The difference between an aerial image and a orthophoto

When using aeronautical data, it is necessary to distinguish between individual methods of data processing.

When using aeronautical data, it is necessary to distinguish between individual methods of data processing. This has a major impact on their accuracy. The most common output is orthophoto… or not?

Aerial shot
It does not correct distortion (visible facades of houses). In the case of a continuous display, only a simple transformation is performed using the edges of the images so that they follow each other.

Example: raw data from airplanes and drones, historical images
False orthophoto
It is created by correcting images for distortion caused by the tilt of the image relative to the terrain and is performed only for points at the terrain level. A terrain model is usually used as a basis for correction.

Example: Orthophoto ČÚZK
True orthophoto
Image converted to a rectangular projection (points on the terrain and other objects) A surface model is used for this method of correction. A variant that is more difficult to process, but more reliable in terms of measuring distances between objects.

Example: G3D data

True orthophoto
False orthophoto

Measurement and vectorization
During measurements and vectorization from aerial images, serious errors can occur due to image distortion, see figures.
In the true orthophoto, the heel of object A and its top A ‘ are in the same position.
In the false orthophoto (or aerial image), there was a significant difference in the position of the image between the heel (A) and the top of the object (A ‘).

Conclusion on vectorization:
• From the right orthophoto it is possible to vectorize in all levels
• Objects can be vectorized from false orthophoto only at ground level
• It is not advisable to vectorize from an aerial image at all

When will you meet?
For high-resolution images, a true orthophoto (typically drone data) is usually used. For medium and lower resolution images, a false orthophoto (aircraft, satellites) is used. This is due to the requirements for accuracy and then also the size of the perspective distortion.
Close-ups will be very distorted and need to be corrected. Otherwise, they are almost impossible to combine into a false orthophoto.
Images from a greater distance / height have less perspective distortion and are therefore less distracting and "just" correct the tilt errors of the image.

Author: Ing. Ondřej Kočí

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