The Depth of Insight: Exploring Water with Topographic LiDAR

In the realm of geographic and environmental sciences, Light Detection and Ranging (LiDAR) technology has become a pivotal tool for mapping and understanding the physical world. Among its applications, the distinction between topographic and bathymetric LiDAR is crucial, especially when it comes to water penetration capabilities, a topic of increasing interest for researchers and professionals alike.

Understanding Topographic LiDAR

Topographic LiDAR, the more commonly used variant of the technology, primarily utilizes a near-infrared laser. This type of LiDAR is adept at mapping terrestrial environments, capturing detailed information about the earth's surface features. It operates effectively over vegetation, buildings, and other structures, making it invaluable for urban planning, forestry, and land management.

However, when it comes to water, the characteristics of near-infrared light pose limitations. Water absorbs this wavelength of light rather efficiently, which significantly restricts the penetration depth of topographic LiDAR. As a result, topographic LiDAR can provide surface-level water data, such as the extent of lakes, rivers, and floodplains, but it struggles to penetrate and map beneath the water surface. The maximum depth to which topographic LiDAR can penetrate water is minimal, often limited to shallow water bodies with clear conditions.

The Role of Frequency

The key to understanding the limitations and capabilities of LiDAR in water lies in the concept of frequency. Near-infrared light, with a wavelength typically around 1550 nm for topographic applications, is not optimal for penetrating water due to its high absorption rate. In contrast, bathymetric LiDAR systems use green light, around 532 nm, which can penetrate water more effectively. The frequency of the laser light used in bathymetric LiDAR is closer to the visible spectrum, where water absorption is significantly less, allowing it to measure depths by reaching the seafloor or riverbeds.

Scattering and Refraction: The Challenges of Light in Water

The interaction of light with water is influenced by scattering and refraction, phenomena that can affect the accuracy and reliability of LiDAR data. When a laser beam enters water, its path is altered due to refraction, the change in light's direction caused by a change in its speed. This effect is more pronounced in bathymetric LiDAR due to the transition from air to water but is a consideration for topographic LiDAR at the water's surface as well.

Scattering, on the other hand, occurs when light is forced to deviate from a straight trajectory due to interactions with particles within the water. This can dilute the strength of the signal that returns to the LiDAR sensor, impacting the clarity and detail of the data collected. Clear water conditions are thus essential for minimizing scattering and maximizing the penetration depth of bathymetric LiDAR.

Can We Rely on Topographic LiDAR for Water?

Given its limitations in water penetration, topographic LiDAR is not the tool of choice for underwater mapping. Its strength lies in mapping the land surface and shallow water bodies where depth information is not critical. For detailed underwater topography, bathymetric LiDAR, with its water-penetrating capabilities, is the appropriate technology.

However, topographic LiDAR can still play a role in hydrological studies, such as delineating floodplains, monitoring surface water extents, and contributing to erosion and sediment transport studies. Its applications in these areas are complemented by bathymetric LiDAR for a comprehensive understanding of both terrestrial and aquatic environments.

Conclusion

While topographic LiDAR offers unparalleled precision in mapping terrestrial landscapes, its capabilities in water are inherently limited by the properties of near-infrared light. For endeavors that require underwater mapping, turning to bathymetric LiDAR, with its green light penetration, provides a more effective solution. Understanding the strengths and limitations of each LiDAR type is essential for their appropriate application in the vast and varied field of geographic information science. Together, they form a powerful duo for mapping and understanding our planet's surface and beneath.

Reference :

[1] Mader, D., Richter, K., Westfeld, P. et al. Correction to: Potential of a Non-linear Full-Waveform Stacking Technique in Airborne LiDAR Bathymetry. PFG 90, 495–496 (2022).