Introduction to LiDAR

LiDAR stands for Light Detection and Ranging. LiDAR uses light in the form of a pulsed laser to measure variable distances to the Earth. These light pulses—combined with other data recorded by the airborne system—generate precise, three-dimensional information about the shape of the Earth and its surface characteristics. But when it comes to water, does LiDAR hold the same prowess? 

The Basics of LiDAR

LiDAR operates by emitting laser pulses towards the ground surface, which reflect back to the sensor. The reflections are then measured by the amount of time the each laser return took to reach the sensor. These return times are then calculated into distances offering very accurate distances to allow the creation of a point cloud. LiDAR technology is particularly useful when used on thin structures or to penetrate vegetation for land surveying and mapping. When used in combination with photogrammetry, the surveyor can capture high accuracy ground points below vegetation, thin structures such as power lines, as well as combine image textures from photogrammetry for highly detailed point clouds and digital twins.

LiDAR's challenge with Water

Water poses a challenge for LiDAR technology When LiDAR’s laser pulses encounter water, they are significantly absorbed and scattered by the water molecules.

Penetration Depth

The degree to which LiDAR can penetrate water depends on several factors, including the wavelength of the laser light, the clarity of the water, and the angle of incidence. Generally, shorter wavelengths (such as those in the blue-green spectrum) can penetrate water better than the longer wavelengths (such as infrared) that are commonly used in LiDAR systems for terrestrial mapping.

In clear conditions and with optimal wavelengths, LiDAR can penetrate the water surface to some extent, allowing for the mapping of shallow underwater features. However, this penetration is limited to relatively shallow depths—often no more than a few meters in clear water and even less in turbid conditions.

Alternatives for Underwater Mapping

For detailed underwater mapping, alternative technologies are often used in conjunction with or instead of LiDAR. Bathymetric LiDAR is one such technology that is intended to map the seafloor by penetrating the water's surface.. Bathymetric LiDAR systems use green light, which has a better penetration capability in water compared to the infrared light used in traditional LiDAR. Another alternative is sonar (Sound Navigation and Ranging), which uses sound waves instead of light to map underwater topographies.

Conclusion

In conclusion, the typical LiDAR sensors we use on small drones and UAV's for land surveying and mapping are not designed for underwater mapping, due to the higher wavelength being a limitation to penetrate water.

However, Bathymetric LiDAR Sensors are for drones are designed specifically for mapping water and shorelines, combining both topographic and bathymetric technologies.