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LiDAR & Photogrammetry
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LiDAR & Photogrammetry
By: Arwa Shtaya
Introduction
LiDAR (Light Detection And Ranging)
Is a remote-sensing technology that uses pulsed laser energy (light) to measure ranges (distance).
Introduction
Engineers and earth scientists use LiDAR to accurately and precisely map and measure natural and constructed features on the earth’s surface, within buildings, underground, and in shallow water.
What is LiDAR
LiDAR systems are active systems because they emit pulses of light (the laser beams) and detect the reflected light. This characteristic allows LiDAR data to be collected at night when the air is usually clearer and the sky contains less air traffic than in the daytime.
What is
LiDAR
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RADAR V.S. LiDAR
RADAR V.S.
LiDAR
The difference between LiDAR (Light Detection And Ranging) and RADAR (Radio Detection And Ranging) is their wavelength.
(LiDAR) is a light-based remote sensing technology. In the case of Yellowscan, the idea behind LiDAR is quite simple: point a small infrared laser beam at a surface and measure the time it takes for the laser to return to its source
LiDAR is a compact solution that enables a high level of accuracy for 3D mapping. At a distance of 100 meters, Yellowscan LiDAR systems have a resolution of a few centimeters.
The RADAR system works in much the same way as LiDAR, with the big difference that it uses radio waves instead of laser or LED light. It transmits radio waves from a rotating or fixed antenna and measure the time of flight of the reflected signal.
The wavelength of RADAR is between 30cm and 3mm, while LiDAR has a micrometer range wavelength (Yellowscan LiDARs work at 903 and 905 nm).
With its wavelength, the RADAR can detect objects at long distance and through fog or clouds. But its lateral resolution is limited by the size of the antenna. The resolution of standard RADAR is several meters at a distance of 100 meters.
This is why LiDAR is used for laser altimetry and contour mapping. Radar, on the other hand, is used for aircraft anti-collision systems, air traffic control or radar astronomy.
LiDAR is analogous to RADAR
• Except that it is based on discrete pulses of laser light.
• The three-dimensional coordinates (e.g., x,y,z) of the target objects are computed from:
1. the time difference between the laser pulse being emitted and returned
2. the angle at which the pulse was “fired,”
3. the absolute location of the sensor on or above the surface of the Earth
• LiDAR uses shorter waves of radar that return with more details, it can be used to create a DEM
• LiDAR is used in ground and spaces
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Components of a LiDAR system
Components of a LiDAR system
1. Laser scanner
2. High precision watch
3. GPS
4. Inertial navigation Measurement Unit
5. Data storage and management systems
6. GPS ground station
7. Scanner
1. Frequency: 50 kHz - 200 kHz pulses/sec
2. wavelength:
- Infrared: 1500 - 2000 nanometers for meteorology
- Near infrared: 1040-1060 nanometers for ground mapping
- Green and blue: 500-600 nanometers to measure depths
- Ultraviolet rays: 250 nanometers, meteorological
- Low Wattage <1W
LiDAR pulses components
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- A light pulse is emitted and the required time recording begins.
- This pulse is reflected from the inverter or the object and the time required for the return of the pulse is accurately recorded.
- The time between departure and return can be expressed as distance.
- By knowing the location, position and direction of the sensor, the X,Y,Z of the inverter or object can be calculated.
How does LiDAR work?
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Based on the Platform:
1. Ground-based LIDAR
2. Airborne LIDAR
3. Spaceborne LIDAR
LiDAR System Types
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The difference between LiDAR and Photogrammetry
LiDAR has one big advantage over photogrammetry:
it produces its own light. That means it is not affected by weather conditions, such as cloud cover and changing lighting conditions which can greatly disrupt aerial data collection for photogrammetry. Both terrestrial and aerial 3D laser scanning benefit from this, whereas photogrammetry using drones or handheld cameras does suffer in poor lighting.
LiDAR and Photogrammetry
LiDAR can penetrate the spaces between pieces of foliage and pick up small details. The laser pulse will see in-between leaves and give a measurement straight to the tree trunk or ground beneath the tree, whereas photogrammetry depends on photos, reconstructing only what is visible at the surface.
In addition, photogrammetry benefits from a range of outputs, including colorized point clouds, textured meshes, and Orthomosaics, while LiDAR only produces a point cloud.
Photogrammetry and LiDAR in action
LiDAR can be used to perform similar tasks to photogrammetry and can strengthen its outputs. The use of both techniques in combination means that LiDAR, especially terrestrial laser scanning, can add details that aerial photogrammetry data may have missed. Thus, when LiDAR and photogrammetry are combined they bring more detail to a project that may not have been achieved by either individually.
LiDAR and photogrammetry point clouds are often massive files made of billions of points. Manipulating and seamlessly navigating this data has been a challenge that PIX4D survey now solves, allowing users to handle these billions of points.
Now, LiDAR-equipped devices are becoming more widespread, making the technology avaiable to many by turning popular consumer products into devices ready for accurate 3D modelling.