Introduction to LiDAR

  • Active remote sensing with visible or near-infrared lasers to measure distances

  • Can be mounted on tripods, cars, planes, helicopters, drones, satellites, and more

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How do LiDAR systems measure distance?

Three primary methods:

  1. By measuring the time-of-flight of laser pulses.

  2. By measuring the time-of-flight of single photons.

  3. By using interferometry to count the fractional number of wavelengths between a scanner and a target.

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How is LiDAR used?

  • Terrestrial Laser Scanning (TLS)

  • Mobile Laser Scanning (MLS)

  • Airborne Laser Scanning (ALS): also called Airborne Laser Swath Mapping (ALSM)

  • Unmanned Laser Scanning (ULS)

Discrete vs. full-waveform

  • Outgoing pulses are not instantaneous — they have a finite width and height (approximated by a Gaussian)

  • Return energy is not instantaneous either (see picture)

  • The “full waveform” output (see picture) is usually simplified down to single points (discrete-return)

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Georeferencing

  • LiDAR data is usually only useful if placed in a larger project or global context.

  • “Registering” LiDAR data to a global coordinate system is called georeferencing.

  • Georeferencing static (TLS) scans is usually done via known control points, either benchmarks or GNSS survey points.

  • Georeferencing mobile or airborne (MLS/TLS) scans is done by combining LiDAR data with position and orientation information from a GNSS/IMU mounted alongside the LiDAR scanner.

How is LiDAR processed?

  • Vendor-specific software (e.g. RiSCAN PRO and RiPROCESS, from RIEGL)

  • Other commercial softwares (e.g. TerraStation, QT Modeler)

  • Mixed-source software (e.g. LASTools)

  • Open-source software (e.g. CloudCompare, PDAL, laspy)