Dr. Réjean Simard
Lasermap Asia Sdn. Bhd.
Suite 2M, 2nd Floor, 2330 Century Square, Jalan Usahawan
63000 Cyberjaya, Selangor D.E., Malaysia
Tel : +603 8318 0052, Fax: +603 8318 0049
Email: [email protected]
Web: www.lasermap.com
Mr. Pierre Belanger
GPR Consultants Inc. / Lasermap Image Plus
85 chemin Grande Côte
Boisbriand, Quebec, Canada, J7G 1C4
Tel : +1 450 437 2922, Fax : +1 450 437 2923
Email: [email protected]
Mr. Mohamed Razali Mohamed
Malsat Sdn. Bhd.
No. 29-3 Jalan USJ 21/1
47630 UEP Subang Jaya, Selangor D.E., Malaysia
Tel: +603 8023 3751, Fax: +603 8023 3752
Email: [email protected]
Ir. Dr. Mohd Asbi Othman
Mohd Asbi & Associates Sdn. Bhd.
B-3-15, Megan Phileo Avenue
12, Jalan Yap Kwan Seng
50450 Kuala Lumpur, Malaysia
Tel: +603 2166 8200, Fax: +603 2166 5355
Email: [email protected]
Introduction
This presentation is to provide a brief introduction to airborne LiDAR technology, show some samples of what sort of products can be developed and for which applications, and finally to review a project completed in Malaysia in the year 2002 for landslide risk assessment in rugged terrain in Sabah.
LIDAR Technology
Airborne LiDAR systems are composed of three separate technologies: a laser scanner, an Inertial Measurement Unit (IMU) and a Global Positioning System (GPS) all configured together with a computer system which ensures that all of the data collected are correlated with the same time stamp – this is extremely important as all of the components require extremely accurate timing (to the millisecond).
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.
Airborne laser profiling has been around for much longer, but the absolute position of the aircraft was always very difficult to determine without a sophisticated GPS. Continued research over the last five years has meant that LiDAR systems (especially those made by the main manufacturers) have become quite robust and very dependable. Individual units still seem to have quirks and many older units were somewhat less than reliable, and like everything else in life, you get what you pay for.
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. There are several causes for this, some of them relate to atmospheric conditions, curvature of the earth, geoids and spheroid separations (the spheroid for the map projection is not based on the same centre of the earth as the satellite orbit) etc. 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 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 procedure for a LiDAR survey is to fly an aircraft (or helicopter) over an area and to operate laser scans from side to side. The receiver picks up the laser pulses reflection value of the target and records the time it takes from emission to when it is received back at the receiver. If this time is divided by two and multiplied by the speed of light, then that is the distance from the aircraft to the ground. The inertial system keeps track of the rotations of the aircraft in the three axes (along the line of flight around the wings and crab) and the GPS keeps track of the actual location of the aircraft in space. The inertial system also keeps track of location using accelerometers, but inertial systems are notorious for gradually losing sense of position, so the data are actually updated every 0.5second using the GPS.
The direct result of a LiDFAR 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 newer LiDAR models like the Optech 2050 (see Figure 1 below) owned by LaserMap which also produce an infrared laser intensity map.
Figure 1. Latest OPTECH 2050 LiDAR operated by LaserMap
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 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. While the beams of topographic LiDARs are in the infrared end of the spectrum, and cannot be seen, the infrared energy tends to be reflected similarly to visual light; that is more energy is reflected from lighter colour objects and less from darker colour objects. So the image is similar to an infrared black and white 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. If one flies higher, the same numbers of LiDAR pulses are recorded from a much wider swath meaning that the data points are spaced further apart. Secondly if the angle is increased then at the extremities of the LiDAR beam it starts to hit tree trunks or the sides of objects rather than the ground
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 we 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 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.
LIDAR Applications
LiDAR can be applied in a multitude of applications requiring large scale mapping with most of them related to infrastructure development or maintenance. The following table presents the main applications areas.
