LiDAR is a mapping technology that uses laser light to measure the shape of Earth’s surface. From an aircraft or drone, it can scan forests, coastlines, cities, and river valleys with millions of points. This matters because many landforms and human features are hard to measure from the ground or from ordinary photos.
LiDAR helps geographers make accurate elevation maps, flood models, forest surveys, and archaeological maps.
Key Facts
- LiDAR stands for Light Detection and Ranging.
- Distance is found using d = ct/2, where c is the speed of light and t is the round-trip travel time.
- The speed of light is about c = 3.0 x 10^8 m/s.
- A point cloud is a set of millions or billions of measured x, y, z locations.
- A digital elevation model shows bare ground elevation after buildings, trees, and other surface objects are removed.
- Higher point density usually gives more detail, such as 10 points/m^2 showing smaller features than 1 point/m^2.
Vocabulary
- LiDAR
- A remote sensing method that uses laser pulses to measure distances and build detailed maps of surfaces.
- Point cloud
- A large collection of 3D points that records the positions of surfaces hit by LiDAR laser pulses.
- Digital elevation model
- A map or dataset that represents the height of the bare ground across an area.
- Return
- A detected laser reflection from a surface such as a tree canopy, roof, or ground.
- Resolution
- The level of detail in a map or dataset, often related to the spacing between measurements.
Common Mistakes to Avoid
- Thinking LiDAR is just a photograph is wrong because LiDAR records measured distances, not only colors or visual images.
- Forgetting to divide travel time by 2 is wrong because the laser pulse travels to the ground and back to the sensor.
- Assuming LiDAR always maps only the ground is wrong because pulses can reflect from tree tops, buildings, power lines, and the ground.
- Treating every point as perfectly accurate is wrong because GPS error, aircraft motion, vegetation, water, and data processing can affect measurements.
Practice Questions
- 1 A LiDAR pulse takes 0.0000020 s to travel from a drone to the ground and back. Using c = 3.0 x 10^8 m/s, how far below the drone is the ground?
- 2 A drone maps a 200 m by 150 m field with an average point density of 8 points/m^2. How many LiDAR points are collected over the field?
- 3 A forested hillside is scanned by LiDAR and by a normal aerial photograph. Explain why LiDAR may reveal old roadbeds or terraces under the trees better than the photograph.