Hi George,
I never use undistort. Undistort is a function which uses the interior parameters calculated by SFM (Structure from Motion) to create a new photograph that is free of lens distortion. It can, for instance, be used to map something when the image was taken in a vertical way and no relief was present. However, besides the lens distortions, geometric errors are also induced by the topographical relief and the tilt of the camera axis.
You have to imagine that a camera is placed at a certain location in space (in the air or on the ground) and is pointed in a certain direction. The location defines the projection centre O with three coordinates (X, Y, Z) and the direction is defined by three rotation angles roll, pitch and yaw (omega, phi, kappa). Together, these six parameters establish the so-called exterior/outer orientation. Other terms for that are camera extrinsics or simply pose. When including the principal distance (which is part of the interior orientation), the position of the image is unequivocally defined. During a vertical photography flight, omega and phi are near to zero. When they equal zero, the result is a perfect nadir/vertical photo that does not need any correction for tilt displacement. The more tilted the photographic axis with respect to the ground surface, the more corrections need to be dialled in. The projective transformation of a tilted aerial image to a horizontal plane to remove these tilt displacements (and thus scale differences) is called (planar) rectification.
However, any (even tilt-free) aerial photograph will contain displacements due to topographic relief and other height differences. Thus any feature lying below or above the horizontal reference surface will be misplaced in a planar rectification due to the central perspective of the air photo and the resulting relief displacements.
The geometric correction of a photo aims to compensate for most of these deformations. The result of such a correction must be an image with a geometric integrity like a map, i.e. an orthogonal projection to the horizontal reference plane. Just as rectification denotes the process of removing tilt from a photograph, relief displacements and other geometrical deformations (such as optical distortions) can be corrected through the process of orthorectification or differential rectification .
PhotoScan enables the production of true orthophotographs, since it computes the internal camera parameters and a digital surface model, which provides data on the terrain differences. Since also the exterior orientation of the cameras is calculated, all variables are known to compute an orthophoto. So, no need to undistort the photographs in a separate step. You can work with your original, distorted images since Photoscan bases all its calculations on them.
In a few weeks, a quite extensive book chapter should be published in which I detail all the parameters influencing an (aerial) image and how they can be taken into account using a standard photogrammetric and Structure from motion approach. As soon as I receive the final version, I can post it here if you like.
What do you mean by: "Does the problem anyhow concern with ever oblique edges of the final mosaic?".
Greetings,
Geert
Topic: Lens correction (aerial photography) (Read 23883 times)
