  Flatness of a plate is a parameter has been put under consideration for long
time. Factors influencing the accuracy of this parameter have been recognized
and examined carefully but placed scatterringly. Beside that those reports have
not been always in harmonization with Guide for expression of uncertainty
measurement (GUM). Furthermore, mathematical equations describing clearly the
flatness measurement have not been seen in those reports also. We have
collected those influencing factors for systematic reference purpose,
re-written the equation describing the profile measurement of the plate
topography, and proposed an equation for flatness determination. An
illustrative numerical example will be also shown.
