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Lidar



LiDAR (Light Detection and Ranging) is an active remote sensing technology that measures distances by illuminating a target with laser pulses and analyzing the time it takes for the reflected light to return. Unlike passive systems (e.g., cameras, multispectral sensors), LiDAR provides its own energy source (laser), allowing it to operate both day and night and even penetrate through vegetation canopies.

🔹 How LiDAR Works (Step-by-Step Process)

  1. Laser Pulse Emission

    • The system emits rapid, short pulses of laser light (commonly in the near-infrared wavelength, 1064 nm).

    • Some systems emit up to hundreds of thousands of pulses per second.

  2. Interaction with Target Surface

    • The laser beam strikes objects such as vegetation, buildings, or bare ground.

    • Depending on the object's structure, part of the pulse may scatter or reflect.

  3. Return Signal Detection

    • The sensor records multiple returns:

      • First Return → typically vegetation canopy tops.

      • Intermediate Returns → branches, understory.

      • Last Return → bare ground or solid surface.

    • This capability makes LiDAR extremely effective for forest structure and terrain mapping.

  4. Time of Flight Measurement

    • Distance is calculated using the formula:
      [
      D = \frac{c \cdot t}{2}
      ]
      Where:

      • (D) = distance to the object

      • (c) = speed of light (3 × 10⁸ m/s)

      • (t) = time taken for the pulse to travel to the object and back

  5. Positioning and Orientation

    • GPS (Global Positioning System) determines the precise location of the sensor platform (aircraft, drone, or ground vehicle).

    • IMU (Inertial Measurement Unit) records roll, pitch, and yaw movements of the platform, ensuring spatial accuracy.

  6. 3D Point Cloud Generation

    • Each measured distance, combined with GPS and IMU data, generates a 3D point in space.

    • Millions of such points create a point cloud, a dense dataset representing the terrain and objects.

    • Point density is often measured in points per square meter (ppsm).

🔹 Key LiDAR Products & Terminologies

  • Digital Elevation Model (DEM): Bare-earth surface derived by removing vegetation and structures.

  • Digital Surface Model (DSM): Elevation of natural and built surfaces (trees, buildings, etc.).

  • Canopy Height Model (CHM): Difference between DSM and DEM, representing vegetation height.

  • Intensity Data: Strength of the returned laser pulse, useful for classifying materials.

  • Full-Waveform LiDAR: Records the complete return signal, giving detailed vertical structure information.

  • Discrete Return LiDAR: Captures only a limited number of returns (e.g., first, last).

🔹 Applications of LiDAR

  • Topography & Terrain Mapping: Producing accurate DEMs and contour maps.

  • Forestry: Estimating tree height, canopy density, and biomass.

  • Urban Mapping: 3D models of buildings and infrastructure.

  • Hydrology: Mapping floodplains, river channels, and coastal erosion.

  • Archaeology: Revealing hidden structures beneath vegetation.

  • Disaster Management: Landslide mapping, post-earthquake damage assessment.

  • Transportation: Road and railway corridor mapping.

🔹 Advantages of LiDAR

  • High vertical accuracy (±10–15 cm in many systems).

  • Ability to penetrate vegetation to map bare-earth terrain.

  • Day-and-night operation.

  • Generates highly detailed 3D spatial data.

🔹 Limitations of LiDAR

  • High cost of equipment and data acquisition.

  • Weather sensitivity (rain, fog, or snow can scatter laser pulses).

  • Requires specialized processing software.



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