 Solution

Measure a genuinely flat calibration reference over the same scan range, scanner position and operating conditions, then subtract its reproducible bow background from the specimen data. Retain reference flat regions outside the feature if available, fitting only those regions rather than the bulge. A scanner with independently measured vertical motion or suitable closed-loop correction reduces the instrumental uncertainty. Translating or rotating the specimen checks whether a feature follows the specimen or the scanner coordinates; independent optical or stylus profilometry can corroborate its height. For a rotationally symmetric parabola, rotation alone is not sufficient. Report drift and calibration uncertainty as part of the $200\mathrm{nm}$ height measurement.