Flat surface
From Self-sufficiency
To check if a surface is flat:
It only works for a polished slab
This works rather well as a quick check.
Find a straight line some distance away that you can see at an oblique angle reflected on the slab.
Its remarkably sensitive even to slight bow Move your view point to make that line traverse the length and width of the slab.
Any imperfections in flatness show up as a kink in the straight line.
Its remarkably sensitive even to slight bow.
