Any air photo (vertical or oblique) can be orthorectified to remove ground and lens distortions. Unfortunately, when off-nadir (oblique) air photos are orthorectified the above ground features displayed on the oblique photo appear to elongate. The level of elongation depends on the angle of the off-nadir sensor:
- less than 20 degrees, minimal distortion (similar to vertical photos).
- 20-40 degrees varying levels of distortion.
- more than a 40 degree angle, above ground features appear very distorted.
To properly view off-nadir / oblique photos as accurate georeferenced imagery, they should be positioned based on photogrammetric methods using the photo exterior orientation rather than orthorectifying the image. As you can see in the example below,
when an oblique photo is orthorectified, above ground objects elongate as they are projected across the DEM from a side view.
Flyeye was designed to address this exact issue. The Flyeye software uses the photo exterior orientation and the DEM to “ortho” the photo in real-time. The photo is ortho positioned, but the photo is not draped over the DEM, therefore no photo elongation. Just like an orthorectified photo, every pixel on the Flyeye displayed image has an x, y, and z value.