 Solution

Common processing fits and subtracts a low-order polynomial background, often a quadratic surface or a polynomial for each scan line. This AFM image flattening removes scanner curvature on an assumed flat sample. Here the desired specimen signal is itself roughly quadratic, so such subtraction can remove the actual $200\mathrm{nm}$ bulge along with the artefact. The two contributions cannot be separated by polynomial fitting to this image alone. A plane subtraction removes tilt but does not independently calibrate curvature.