= 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. \b[The two contributions cannot be separated by polynomial fitting to this image alone.] A plane subtraction removes tilt but does not independently calibrate curvature.
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