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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). The camera should be maintained as much possible at a constant height over the terrain / object to ensure the desired GSD.
The overlap and flight height have to be adapted depending on the terrain.
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.
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.
For projects with multiple flights 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.
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.
If the Airinov Aircalib target is used the images of the calibration target can be added to the project along with other images and it will be automatically recognized as a calibration image. It is also possible to manually introduce the calibration image information.
To manually introduce the calibration target images:
- In the Menu bar, click Process > Processing Options. The Processing Options pop-up appears.
- Click 3. DSM, Orthomosaic and Index.
- Under Radiometric Processing and Calibration click the Calibrate button.
- The Radiometric Calibration pop-up appears.
- Import the calibration target image.
- Draw a region on the image over the lambertian area of the target.
- Set the Albedo values provided by the manufacturer.
- Click OK.
Environmental Flight Conditions
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.