Flatness - Surface vs Feature of Size

You are probably familiar with seeing Surface Flatness called out on a drawing and understanding how to inspect for that. But are you familiar with Flatness for a Feature of Size? This video discusses how both types of Flatness are shown on a drawing and their implications on the design and inspection of a part.
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[@JD-uy5js]

Very good explanation thank you!

[@arnolddecker8323]

The other way to measure flatness is with an optical flat and monochromatic light source.

[@harikumar4757]

Sir, Geometrical tolerance for surface flatness mentioned 0.05 in feature control frame, but how it is changed to 0.03 , saying all points lie between two plane 0.03 apart on surface .

[@Gdandtbasics]

Thank you for the comment. This is an error on our part, that is a typo. If the Feature Control Frame states 0.05 the tolerance zone should be two parallel planes at 0.05 apart, not 0.03.

[@harikumar4757]

Thank you very much, for clarifying doubt.

[@haribaskarperiasamy2927]

Can you please explain how Flatness per area calculation?