Airborne Laser Scanning (ALS) PDF

GEOMATICS FOR URBAN AND REGIONAL ANALYSIS (2024-2025) Gabriele Bitelli A preliminary basic consideration about Z coordinates Geomatics for Urban and Regional Analysis Map: 2D (East, North), 2.5D (E,N,Z)...

GEOMATICS FOR URBAN AND REGIONAL ANALYSIS (2024-2025) Gabriele Bitelli A preliminary basic consideration about Z coordinates Geomatics for Urban and Regional Analysis Map: 2D (East, North), 2.5D (E,N,Z) Real world /3D models: true 3D Gabriele Bitelli ALS: Airborne Laser Scanning Systems (LiDAR) In a map we usually see and can deduce the 2D coordinates of the points, and for some objects In the real world, and in the 3D (height points, contour points) also the height models, for each pair of North, East (one single value) above mean sea level. In this coordinates we could have even case we speak of a 2.5 D representation. more than one value of Z Definition LiDAR (LIght Detection And Ranging) refers to a remote sensing technology that emits intense, focused beams of light and measures the time it takes for the reflections to be detected by the sensor. This information is used to compute ranges, or distances, to objects. The three-dimensional coordinates (e.g., x,y,z or latitude, Digital Terrain Model longitude, and elevation) of the target objects, in the form of a Digital Elevation Model cloud of points, are computed from three data: vs 1) the time difference between the laser pulse being emitted and Digital Surface Model returned → distance (range) “laser - point on the ground” 2) the angle at which the pulse was “fired” 3) the absolute location of the sensor on the surface of the Earth LiDAR Measures Objects From “Line of Sight” The cloud of 3D points Point Cloud 3D coordinates + intensity xn, yn, zn, in Example: 2 km x 2 km area with 3/4 points per m2 will contain 9-12 million points. 3-m2 – GEOMATICS FOR URBAN AND REGIONAL ANALYSIS (2024-2025) 1 Gabriele Bitelli  GEOMATICS FOR URBAN AND REGIONAL ANALYSIS (2024-2025) Gabriele Bitelli A main use of ALS is to provide accurate 3D representation of the terrain. Widely available digital topography (digital elevation models - DEMs) are too coarse to provide representation of small geomorphic features / process. USGS 30 m DEM (each cell 30x30 m2) was the best available US coverage a few decades ago USGS 10 m DEM ALS system components LiDAR / ALS data Laser scanner, transmitter and receiver: to calculate the DEMs at resolutions not distance (range) between the previously possible. emitter and the ground point – sub-meter resolution hit by the laser beam Applicable to: GNSS receiver: to determine – Geomorphology in post-processing the position of the antenna and – Landslide & flood hazards then of the laser sensor – Forestry/Ecology (kinematic positioning, – Civil Engineering requires a GNSS master – Urban planning station on the ground) – Volcanology IMU/INS to determine the One of the hottest tools in the aircraft/laser system attitude Geosciences, widely available (three angles: Pitch, roll, Yaw) only in the last 10-15 years. Optional: RGB camera Typical flight height: 700‐1200 m > 30,000 points/second Aerial LiDAR Platforms and Application Diversity Ground samples: multiple points/sq. meter ~ 15 cm ground point position accuracy ~$300 ‐ $500 per sq. km acquisition cost GEOMATICS FOR URBAN AND REGIONAL ANALYSIS (2024-2025) 2 Gabriele Bitelli  GEOMATICS FOR URBAN AND REGIONAL ANALYSIS (2024-2025) Gabriele Bitelli History The accuracy of the sensors – 1970’s Development began (NASA) – 1980’s GPS incorporated The various sensor components fitted in the LiDAR – 1990’s Commercial LIDAR instrument possess different precision. – 2000’s Maturing of technologies: GPS positional accuracy increases For example, in a typical laser sensor the range accuracy is cm level accuracy 1-5 cm, the GNSS accuracy 2-5 cm, scan angle measuring INS orientation accuracy increases accuracy is 0.01°, INS accuracy for pitch/roll is