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An Airborne Lidar Mapping (ALM) system consists of a laser light source that projects a laser beam onto a spinning or scanning mirror, thus sending a brief pulse of laser light to the earth's surface. The mirror is rotated or scanned at a very fast rate, enabling the system to project tens of thousands of laser pulses per second. This in turn creates a dense swath of laser points on the ground. Current ALM systems produce between 50,000 and 100,000 spot elevations per second. The ALM system subsequently detects the reflected laser light pulse, computes a range and circular measurement and subsequently determines the x, y and z position of each reflection point. In addition to the first return, a modern ALM system can also record as many as five additional returns from the same pulse. First returns define reflective surface models, second returns are used to characterize tree canopies and the third return produces a bare earth model. Additional returns further define and delineate man-made features or increase the likelihood of penetration through dense canopy. The intensity of the returns can also be recorded and analyzed to provide valuable information such as tree species and health or surface characteristics.
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