initial LiDAR data can be further enhanced using additional post-processing, some of which can be automated and some are manual. Further processing utilises the multiple return signals from each laser pulse. By evaluating the time differences between the multiple return signals the post-processing system can differentiate between buildings and other structures, vegetation, and the ground surface. This process is used to remove surface features to produce bare earth models (DTM) and other enhanced data products.

It is also possible to do selective feature extraction, for example, the removal of trees and other vegetation to leave just the buildings.

Ground-based LiDAR

Ground-based LiDAR systems are very similar, only that an IMU is not required as the LiDAR is usually mounted on a tripod which the LiDAR sensor rotates 360 degress around. The pulsed laser beam is reflected from objects such as building fronts, lamp posts, vegetation, cars and even people.

The return pulses are recorded and the distance between the sensor and the object is calculated.

The data produced is in a 'point cloud' format, which is a 3-dimensional array of points, each having x, y and z positions relative to a chosen coordinate system.