LiDAR is a non-destructive remote sensing technique for the measurement of distances and provides a relatively novel tool for generating a unique and comprehensive mathematical description of the target (e.g. vegetation and infrastructure). LiDAR measures the properties of scattered light to determine the range and other information of a distant target (Wehr and Lohr 1999). The prevalent method is the use of laser pulses, with frequencies in excess of 150 kHz. The distance between the sensor and the target can be measured by either measuring the time that a laser pulse takes to travel between the sensor and the target (time-of-flight LiDAR) or measuring the phase difference between the incident and reflected laser beams (phase-shift measurement LiDAR).
Laser scanning is the use of opto-mechanical assembly to scan a certain area on the ground with laser beam (photons ) as the sensing carrier (Wehr and Lohr 1999). It is an active system in that the assembly emits the sensing photons and collects the reflected photons through a sensor. The intensity of the reflected light depends on the reflecting surface and can therefore be used in sensing the type of topography. Since the laser consists of a narrow beam of light (narrow instantaneous field of view), laser scanning involves deflecting the beam in a pre-defined pattern to cover the region of interest in the lateral direction.
The forward motion of the assembly completes the full sampling coverage of the region with high point density (see lower part of Figure 1.5). A laser scanning assembly not only detects the properties of the illuminated region on the ground but also measures the distance between that region on the ground and the assembly, a process called ranging. This is determined by time of flight measurements, that is, by measuring the time the photons are emitted to the time they arrive in the sensor. If is the distance, called the range, from the ranging unit to the object’s surface, and is the speed of light, the traveling time of the photon can be determined as,
A laser scanner works in the same way as a RADAR (RAdio Detection And Ranging) except that the ranging signal is a laser beam in the near infrared to visible wavelengths. For this reason, laser scanner is also known as LADAR (LAser Detection And Ranging) or LiDAR (Light Detection And Ranging). LiDAR data can be used in conjunction with an onboard GPS and Inertial Navigation System (INS) for precise determination of locations (Lang, et al. 2010).
When the laser was invented, it was called a solution seeking for a problem. Nowadays, lasers are ubiquitous in our technology dependent society. The advantage of using laser in ranging is its susceptibility to be produced in high energy pulses at short time intervals (see Section 1.2.1) and its highly collimated (narrow) beam (due to spatial coherence) requires a narrow aperture. Soon after the invention of the laser, its precise ranging potential was immediately realized. When pulsed lasers with high repetition rates became available, scanning laser systems were soon contemplated. The use of lasers for remote sensing was conceptualized in the 1960s (Ritchie 1996) but rapid growth of the use of commercial airborne LiDAR with fine spatial resolution was realized only in the latter half of the 1990s (Lang, et al. 2010).
The primary components of a laser scanning assembly are shown in Figure 1.5. They can be subdivided into the following:
- Laser ranging unit – consists of the emitting laser and the electro-optical receiver
- Opto-mechanical scanner – component responsible for the scanning action of the laser
- Control and processing unit
The two popular types of lasers, namely the pulsed laser and continuous-wave laser, provide two ways of laser ranging: pulsed ranging and continuous-wave ranging, respectively.