how is flatness tolerance measured
The flatness tolerance references two parallel planes (parallel to the surface that it is called out on) that define a zone where the entire reference surface must lie. Flatness tolerance is always less than the dimensional tolerance associated with it. Tolerance Zone: Two Sets of Parallel Planes where the entire referenced surface must lie. Gauging / Measurement: Flatness is can be measured using a height gauge run across the surface of the part if only the reference feature is held parallel.