Okay, it may be the bourbon doing the thinking, but I think I've come up with a slow, tedious, and probably impractical way to at least check the flatness of a granite surface plate with just a hand scraper and two cast iron straightedges at least as long as the long diagonal of the plate. It's more than possible I'm flat wrong about this, but please check my thinking. To create a truly flat surface plate by hand with no other reference, you actually create three, using what Connelly calls "automatic generation of gages" (chapter 21 in Machine Tool Reconditioning). Basically, repetitively scraping each plate to match the others. Connelly calls this the "Principle of the Symmetrical Distribution of Errors." (Tom Lipton demonstrated the process in a series of videos while creating some small lapping plates.) Basically, "two things equal to a third thing are equal to each other." I suspect it would take months to create three plates of any reasonable size this way, but that's beside the point. A relatively "quick" way to just test your granite plate might be as follows: Scrape in straightedge A as accurately as you can, bluing up across one of the long diagonals on the plate. Alcohol "bluing" and at least 40 PPI is required in the final stages. "Blue" up with alcohol along the other diagonal. If the straightedge doesn't mark up evenly, your plate isn't flat. Repeat step two in the horizontal and vertical directions. Again, if it doesn't mark up evenly, your plate isn't very flat. Repeat steps 1-3 with straightedge B. Now, use alcohol to mark up A against B. Any unevenness in the markup will indicate a curve in the plate. If there are no marks in the middle, the plate has a bulge in the middle. If "hinging" the two straightedges together shows they have a high spot in the middle, then your plate is concave. Step 5 effectively doubles any error, so I think you should be able to detect defects in flatness down to 0.0001" or so. This is just a thought got experiment, though. I've not actually tried this. What do you think? Is my reasoning sound?