13. Remote Sensing and LiDAR
Remote Sensing and LiDAR
This domain tests electromagnetic sensing, platforms, resolution types, image classification and LiDAR ranging. Expect distinctions between spatial, spectral, temporal and radiometric resolution, and between DEM, DSM and point clouds.
Core rules
- Passive sensors record reflected or emitted radiation; active sensors such as LiDAR supply their own energy.
- Spatial resolution is ground sample distance; spectral resolution is the number and width of bands.
- Temporal resolution is revisit frequency; radiometric resolution is bit depth.
- LiDAR range = c x dt/2, where dt is the two-way travel time.
- A DSM includes vegetation and buildings; a DEM represents bare earth; a point cloud preserves both.
Worked example
A LiDAR pulse returns after 20 ns. Range = (3.0 x 10^8 m/s x 20 x 10^-9 s)/2 = 3.0 m. If the aircraft is at 1,000 m above the datum and the range is 700 m, the reflecting surface is about 300 m above the datum, ignoring atmospheric and angular corrections. The division by two is essential because the pulse travels out and back.
Traps
Forgetting the factor of two; confusing resolution types; treating a DSM as a DEM; assuming LiDAR sees through water; ignoring classification errors in point clouds.