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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.

Scanner bow is the relevant artefact. A piezoelectric tube scanner bends to produce lateral displacement; its moving end also follows a curved vertical trajectory. Cross-coupling of lateral and vertical motion therefore gives a roughly quadratic height background even over a flat reference. Over a wide scan it can mimic or conceal a genuine parabolic sample profile; its sign depends on scanner geometry and height convention.