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Airborne Laser Scanning – Cost effective spatial data 
First Return: Measures the distance to the first object encountered … in this case, the tree foliage Last Return: Measures the distance to the last object … in this case, the ground By acquiring first and last return data simultaneously, it is possible to measure both the tree-heights and the topography of the ground beneath in a single pass Information from the three ALS components is combined with ground base station GPS data to produce XYZ co-ordinates of the reflected points. These are then separated automatically into points reflected from the ground and those reflected from above ground features. The profile below shows ALS points reflected from power lines, nearby vegetation and the ground. 
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