Coastal erosion analysisFlood risk mappingForestry applicationsGeo-reference location structuresGIS and high tech aerial surveysGolf & resort planningHydropower projects Large-scale civil engineering projectsLandslide risk mapping Defence applicationsMovie animation productionOpen pit miningPipelines constructionRoadway corridor planningTelecommunicationsTopographic surveysTransmission lines
Figure 2 shows one sample type of data collected by LaserMap over the years – an isometric view of a terrain model for a downtown in Bun Dang City in Korea. These data are collected over a dense urban area and can be used, for example, for radio frequency coverage analysis for optimal telecommunication antenna location; or by security forces for disaster preparedness and mitigation.
Figure 2. Isometric view of LiDAR DTM over Bun Dang City, Korea
© 2002, LaserMap Image Plus. All rights reserved
Over the recent years, flood risk mapping has been a significant part of LaserMap’s work in Canada. For example, on the Canadian USA border in Manitoba, in central Canada, a large area was flooded in the Red River flood plain. The area shown below in Figure 3 is about eight kilometres wide. This area was subject to wide ranging floods after a winter of heavy snowfall followed by lots of spring rain in 1977. Using LiDAR the engineers were able to see where the flood lines were located and then try to take various measures to reduce risk in the future.
Figure 3. Color coded LiDAR heights by 1m interval over Red River plain
© 2002, LaserMap Image Plus. All rights reserved
LIDAR Survey in Sabah, Malaysia
Closer to south-east Asia, here in Malaysia, LaserMap was involved in several LiDAR surveys for Public Work Malaysia (JKR). The following example was completed in Sabah during the first months of the year 2002.
The mountains in Sabah are steep and covered in tropical vegetation. Indeed, this area in the northern part of the island of Borneo, a few degrees north of the equator, has the highest mountains in south-east Asia, reaching 4,175 meters.
The two main cities of Sabah are Kota Kinabalu and Sandakan, which are joined by a 300 kilometres long road running through the mountains and forming the economic lifeline between Sabah’s two main manufacturing and trading hubs. A significant problem with the road, however, is much of the mountainous area through which it passes is covered with a layer of soil over the bedrock. In the heavy rainy season it is not unusual for landslides to occur which block the road, sometimes for several weeks or longer. When this happens the only way to ship goods between the two main centers is by air, which is expensive, or by sea, which is not only slow but also dangerous – as in this area shipping is often subject to attacks by pirates. In reality, neither of these methods is economic or practical for everyday commerce. Mitigating the possibilities of landslides is a prime concern for the government of Malaysia, not only for maintaining the road and keeping it open year round, but also to reduce the risk of injury and loss for those using the road during the rainy season.
JKR, therefore, issued a mandate to the local civil engineering company ZNA, together with MALSAT for the mapping portion of the work, for a study to determine where a relocation of the roadway would reduce the possibilities of blockages by future slides. In order to do this effectively a good terrain model of the ground was needed.
MALSAT contracted a local supplier of aerial imagery and LaserMap Image Plus of Canada to be partners in providing the raw data for the study. Because of the local cloud cover for much of the time, optical imagery was difficult to obtain. In addition, in areas of dense tropical vegetation the aerial photography shows mainly the tops of trees, hence very little of the ground can be seen. This is where the Canadian company, LaserMap, which operates airborne LiDAR systems was able to provide the needed measurements to produce real ground information.
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 and an inertial measurement unit, which are integral parts of the equipment, allow for continuous monitoring of the position of the aircraft and its attitude (how much off vertical the aircraft and LiDAR system is). 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.
LaserMap, which has now opened a subsidiary, LaserMap Asia, in Kuala Lumpur, used an Optech LiDAR system for the survey. Optech Inc. is the largest supplier of LiDAR systems in the world and also happens to be a Canadian company.
Currently airborne LiDAR is the only technology which allows measurements to be taken directly to points on the ground with precision and efficiency from an aircraft. The speedy survey made possible by being mounted in an aircraft means the work can be done in days rather than months for a traditional ground survey. Indeed, in areas of steep sided hills and heavy vegetation it is, in reality, the only survey technology which works with an accuracy suitable for engineering.
