Nadir images are better for generating a more accurate orthomosaic. The orthomosaic generation is based on orthorectification which removes the perspective distortions from the images. The orthorectification process creates an image where every pixel looks like it has been taken directly from above (nadir).
In oblique images, on one side, because the camera’s optical axis is typically pointed at an angle of 45-degrees or more, the effects of building alignment, elevation differences, and non-square pixels are all exaggerated.
On the other side, by taking images from an angle of 45° you can capture more detail on the sides of objects, which improves the quality of the 3D model.
For flat terrain, nadir images taken during a grid flight should be enough for reconstruction. One has to keep in mind that images taken from neighboring flight lines will also capture the “sides” of features, and for land reconstruction, this is sufficient for usual applications.
However, to capture complex 3D structures, it can indeed be interesting to combine both nadir and oblique flights.
Keep in mind that, when working on the images in Pix4D, first process each flight separately, then merge the projects into one (see Merging Projects).
About flight height, check out the following articles:
Regarding elevation. we have two types of output:
- digital surface model DSM: based on the densified point cloud
- digital terrain model DTM: generated by using the point cloud classification to filter out objects to keep only points recognized as terrain. Its resolution is 5*GSD of the project.