Charles: with an autocollimator one can determine flatness of as small as an 18x 24 surface plate-- with 2 reflector mirrors (using the British flag pattern) right down to fractions of a millionth of an inch. It is a slow process (about an hr.) to inspect and document, though. An auto collimator like mine -( Hilger Watts) normally has a range of not more than 8 ft. I use the tilting level to level my planers (4ft and 8 ft ) to a plane of .001 and the jig transit to a plumb of the same value, but their range is about 100 ft - after that distance, the planet's curvature gets to be a factor. Regards, John