terminology - What is the difference between Key-Point, Tie-Point in Photogrammetry?

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?

Answer

I found this page in the Pix4D documentation which states:

Keypoints are points of interest (high contrast, interesting texture) on the images that can be easily recognized. The number of keypoints depends on:

• The size of the images.


• The visual content.

A 14MP image will generate between 5'000 and 50'000 keypoints per image.

And this page which defines manual and automatic tie points.

An Automatic Tie Points is a 3D point and its corresponding 2D keypoints that were automatically detected in the images and used to compute its 3D position.

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.