Dr. Réjean Simard
Lasermap Asia, Cyberjaya, Malaysia
Mr. Pierre Bélanger
GPR Consultants/Lasermap, Boisbriand, Canada
Airborne LiDAR surveys have the inherent ability to produce large-scale topographic maps and very accurate and dense 3-D data over the ground (bare-earth and vegetation as well). The accuracy of the data allows the production of maps at scales as large as 1:1000, which can be merged with colour orthophotos. The fact that laser intensity maps can be produced simultaneously with a 3-D terrain layer holds considerable promise for cartographic production activities since it considerably reduces the data integration efforts needed to merge topographic data with imagery.
LiDAR’s ability to penetrate vegetation is critical to the production of large-scale maps. In comparison with other survey methods, LiDAR is the only precise option because it makes direct physical measurements and thus does not need tree height estimates nor require vegetation removal.
The dense measurements obtained by LiDAR on the ground (as high as one point per square meter in some cases) mean that fine-resolution Digital Elevation Models (DEMs) can be derived and contour lines automatically generated with one-meter intervals. The high quality of LiDAR-derived DEMs is sufficient to enable the development of reliable and precise value-added products (such as flood or landslide risk maps) for civil and environmental engineers. This is also true for many other engineering applications related to power lines and pipeline and dam construction, just to name a few.
Finally, the fact that large-scale maps can be produced with airborne LiDAR in much shorter time periods than with conventional methods makes it an ideal tool for mapping highly dynamic environments such as urban areas. The timesaving factor associated with this technology has been demonstrated to have the greatest positive financial impact, particularly for high-cost applications such as road construction.
The presentation will highlight airborne LiDAR’s characteristics for large-scale mapping and describe the results obtained in some of Lasermap’s survey campaigns.
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 millisecond, the post processing of the data allows the construction of a detailed digital terrain model.
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.
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) owned by Lasermap Asia 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.
Fig. 1 Optech LiDAR ALTM 2050