LiDAR
In the past, analytical photogrammetry was the main measurement technique for gathering topographic information of large areas with height accuracy in the decimeter or meter range. Compared with traditional methods, laser altimetry, with its increasing operationality, has been developed into a cost and time effective alternative for the acquisition of digital elevation models, for geospatial analyses and planning. Highly automated measurement techniques like LiDAR, Airborne Laser Scanning (ALS), automated image matching, and interferometric synthetic aperture radar (InSAR) had become prime technologies for the rapid collection of large-scale height data for various applications.
LiDAR is an active remote sensing technique that utilizes a narrow laser beam for a high frequent range determination to illuminated objects. Similar to radar technology, LiDAR measures a time delay between transmitting electromagnetic pulse and receiving reflected signal to calculate the distance form observed object. Compared to radar, much higher precision is achieved since it uses a signal with shorter wavelength (ultra-violet, near infra-red). Based on range measurement, the position of a distant object is determined, usually using Global Positioning System (GPS). Such sensor system permits measurements in a short time (over 200.000 measurements per second), allowing aircraft to fly over a large geographic area (to 90 km2/h) collecting a height field with dense sampling (over 24 points/m2) and high accuracy (to 3cm exactly).
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