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Sample tilt and scanner bow are other common backgrounds that AFM image flattening can remove. Fit a plane $b=a+bx+cy$ to a genuinely flat reference area and subtract it to remove tilt. A low-order curved fit can approximate the vertical background produced by a piezoelectric scanner's non-planar motion.

Line-dependent offsets or slow drift can sometimes be removed by subtracting a reference level from each scan line. Mask pits, particles and true steps so that they do not bias the reference. An independently measured flat calibration specimen and scans in different orientations help distinguish instrumental background from actual specimen curvature.

AFM image flattening estimates an unwanted background and subtracts it from the recorded height:A plane removes overall tilt; a line-by-line constant or low-order fit removes line offsets or curvature. For a tip-change height jump, align the baselines of unchanged reference regions on either side of the event, using a piecewise offset or appropriately masked line fits. A single global plane cannot in general remove an abrupt step.

Exclude genuine features from the fit, use the same terrace as a reference where possible, and retain the unprocessed data. The corrected image has a consistent relative background; it does not establish that the interaction force, tip shape or absolute height calibration remained unchanged. In particular, flattening is a correction for the offset component of a tip-change artefact, not a repair of every consequence of a changed tip.

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.

An apparent image change caused by alteration of the effective probe apex during acquisition. A shifted height baseline may be corrected using a suitable reference, but changed geometric access, duplicated features and lost data are not generally repaired by AFM image flattening.