Detrending DEM in floodplain?

Question

I am after the same solution to Removing elevation trend over sloped surfaces?

I am following the steps outlined, but in the last step it states:

In this expression [X'] refers to the the x-coordinate grid in the original coordinates and [Y'] refers to the original y-coordinate grid.

I'm not sure what is meant by the original coordinate grids. It doesn't look as if these are the original x and y values.

What are the coordinate grid values, and how I might obtain them?

That Q&A is quite old so perhaps there is a new method for carrying out this operation.

Answer 1

While I wasn't able to address this part of the question exactly, I followed Whited el al. (2007) https://onlinelibrary.wiley.com/doi/full/10.1002/rra.1585 for the DEM detrending process. This gave me the desired results of removing the longitudinal gradient from the riverscape.