the survey GPS data during post-processing, one can expect that the position is
accurate within 0.05 m in all directions (Katzenbeisser 2003). Such advances in
the use of GPS, alongside the fine precision afforded by IMU units, have served to
decrease the cost and increase the accuracy of LiDAR as a survey technology.
LiDAR sensors can be categorized into three types: (1) profiling, (2) discrete
return, and (3) waveform. Profiling sensors are the simplest and the sensor records
only one return at fairly course sample densities along a narrow swath. Discrete
return instruments are more advanced and employ a type of laser transceiver that
records multiple returns (typically *5) for each pulse of light sent towards the
target, while waveform instruments record a digitized profile of the full return
pulse. The addition of a scanning component (which may be zig-zag, parallel,
elliptical, etc.) greatly increases the ground covered beneath each flight line and is
typical for more modern systems. The mode of delivery of the laser light can also
be used to characterize the technology. Most commonly used for ranging applications, ‘pulsed’ LiDAR consists of a pulsed laser transmitter, an optical telescope
receiver that amplifies the backscatter, and photomultiplier receiver tube to convert
optical energy into electrical impulses. Range to object is determined by recording
the time taken by the transmitted pulse to the target and back. By contrast,
‘continuous wave’ LiDAR ranges by modulating the intensity of the laser light.
Here, travel time is directly proportional to the phase difference between the
received and transmitted sinusoidal laser signal. It is important to appreciate that
while advanced scanning LiDAR can yield data which, if processed, can be viewed
as an image; more simple profilers only deliver widely spaced range postings along
a narrow corridor. In order that LiDAR data not be misinterpreted, it is paramount
that the user be fully informed as to the specifics of the instrument used to acquire
the soundings and the associated caveats with the dataset.
5.2 Physical Principles
5.2.1 Aircraft-Deployed LiDAR
Bathymetric LiDAR instruments are typically aircraft-mounted, operate on the
transmission of green laser light (typically 532 nm), and record the intensity of
back-scattered energy. In addition to a green laser, most bathymetric LiDAR
systems also operate a separate near-infrared laser pulse. This is used to find the
range to the water surface, but some designs employ the same green laser pulse to
range to the sea surface as well as the sea bottom. Only systems with short laser
pulses can follow this strategy without impacting the accuracy and precision of the
data. Common to all of these designs is that the green laser pulse propagates
through the water column and reflects off the sea bottom. The water depth is
calculated from the time-lapse between the water surface return and the bottom
return (Fig. 5.1). Dividing the travel-time of a reflected laser pulse by two, and
5 LiDAR Overview
117
accurate within 0.05 m in all directions (Katzenbeisser 2003). Such advances in
the use of GPS, alongside the fine precision afforded by IMU units, have served to
decrease the cost and increase the accuracy of LiDAR as a survey technology.
LiDAR sensors can be categorized into three types: (1) profiling, (2) discrete
return, and (3) waveform. Profiling sensors are the simplest and the sensor records
only one return at fairly course sample densities along a narrow swath. Discrete
return instruments are more advanced and employ a type of laser transceiver that
records multiple returns (typically *5) for each pulse of light sent towards the
target, while waveform instruments record a digitized profile of the full return
pulse. The addition of a scanning component (which may be zig-zag, parallel,
elliptical, etc.) greatly increases the ground covered beneath each flight line and is
typical for more modern systems. The mode of delivery of the laser light can also
be used to characterize the technology. Most commonly used for ranging applications, ‘pulsed’ LiDAR consists of a pulsed laser transmitter, an optical telescope
receiver that amplifies the backscatter, and photomultiplier receiver tube to convert
optical energy into electrical impulses. Range to object is determined by recording
the time taken by the transmitted pulse to the target and back. By contrast,
‘continuous wave’ LiDAR ranges by modulating the intensity of the laser light.
Here, travel time is directly proportional to the phase difference between the
received and transmitted sinusoidal laser signal. It is important to appreciate that
while advanced scanning LiDAR can yield data which, if processed, can be viewed
as an image; more simple profilers only deliver widely spaced range postings along
a narrow corridor. In order that LiDAR data not be misinterpreted, it is paramount
that the user be fully informed as to the specifics of the instrument used to acquire
the soundings and the associated caveats with the dataset.
5.2 Physical Principles
5.2.1 Aircraft-Deployed LiDAR
Bathymetric LiDAR instruments are typically aircraft-mounted, operate on the
transmission of green laser light (typically 532 nm), and record the intensity of
back-scattered energy. In addition to a green laser, most bathymetric LiDAR
systems also operate a separate near-infrared laser pulse. This is used to find the
range to the water surface, but some designs employ the same green laser pulse to
range to the sea surface as well as the sea bottom. Only systems with short laser
pulses can follow this strategy without impacting the accuracy and precision of the
data. Common to all of these designs is that the green laser pulse propagates
through the water column and reflects off the sea bottom. The water depth is
calculated from the time-lapse between the water surface return and the bottom
return (Fig. 5.1). Dividing the travel-time of a reflected laser pulse by two, and
5 LiDAR Overview
117