GD&T Flatness is a common symbol that control how
flat is a surface of a feature regardless of any other datums.
Usually, a value will be indicated in the control box to identify
the require flatness.
Flatness can be easily measured using a height gauge or a
dial gauge run across the surface of the part. High Gauge
can be seat on the surface to be measure if the surface is
vs parallelism Flatness and parallelism is never the same.
Flatness measures the condition of a surface in one plane
and it is a feature compare to it self within the same plane.
While Parallelism compare the
condition of a surface or axis from another datum plan or