Remote Sensing: LiDAR and UAS
What the exam tests
Remote sensing items test LiDAR returns, point density, classification, UAS photogrammetry, ground control, and accuracy. Expect a density or product-selection question.
LiDAR
- A pulse can return multiple times; first returns hit canopy, later returns can reach ground.
- Point density depends on pulse repetition frequency, flying speed, and swath width.
- Ground classification filters nonground points to make a DTM; the unclassified surface is a DSM.
- Intensity and waveform data help distinguish surface materials and structure.
UAS photogrammetry
Structure-from-motion uses overlapping images and ground control. RTK/PPK reduces but does not eliminate the need for checkpoints. Accuracy depends on camera calibration, overlap, surface texture, and control distribution. Commercial operation falls under applicable aviation rules such as Part 107 in the United States.
Worked example
A LiDAR system fires 100,000 pulses per second. The aircraft flies 100 m/s with a 500 m swath. Pulses per meter along track = 100,000 ÷ 100 = 1,000. Density = 1,000 ÷ 500 = 2 pulses per square meter, before accounting for overlap and scan pattern.
Traps
- Using a DSM for ground volumes under vegetation.
- Assuming RTK alone guarantees accuracy without checkpoints.
- Confusing point density with point spacing.
- Ignoring scan angle and overlap effects on density.
- Treating a UAS orthophoto as a survey-grade boundary.