I'm looking into the reports & the Mechanism of UAV Photogrammetry software like Pix4D and AgiSoft.
These two words are mentioned in a lot of support documentation, but they are defined no-where.
From my studies, I remember that tie-points are points marking the same physical feature in two or more images.
What does key-point mean? Is it the same?
A 14MP image will generate between 5'000 and 50'000 keypoints per image.
And this page which defines manual and automatic tie points.
A Manual Tie Point is a point without 3D coordinates that is marked by the user in the images. It can be used to asses and improve the reconstruction accuracy.
Key-Points are what is being recognized between the images. In Pix4D for example if there are dominant features in the images, it would be easier for software to recognize and tie them together, which would generally result higher key-points.
Therefore for snow, high-densed vegetation and water, you usually end up with lower key-points because it is difficult for software to tie the images with each other.
Moreover in Pix4D processing options like image scale and calibration method also affects the number of key-points. You can also force Pix4D to limit the number of key-points.
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