"This is why stabilisation is of key importance for LiDAR data capture:
The most important criteria in a LiDAR capture project is point density. Most projects come with very clear specifications regarding the minimum required point density of aproject, e.g. a minimum 8 points per square metre that has to be respected for any e.g. 25m by 25m area tested. The point density is mainly depending on the LiDAR equipment capability, the flying height above ground and the ground speed of the aircraft. So planning a LiDAR mission has to take in account many known parameters such as:
- Scan rate (frequency) and scan pattern of the LiDAR equipment (e.g. scanlines perpendicular to the flight direction for a scanner that is using a rotating mirror, or asinusoidal or zig‐zag pattern for a scanner that uses an oscillating mirror)
- Terrain height variation (requires a digital terrain model for planning)
- Altitude restrictions and other air space restrictions imposed by air traffic controllers
Apart from the known factors, there are many "known unknowns" that have to be dealt with when planning a mission:
- Ground surface properties (some surfaces such as dark asphalt reflect less of the laser pulse energy, so one can expect a lower point density on such surfaces)
- Meteorological situation, high air humidity or surface humidity (dew) will degrade the reflected signal drastically
- Wind conditions, where tail wind can cause a higher than planned ground speed and thus a lower point density
Another unpredictable factor is turbulence. Turbulence can lead to nose‐up and nose‐down motion of the aircraft, which will drastically influence the point density of the scanned data:
- During sudden nose‐up motion of the aircraft, the scanlines (perpendicular to the flight direction) on the ground will be further apart due to the pitch rotationduring the motion. Thus, any nose‐up motion of the aircraft will temporarily reduce the point density for as long as the nose‐up motion lasts. This effect can lead todata gaps and requires re‐flying such areas.
- During sudden nose‐down motion of the aircraft, the scanlines on the ground will be closer together, so any nose‐down motion of the aircraft will temporarily increase the point density for as long as the nose‐down motion lasts. This is less of a problem for project fulfillment.
Any pitch motion during LiDAR data capture will lead to a deviation from the planned point density.
The usual way of addressing the "known unknown" factors is by adding a margin to the planned point density, e.g. planning for 10pts per square metre for a 8pts persquare metre, in order to accommodate point density variations induced by the unknown factors and then try to manage the meteorological conditions as best as possible.
The big advantage of having the gimbal is that:
- Smaller margins are required to fulfill point density requirements, which reduces flying time and project costs
- The risk for data gaps is decreased substantially, which reduces cost for re‐flying"
(Dragan Vogel, Swiss Flight Services SA)