In this instance, it was possible to fly the LiDAR aircraft at approximately 750 meters above the ground and below the persistent cloud cover. This is several hundred meters higher than required for the infrared laser to be used safely. Class 4 lasers, which are those most often used in this type of equipment, while invisible to the human eye, are certainly still a danger to vision at close range, but the Optech systems are eye safe when flown more than 300 meters above the ground. It would, incidentally, take repeated, direct shots in the eye to cause damage, and because the aircraft is moving at approximately 200 kilometers an hour, this is impossible. But there is anyway an automatic safety shut-off if the laser beam contacts an object less than the safe distance away.
The laser is directed by a mirror which swings rapidly from side to side to provide a swath of ground points, which for this survey was 530 meters wide. The model used was an Optech 1020 with a laser firing rate of 5,000 measurements a second.
By adjusting the forward speed of the aircraft and overlapping lines of data capture to form a block of data, the resulting terrain model produced data points that were spaced approximately one meter apart. While not every single laser shot penetrates to the ground because it hits some impenetrable vegetation, the survey provided a very detailed ground model.
To ensure the airborne survey is on the proper local datum, a ground based survey-grade, global positioning system is located on ground control points for the duration of the airborne survey and records the same satellite data as the aircraft’s GPS. This allows a three dimensional transformation of the LiDAR survey data into the ground coordinates used by JKR. The LiDAR unit records all data in the World Geodetic System based on the GPS satellites, and as with any GPS survey this needs to be transformed by special software into the local survey system.
The actual survey took an elapsed time of one month including mobilizing the crews, setting up the base GPS stations, planning and coordinating flight plans. The processing of the LiDAR data was completed in less than two months. The final result was a block of digital terrain data accurate to 30 cm for the length of the road (a portion of the result is shown in Figure 4 below). This allowed the engineering company to assess the degree of slopes, the hydrology, and the high-risk areas so that the re-engineering of the road will avoid the most likely landslide areas and provide a safe and continuously open road throughout the year.
Figure 4. Isometric view of LiDAR DTM over Sabah, Malaysia © 2002, LaserMap Image Plus. All rights reserved
Public Work Malaysia assigned a foreign independent surveying firm to check the conducted LiDAR survey with cross sections in several places and found that the results exceeded the specifications for the survey more than 99% of the time.
As mentioned, this project was actually the first major LiDAR projects in Malaysia completed by LaserMap. Future projects contracted by LaserMap Asia will have the advantage of using a new Optech 2050 LiDAR system, acquired in 2003, which provides ten times as much data compared with the previous system. The added capability will provide even more uses for airborne LiDAR in the future especially in areas where traditional aerial photography or satellite imagery is hard to obtain.
Conclusion
Airborne LIDAR technology is now a proven method for acquiring accurate digital terrain model data and associated imagery under a wide range of conditions. As an active sensor it can be used when other remote sensing tools will not work. But LIDAR technology is quite new and is difficult to master. Few companies around the world own the necessary equipment and even fewer offer data acquisition services that actually meet the needs of their clients.
We believe that access to a technology that enables the acquisition and fusion of baseline cartographic data and digital photos in much shorter time periods when compared to conventional methods can speed up the initiation of projects related to road construction and safety, mineral exploration, natural resources planning and exploitation, construction of infrastructure, environmental impact assessments and so on.
This new source of data bundled with advanced visualization software is an essential ingredient for any major development project that needs high-resolution and high-precision cartographic information, be it road or railway construction or improvement, telecommunication transmission lines, power lines, high-precision digital topography (Digital Terrain Models) for slope analysis in landslide risk mapping or for flood risk mapping, interactive fly-through simulations for land developers, and so on.
References
- Fowler, Robert. Topographic Lidar, Chapter 7, Digital Elevation Model Technologies and Applications, Editor Dr. David Maune, published by The American Society of Photogrammetry and Remote Sensing, 2001
- Hartman, Jim. P.Eng. LiDAR Surveying: A Tool for Flood Risk Mapping and Engineering, Earth Observation Magazine June 2003 pp 32-36.
- Fowler, Robert. Lidar Processing: It’s Not Black Magic, Earth Observation Magazine, October 2001 pp 33-35.
