Interpolated DEM RMSE

Question

I'm trying to figure out the 'true' error associated with interpolated cells in a DEM, which itself is known to have a RMSE of 0.15m.

1. DEM has RMSE of 0.15

2. Interpolation model shown by cross-validation to have an RMSE of 0.1

What is the RMSE of any interpolated cells?

Is it:

0.15 + 0.1 or 0.15 * 0.1 or even,

Answer 1

I'm going to hazard the last of my suggestions was correct based on this extract from the following guidance.

See: enter link description here