LIDAR Surveys for Road Design in Thailand
INTRODUCTION TO LIDAR TECHNOLOGY
The relatively new LiDAR technology uses a rapidly firing laser installed in an aircraft for
measuring points on the ground. The strength of the laser and its narrow, infrared beam allow it
to penetrate between much of the leaves and branches and often receive a return reflection from
the ground. A Global Positioning System (GPS) and an Inertial Measurement Unit (IMU),
which are integral parts of the equipment, allow for continuous monitoring of the position of the
aircraft and its attitude. Using all of this equipment together with timing coordinated to the
milli-second, the post processing of the data allows the construction of a detailed digital terrain
model.
The components for LiDAR technology have been around for many years. Lasers were
invented in 1958. Inertial technology has been around for a long time and GPS has been around
commercially for over 15 years. The challenges faced by user of LiDAR technology is putting
all of these technology-components together and making them work together – at the same time
ensuring that it is small enough for use in an aircraft or helicopter. In reality this has only been
achieved commercially in the last eight years or so.
The major limiting factor for the technology has always been (and still is) the airborne GPS and
it is only in the last ten years that GPS systems have become accurate enough to provide
airborne positions better than 10cm.
With respect to the use of a GPS on board the aircraft, it is necessary to provide a link to a
ground GPS station on a known control point. The ground station should be located on or
close to the project site - where the aircraft is flying. This is to ensure that the aircraft record the
same satellites signal as the ground station. If the ground station is located further away from
the aircraft or project site then it is quite possible some of the satellites recorded by the ground
station will be different from those recorded by the aircraft GPS. There are a number of other
reasons also: absolute accuracy tends to diminish the further away the aircraft is from the
ground station.
Just to clarify a point: the accuracy of the system does not change when it is further away from
the ground station; it produces data within the same relative accuracy parameters all of the time
– but the accuracy in relation to the absolute ground position changes when the airplane get
further away from a ground station.
The direct result of a LiDAR survey is actually a
set of points which consist of easting northing
elevation obtained at the rate of 3 million points
per minute (meaning a spatial density as small as
1m apart) as in the case of LiDAR models like
the Optech 2050 (see Figure 1 shown left) owned
by LaserMap which also produce infrared laser
intensity maps.
The point data are then post processed and
classified into three main classes of points. The
last return ground, the first return tops of
vegetation or buildings or structures.
Ground bare earth points can be used as a Digital
Terrain Model (DTM) or converted to contours or, as we will see later, a relief model. The
vegetation can be used to determine the heights of trees and using specific software calculate
biomass or even expected lumber that could be cut in any specific stand.
In addition, the intensity feature allows the brightness of the reflected return to be recorded as a
value between 0 and 255. This can then be rendered to produce an image of what is on the
ground which is similar to an infrared photo. While this is not close to photographic colour
quality it does allow interpretation of what is on the ground.
LiDAR missions are planned very similarly to aerial photo missions. However, the LiDAR
aircraft is usually flying much lower (between 1000-3000 metres) and lines are spaced closer
together as the beam width is relatively narrow. LiDAR data can (and often are) used together
with standard air photo or a more advanced CCD camera to produce digitally rectified images or
othophotos. The DTM is used to rectify the image taking out the distortions caused by relief.
This saves time and money compared to collecting a terrain model by photogrammetry.
However, it should be noted that it is rare that a LiDAR system and a precision aerial camera are
flown at the same time, as the swath width covered by the camera is not the same as that
covered by the LiDAR. But Lasermap GPR can fly LiDAR and CCD camera at the same time
on the same platform.
The following is a list of the main advantages of using LiDAR as a survey technology:
- The data are all collected numerically.
- The laser is an active sensor so it does not require specific sunlight conditions or even
daylight; it can be flown under the clouds so well suited to tropical environment,
- It is an aerial survey, so data are collected quickly and accurately and do not need field
intervention.
- The automated processing helps speed data analysis
- The high precision of the data allows its use for planning and detailed engineering
- It provides data in areas difficult to access or where it is environmentally sensitive
- And because the data are generic by nature (digital) they can be used in many different
software packages and used to generate different views.
