When planning a project over a terrain with height variations, there are 3 image acquisition plans possibilities:
- Single flight following the terrain elevation variation (recommended)
- Multiple flights at different constant altitudes
- Single flight at a constant altitude regardless of the terrain changes in altitude (not recommended)
It is the recommended method since the GSD will be constant for the project since the height difference between the UAV and the terrain is maintained.
|Figure 1. Single flight following the terrain height variations.|
Perform multiple flights at different altitudes with the flight lines perpendicular to the slope. It is important that the there is overlap between the flights.
The GSD will be aproximately constant for the project since the height difference between the UAV and the terrain is maintained.
|Figure 2. Multiple flights at different altitude.|
If the height difference between the UAV and the terrain is not maintained (keeping constant flight altitude):
- It will affect the GSD: The GSD will not be the same over the whole project a cause of the variations in elevation of the terrain. In Figure 3, the obtained GSD in A has less resolution than in B.
- It will affect the overlap: The overlap will be lower in the higher point B, if the image rate is the maintained.
- It may affect the reconstruction or even fail, if the height difference between the higher and lower point is twice or higher (if the highest GSD exceed twice the lowest GSD)
|Figure 3. Constant flight altitude.|