Why can coregistration result in the appearance of block-like (or patchy) artifacts?

[TreeYu123]

I did not use xdem.coreg.BlockwiseCoreg, but instead applied xdem.coreg.NuthKaab; this blocky effect was not present in the original DEMs.

[adebardo]

Thank you for your question. We'll do our best to answer it, could we get a bit more context?
Such as the input DEMs? Or perhaps your script?

Otherwise, off the top of my head, I suspect a very large difference in resolution.

Thanks in advance.