LASER SCANNING

Laser scanning is also referred to as Airborne Laser Scanning (ALS) and includes technologies
such as LiDAR (Light Detection and Ranging). It is an active remote sensing technology, which
involves the measurement of distance or range between the sensor and the illuminated spot on
the terrain. In functional terms, it may be defined as a system that produces Digital Surface
Models (DSM), which is a 3D representation of a surface or terrain and all features upon it. It is
therefore a method of generating precise and directly georeferenced spatial information about the
shape and surface characteristics of the Earth. It is similar to radar but uses laser light pulses
instead of radio waves and is typically “flown” or collected from planes where it can rapidly
collect points over large areas.

The system emits intense, focused beams of light and measures the time it takes for the
reflections to be detected by the sensor. This information is used to compute ranges, or distances,
to objects. In this manner, ALS/ LiDAR is analogous to radar (radio detecting and ranging),
except that it is based on discrete pulses of laser light. The three-dimensional coordinates (e.g.,
x,y,z or latitude, longitude, and elevation) of the target objects are computed from 1) the time
difference between the laser pulse being emitted and returned, 2) the angle at which the pulse
was “fired,” and 3) the absolute location of the sensor on or above the surface of the Earth. ALS/
LiDAR instruments can rapidly measure the Earth’s surface, at sampling rates greater than 250
kilohertz (i.e., 250,000 pulses per second). The resulting product is a densely spaced network of
highly accurate georeferenced elevation points, often called a point cloud, which can be used to
generate three-dimensional representations of the Earth’s surface and its features. Many
ALS/LiDAR systems operate in the near-infrared region of the electromagnetic spectrum,
although some sensors also operate in the green band to penetrate water and detect bottom
features.

The ability to “see under trees” is a recurring goal when acquiring elevation data using remote
sensing data collected from above the Earth’s surface (e.g., airplanes or satellites). Most of the
larger scale elevation data sets have been generated using remote sensing technologies that
cannot penetrate vegetation. ALS/LiDAR is no exception; however, there are typically enough
individual “points” that, even if only a small percentage of them reach the ground through the
trees, there are usually enough to provide adequate coverage in forested areas. In effect,
ALS/LiDAR is able to see through holes in the canopy or vegetation. Dense forests or areas with
complete coverage (as in a rain forest), however, often have few “openings” and so have poor
ground representation (i.e., all the points fall on trees and mid-canopy vegetation). A rule of
thumb is that if you can look up and see the sky through the trees, then that location can be
measured with LiDAR. For this reason, collecting LiDAR in “leaf off” conditions is
advantageous for measuring ground features in heavily forested areas.

Basic Principles and Techniques

The basic idea is fairly straightforward: measure the time that it takes a laser pulse to strike an
object and return to the sensor (which itself has a known location due to direct georeferencing
systems), determine the distance using the travel time, record the laser angle, and then, from this
information, compute where the reflecting object (e.g., ground, tree, car, etc.) is located in three
dimensions.
In reality, to achieve a high level of accuracy, this process is a bit more complicated since it is
important to know, within a centimeter or so, where the plane is as it flies at 100 to 200 miles
per hour, bumping up and down, while keeping track of hundreds of thousands of light pulses
per second. Fortunately, several technologies—especially the Global Positioning System
(GPS) and precision gyroscopes—came together to make it possible.

Major advancements in Inertial Measuring Units (IMU) or Inertial Navigation Systems (INS)
have been instrumental in making the exact positioning of the plane possible. These systems are
capable of measuring movement in all directions and parlaying these measurements into a
position. They are, however, not perfect, and lose precision after a short time (e.g., 1 second). A
very highly sophisticated GPS unit, which records several types of signals from the GPS
satellites, is used to “update or reset” the INS or IMU every second or so. The GPS positions are
recorded by the plane and also at a ground station with a known position. The ground station
provides a “correction” factor to the GPS position recorded by the plane.
An ALS/LiDAR system uses a powerful laser sensor comprised of a transmitter and a receiver, a
geodetic- quality Global Positioning System (GPS) receiver, and an Inertial Navigation System
(INS) unit or Inertial Measurement Unit (IMU). The technology resembles that used by radar
sensors by which a device emits energy (focused light) and then measures the time it takes to
travel to a target and return to a collector and at the same time compensates for the movement of
the aircraft and the sensor. The laser sensor is precision-mounted to the underside of an aircraft
(helicopter or airplane) similar to the mounting of a precision aerial mapping camera. Once
airborne, the sensor emits rapid pulses of infrared (IR) laser light that are used to determine
ranges to points on the terrain below as shown in the illustration. Most LIDAR systems use a
scanning mirror to generate a swath of light pulses. The swath width depends on the mirror’s
angle of oscillation and the ground-point density depends on such factors as aircraft speed,
system capability for emitting pulses of light, and mirror oscillation rate. Ranges are determined
by computing the amount of time it takes light to leave its source, travel to the ground, and return
to the sensor. The sensing unit’s precise position and attitude, instantaneous mirror angle, and the
collected ranges are used to calculate 3-D positions of terrain points. As many as 50,000
positions, or “mass points,” can be captured every second. Although features such as buildings
and automobiles are included in the accompanying figures, these can be removed from Digital
Surface Models (DSMs) through post-processing filtering techniques. In addition, the ground can
be modeled as a “bare-earth” Digital Elevation Model (DEM). The components of an ALS/
LiDAR system are as shown in the figure below.
 Laser (usually NIR –1064nm) mounted in an aircraft
 Scanning assembly –precisely controlled rotating mirror
 Receiver for recording reflected energy “Returns”
 Aircraft location system incorporating Differential GPS and Inertial Navigation System
 A very fast computer to synchronize and control the whole system

Two distinct families of ALS systems

Waveform systems (large-footprint) - Records the COMPLETE range of the energy pulse
(intensity) reflected by surfaces in the vertical dimension; Samples transects in the horizontal
(X,Y) plane. Waveform systems designed to capture vegetation information are not widely
available Waveform systems include SHOALS, SLICER, LVIS, ICESat

Discrete-return systems (small-footprint or topographic) – SAMPLES the returned energy from

each outgoing laser pulse in the vertical plane (Z) (if the return reflection is strong enough).
Most commercial LiDAR systems are discrete return.

Multiple Returns
Multiple return systems, which are common, can capture up to five returns per pulse (diagram
above). This can increase the amount of data by 30% or more (100,000 pulses/second ≈ 130,000
returns/second) and increases the ability to look at the three-dimensional structure of the
“features above the ground surface,” such as the forest canopy and understory. This is important
in 3D mapping of urban areas and forests as well as determination of crown width in order to
facilitate decision making on appropriate time to log trees.

Advantages of Laser Scanning

 Can be flown day or night since it is an active remote sensing system
 Is independent of surface terrain or texture as is the case with stereo images which use
indirect distance measurement
 Though laser cannot penetrate cloud cover, it can be flown at lower altitudes below the
cloud ceiling hence not weather dependant
 Depending on the laser type and the flying height, the laser beam is very narrow and can
detect very small or narrow objects e.g. power lines due to the small foot print of the
beam
 Multiple return facility is advantageous for mapping of forested areas and urban areas
since one is able to classify the point cloud on return basis
 Laser cannot penetrate leaves but can pass through canopy to the ground, unless the
canopy is very dense
 The high density of ALS data means that the accuracy of elevation data used to generate
DSMs and DEMs is high, to the order of 3cm depending on the point density
Application areas
 Forest surveys
 Flood plain mapping
 Power line and pipe line mapping
 Urban area mapping
 Open pit mining monitoring