Light Detection and Ranging (LiDAR) is a remote sensing technology that uses laser light to measure distances and create detailed three-dimensional maps of surfaces. It is widely employed for mapping and surveying, providing high-resolution spatial data. While powerful for surface mapping, LiDAR primarily interacts with visible surfaces rather than deeply penetrating solid ground.
How LiDAR Interacts with Surfaces
LiDAR systems operate by emitting rapid pulses of laser light towards a target surface. They then measure the precise time it takes for these pulses to reflect and return to the sensor. This “time-of-flight” measurement, combined with the known speed of light, allows the system to calculate the distance to the object. LiDAR uses active sensors, providing their own illumination source, typically in the ultraviolet, visible, or near-infrared regions of the electromagnetic spectrum.
The light pulses emitted by LiDAR interact with the first objects they encounter, generating a “point cloud” of data that represents the 3D shape of the environment. Light does not pass through opaque solid objects. When LiDAR pulses strike solid surfaces like soil or rock, they are either reflected or absorbed at or very near the surface.
Factors Limiting Ground Penetration
LiDAR’s ability to penetrate solid ground is limited by the physical properties of light and its interaction with earth materials. Soil, rock, and most ground materials are opaque to the wavelengths of light used by LiDAR systems. Common aerial and terrestrial LiDAR systems often use near-infrared wavelengths (e.g., 1064 nm, 905 nm, or 1550 nm), which are absorbed or scattered by solid earth.
When a laser pulse encounters the ground, its energy is quickly absorbed or scattered within the first few micrometers or millimeters of the surface. This rapid attenuation prevents light from traveling deeper into the soil or rock. The varied composition of ground materials, including minerals, water content, and organic matter, further influences how effectively light is absorbed and scattered.
What LiDAR Can Penetrate and How Deep
While LiDAR cannot deeply penetrate solid ground, it excels at mapping surfaces obscured by other elements. It can map the bare earth surface beneath dense vegetation, such as forest canopies. LiDAR systems achieve this by emitting multiple laser pulses per second; some pulses find gaps in the foliage and reach the ground, providing elevation data. This allows for the creation of accurate terrain models even in heavily wooded areas where traditional aerial photography would be obstructed.
Certain LiDAR wavelengths can also penetrate water to map shallow underwater topography. Bathymetric LiDAR systems use green light (e.g., 532 nm) because this wavelength is absorbed and scattered less by water than the infrared wavelengths used for terrestrial mapping. In clear water, these systems can penetrate up to 25 meters, or approximately three times the Secchi depth (a measure of water clarity), to map shallow seafloors, lakebeds, or riverbeds. A green light pulse reflects off the water surface and the seabed, allowing for depth calculation based on the time difference between returns.
Limited penetration can also occur through fresh, dry snow or thin ice to reach the underlying surface. This penetration is shallow and depends on the snow’s density, water content, and the ice’s thickness and clarity. Water and snow tend to scatter and absorb laser beams, preventing deep penetration.
Common Applications for Subsurface Mapping
LiDAR’s ability to map terrain beneath obscuring elements like vegetation and water has numerous applications.
- Archaeology: LiDAR helps uncover ancient structures and features hidden under forest canopies, revealing settlements and infrastructure. This technology provides high-resolution data that reveals subtle ground undulations indicative of past human activity.
- Forestry and Ecology: LiDAR provides detailed information for managing forests, assessing biomass, and understanding ecological habitats. It allows for the creation of digital elevation models and provides data on tree height, canopy density, and individual tree characteristics, aiding in forest planning and fire risk assessment.
- Geology and Hydrology: LiDAR data generates detailed topographic maps for flood modeling, landslide risk assessment, and geological surveys. These elevation models help identify flood-prone areas, track changes in landforms, and monitor coastal erosion.
- Urban Planning and Infrastructure: LiDAR maps terrain for construction, utility routing, and managing urban green spaces.
- Disaster Response: LiDAR aids in disaster response by rapidly assessing damage and changes to the landscape after events like earthquakes, floods, or landslides. Its data collection provides information for emergency responders and recovery efforts, even in areas with extensive vegetation.