Reprojection Error: Gradual Selection vs. Chunk Info

[Felix_]

Sorry to dig up this old question again. I thought I had understood the reprojection error, but when trying to remove tie points with bad reprojection errors I saw that the RMS reprojection error from the chunk info was very different from the biggest reprojection error shown under gradual selection.

I understand that the reprojection error is the distance in pixels of the reprojected tie point to the respective key point on an image.

As you can see in the attached figure, the RMS Reprojection error in the chunk info is 2.46 pixels. But when I go to gradual selection and choose "Reprojection error", the largest value where any tie points are selected is "Level 0.84". I would have expected the gradual selection value to be higher.

My best guess for why this is, would be that the Reprojection Error "Level" in the Gradual selection dialogue doesnt refer to the original image resolution but a downscaled version of the image. Or maybe the two values are calculated over a different range of tie points?
Could you please explain why these values are so different?

[James]

It looks like your slider in the gradual selection dialog box is not all the way to the left.

If it was all the way to the left then the max number should read 0.848666 which is the max preprojection error in the units of the keypoint scale.

I don't know what 'units of the keypoint scale' means, but it's what alexey said here https://www.agisoft.com/forum/index.php?topic=11869.msg53289#msg53289 

[Paulo]

Hi,

the maximum reprojection error (first number in Chunk info or 0.848666) corresponds to number in Gradual selection dialog box with slider all the way to the left (as James noted). and is equal to reprojection error in pixels scaled by key point size ie:

max reprojection error = max reprojection error (pix) / key point size

[Felix_]

Aah, that makes a lot of sense now   Thank you guys!