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The image acquisition plan depends on the:
- Type of terrain / object to be reconstructed.
- Ground Sampling Distance (GSD): The GSD required by the project specifications will define the distance (flight height) at which the images have to be taken. For example a GSD of 5 cm means that one pixel in the image represents linearly 5 cm on the ground (5*5 = 25 square centimeters).
- Overlap: The overlap depends on the type of terrain that is mapped and will determine the rate at which the images have to be taken.
A bad image acquisition plan will lead to inaccurate results or processing failure and will require to acquire images again.
All flight plans described below can be flown automatically with the flight planning app Pix4Dcapture available on Android and iOS.
For most cases it is recommended to acquire the images with a regular grid pattern. The recommended overlap is at least 75% frontal overlap (with respect to the flight direction) and at least 60% side overlap (between flying tracks).
Forest, dense vegetation areas and flat terrains with agricultural fields
For forest, dense vegetation areas and flat terrains with agricultural fields the same grid flight plan as in the General Case can be applied. However, it is recommended to increase the overlap to at least 85% frontal overlap and at least 70% side overlap and fly higher so that it is easier to detect similarities between overlapping images. If the overlap is sufficient and best practices are followed, artifacts like stripes, blotches are most likely to be absent from the output maps.
The same grid flight plan as in the General Case can be applied, but we recommend at least 90% frontal and side overlap with thermal images.
If fields are large, multiple flights may be required. For such projects, there should be overlap between the different flight plans and the conditions (sun direction, weather conditions, etc.) should be similar. This could apply to a large field that needs to be covered in several flights. If there are multiple flights, there should be an overlap of one pass between the flights.
Radiometric calibration targets
The calibration target provides an absolute reference, and hence allows the computation of absolute reflectance values. The radiometric calibration target should reflect light equally in all directions (it should be a lambertian surface).
The image of the calibration can be taken before, during or after the flight. It is important for the calibration image(s) to be taken in similar light conditions as the flight. If the images are taken during flying then the target should be large enough to be easily seen on the images. The best practices to capture image of a radiometric calibration target is described in this article.
Environmental flight conditions
The suggestion would be to fly perpendicular to sun direction to avoid the sun from a low angle (around noon). Flying close to sunrise or sunset is not advised. It is recommended to fly either in clear sky conditions or in fully overcast conditions, and for the light conditions not to vary drastically during the flight. The presence of strong wind will also adversely affect the reconstruction due to the motion of the plants.