It is usually recommended to process images captured at the same flight height, as they have the same Ground Sampling Distance (GSD). It means that all images will have the same level of details. This facilitates the matching of keypoints between images and therefore, helps the reconstruction.
The GSD in the overlapping areas between the missions should not differ by more than a factor of two.
Considering that the flight height and the GSD have a linear relationship, for the same project, captured with the same camera, the highest flight height at which images are taken should not exceed two times the lowest flight height:
GSD1 ≤ 2 * GSD2
(Fr * Iw) / (Sw * H1 * 100) ≤ 2 * (Fr * Iw) / (Sw * H2 * 100)
H1 ≤ 2 * H2
- GSD = Ground Sampling Distance [cm/pixel].
- Sw = sensor width [mm].
- H = flight height [m].
- Fr = real focal length [mm].
- Iw = image width [pixel].
For more information on how about how to capture the images for terrain with height variations: Image acquisition plan for terrain with height variations.