|
Airborne LIDAR Surveys - An Economic Technology for Terrain Data Acquisition
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. |