The top surface of this plate is precision ground to achieve a high degree of flatness and the accuracy is +/-.000075". It is a Grade A (Grade 0) plate Granite surface plates meet or exceed Federal Specifications GGG-P-463C and it includes a certificate of inspection. (Note that these are not NIST traceable) Granite plates are used for measuring the flatness and squareness of other objects, laying out parts, calibrating instruments, setting up tools, checking parallelism and perpendicularity, and measuring heights with the use of a height gage They are also used as a work surface for assembling, inspecting, and marking parts. Non-magnetic and non-conductive. Harder than steel and does not nick or burr Surface plates are offered by HHIP in various sizes, with different levels of flatness and accuracy and are checked by an auto collimator for overall accuracy assurance