Questions: LiDAR Principles

Question 1

A LiDAR survey over dense forest records multiple returns from a single pulse. What do the first and last returns typically represent?

• A. First return: ground surface; Last return: top of canopy
• B. First return: top of canopy; Last return: ground surface beneath the vegetation
• C. First return: cloud base; Last return: canopy top
• D. Both returns represent the same surface at different angles

Answer: (B)

The pulse hits the canopy first (first return from uppermost foliage). Portions penetrate through gaps, reflecting off branches (intermediate returns) and eventually the ground (last return). The difference gives canopy height. This multi-return capability is what makes LiDAR uniquely powerful for forestry.

Question 2

LiDAR cannot map the ocean floor because laser light does not penetrate water.

• T. True
• F. False

Answer: False

Bathymetric LiDAR uses green wavelength lasers (~532 nm) that penetrate clear water to approximately 40-70 meters. The system fires both a near-infrared pulse (reflects off the water surface) and a green pulse (penetrates to the bottom), using the time difference to measure depth. This only works in relatively clear water.

Question 3

Explain how LiDAR bare-earth DEMs are created from point clouds that include vegetation and buildings.

Model answer: Ground classification algorithms identify the lowest points in local neighborhoods and iteratively build a surface model, rejecting points above this surface as vegetation or structures. Last-return points are more likely to represent ground. The classified ground points are then interpolated (via TIN or kriging) to create a continuous bare-earth DEM. The challenge is distinguishing low vegetation from ground, requiring adaptive filtering that accounts for terrain slope and point density.

The multi-return capability is essential: without it, separating ground from above-ground objects would require assumptions rather than direct measurements.