Primary form

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Primary form is used in surface metrology to refer to the over-all shape of a surface which can be measured quantitatively, as contrasted with more local or higher-spatial frequency variations in the surface dimensions such as surface waviness or surface roughness or surface finish. Examples of primary form include surface flatness, surface roundness, and surface cylindricity. Each of these classes of primary form can be quantitatively defined in several different ways depending upon the intended purpose of the surface, such a statistical least squares flatness, and overall flatness (aka max-min flatness).