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Laser scanning systems have been recognised as an efficient and reliable source of data acquisition in terrestrial MMSs for 3D mapping and mod- elling. An example of LiDAR data of a road section acquired using Riegl VQ-250 laser scanner in the XP-1 MMS is shown in Figure 2.2. LiDAR

Figure 2.2: XP-1’s LiDAR data of a road section.

technology provides several benefits over conventional sources of data acqui- sition in terms of accuracy, resolution, information content and automation.

The laser scanning system emits a laser pulse which is monochromatic and coherent. The transmitted laser pulse may hit one or more targets which causes one or multiple echo pulses. These echo pulses return to a receiver instrument in the laser scanning system which converts the pulses into digital signals. In Figure 2.3(a), the principle of multiple returns from targets is de- scribed [Rie09]. A laser pulse is transmitted from the laser scanning system

Figure 2.3: Principle of (a) multiple returns from targets and returned (b) echo pulses.

which strikes the tree canopy and produces echo pulses which are returned to the sensor. A fraction of the laser pulse which was not occluded by the tree canopy also strikes the building roof which leads to the return of an additional echo pulse. The returned echo pulses are depicted in more detail

in Figure 2.3(b). The first pulse refers to the transmitted laser pulse. The next n − 1 pulses correspond to reflection from the tree canopy, while the last pulse corresponds to the reflection from the building roof. The laser scan- ning system estimates the exact time position of each pulse as t1...tn for n

targets from an initial time position t0 of the transmitted laser pulse. The

estimated time is used to measure the distance or range in between the laser scanner and target objects using a simple distance-time relation described as

R = s.t/2; (2.1)

where R is the range, s is the speed of light and t is the estimated time interval. The laser scanning system is used in conjunction with navigation sensors which are used to measure the position and orientation parameters of the laser scanner in the global coordinate frame. The navigation mea- surements along with the sensor head orientation and range measurements are then used to obtain the 3D georeferenced information about the target objects.

The time interval of the reflected pulse can be determined based on either Time Of Flight (TOF) or phase shift ranging methods [PT08]. In a TOF method, a laser pulse is transmitted from the laser scanning system which strikes the target object and is reflected back to the sensor as shown in Fig- ure 2.4. A reflected pulse returns to the receiver instrument and the time interval from transmission to return of the pulse is measured. The range is determined from the measured time interval using a simple distance-time re- lation described in Equation2.1. In a phase shift method, the laser scanning system measures the phase difference between the transmitted and reflected pulse, shown in Figure2.5 [PT08]. The time interval is then determined from

Figure 2.4: TOF method

Figure 2.5: Phase shift method.

the measured phase difference as

T = λ(n + ξ/2π) (2.2)

where ξ is the phase difference, λ is the wavelength of the pulse and n is the integer number which are measured using a digital pulse counting technique. The range is estimated from the measured time interval using the distance- time relation described in Equation 2.1.

The TOF method can be used to measure distances from a few hundred metres to several hundred kilometres [PT08]. The phase shift method is only suitable for short distances as an ambiguity can arise during long distance

measurement due to the periodical variation of the phase [Pas10]. The laser scanning system uses a Continuous Wave (CW) transmission in the phase shift method for scanning terrain objects, which requires the use of high power lasers. However, the use of the phase shift method provides better accuracy than the TOF method [PJAA11]. The phase shift method can also be used to measure the direction and velocity of a moving target in addition to the range measurements [BEF+_96].

Laser scanning systems also provide intensity and pulse width informa- tion. Intensity is most commonly described as the maximum amplitude of a reflected pulse [HP07]. In Figure2.6, the intensity of reflected pulse is shown as its maximum amplitude AB. The intensity value depends upon the sur-

Figure 2.6: Intensity and pulse width of reflected pulse.

face characteristic of a target object, distance from the laser scanner to the target object and the incidence angle of laser pulse [JG09]. Intensity values can be used to differentiate terrain objects. However in most cases, they are found to be different for similar terrain objects. Intensity is required to be normalised in order to determine true reflectance values from the terrain objects. The pulse width from the laser scanning system is a recorded time difference between half maximum amplitudes of the pulse. In Figure2.6, the

pulse width is described as the recorded time difference in between points C and D which are at half maximum amplitude positions. It is measured in nanoseconds. The pulse width values vary with the surface roughness of terrain objects [LM10]. This property of the pulse width can be used to classify different terrain objects.