Surface Plate

A surface plate is a precision plate for measuring, layup, and precision tool making. Granite is a common, inexpensive material for a surface plate.

https://en.wikipedia.org/wiki/Surface_plate#Accuracy_and_grade

Grades - AA is under 5/100 of a thousandth accuracy per foot. Tool room grade B is 0.0002 (5 micron).

Comparison

• Surface grinding - 0.001" per foot flatness (25 micron) [1]
• Precision surface grinder - 1 micron (see Air Bearings.
• Surface plate - same as precision surface grinder, as the precision surface grinder is based on surface plate

Sourcing

• Quite affordable for the precision. $116 for grade A, 24x18x3 - [2]