A topographic airborne lidar system is a laser rangefinder integrated with IMU(inertial Measurement Unit), GPS , GNSS, photogrammetric digital cameras and computer to control data recording, provides a reliable, accurate 3D geo-referenced laser point cloud which represent terrestrial landscapes of earth. Airborne Lidar Terrain Mapping (ALTM)is an accepted method of generation precise, accurate, discreet, directly geo-referenced spatial information about the earth surface. Airborne laser Terrain Mapping (ALTM) is an active remote sensing technique providing direct angle and range measurements day and night between the laser shots and the Earth's topography. Helicopter sensor platform and three fixed-wing sensor platforms provide you with options in your data acquisition.
Distance measurements are mapped into 3D point clouds. The elevation accuracy of a topographic lidar measurement is high (between 2 to 15cm), depending on the geometry of illuminated surfaces and survey height, laser precision, etc. Several backscattered echoes can be recorded for a single pulse emission. This is particularly interesting in targets with complex structure, for instance, forested areas. Since lidar systems can measure both the canopy height and the terrain elevation underneath at once, contrary to photogrammetric techniques has difficulty to see the ground in forested area. Moreover, lidar data are well known to be useful in many specific applications such as 3D city modeling, transportation right of way selection, power line survey and Digital Terrain Model generation.
The first commercially available airborne laser scanners provided only one backscattered echo per emitted pulse. The recording of a single return is sufficient if there is only one target. However, even for small laser footprints(0.2 to 2 m), there may be many objects within the travel path of the laser pulse. Individual scattering contributions are generated for each encountered object. Multi-echo or multiple pulse laser scanning systems are designed to record more than one echo. The two first echoes contain about 90% of the total reflected signal power. Real-time detection of more than five returns requires the detection of low intensity signal within noise. Fig. 1 to 4 shows a comparison between first pulse , 2nd, 3rd and last pulse in a forest stand. Compute the first return and last return to get the height of vegetation. Extract the last return will help generating bare earth model from last return of point clouds. Thus, the surface model can be computed. Bare earth model can be derived. From the figure 4 last return, you also can see there are point clouds which penetrate the vegetation and hit the ground which is impossible using photogrammetry sensors.