How flat is a granite worktop?

[Tenson]

What I mean is my 15mm slab of ali is probably the flattest thing I can get my hands on. Gotta be better than a pane of glass or a chunk of MDF for checking 'local' flatness. Probably flatter than the work top material?

What I'm hoping for is a work bench top, not surface plate, but one that I can trust to be flat for general non precision work like checking steel tube for bow when making my next machine. I'm not going to be taking care of it, I'll use it as a general work bench.

If I wanted to check the work top for 'global' flatness, can I expect the edge of a steel rule be reasonably straight?

[JAZZCNC]

If I wanted to check the work top for 'global' flatness, can I expect the edge of a steel rule be reasonably straight?

Not really, it's a ruler, not a straight edge.! . . . if you want to check flatness then you need a proper straight edge.