How are the accuracy grades of granite precision gauges classified?

Accuracy Grade Classification of Granite Precision Gauges (Surface Plates / Platforms)
Domestically, the mainstream standard is GB/T 20428-2021 “Rock Surface Plates”, while internationally, ISO 8512-2 and DIN 876 are commonly used. The core classification is based on flatness tolerance, distinguishing grades 000, 00, 0, 1, and 2, with accuracy decreasing from high to low.
I. National Standard GB/T 20428-2021 Grades (Most Commonly Used)

1. Accuracy Grades (from high to low)
   Grade 000 → Grade 00 → Grade 0 → Grade 1 → Grade 2
   Grade 000, Grade 00: Ultra-precision grade, used for CMM bases, laboratory references, and metrology room standard surface plates
   Grade 0: Precision inspection, laboratories, and high-precision machine tool accessories
   Grade 1: Workshop precision inspection, scribing, and tooling verification
   Grade 2: Ordinary workshop scribing, assembly, and fitter platforms
2. Core Rules for Flatness Tolerance (Unit: μm, taking 600×400mm as an example)
   Grade 000: Flatness ≈ 1.47μm
   Grade 00: ≈ 2.94μm
   Grade 0: ≈ 5.88μm
   Grade 1: ≈ 11.76μm
   Grade 2: ≈ 23.52μm
   Formula: Flatness Tolerance = Coefficient × √(Length × Width)
   Coefficients: 000=0.003; 00=0.006; 0=0.012; 1=0.024; 2=0.048
   II. International Standard Comparison
   ISO 8512-2: Corresponds to grades 00, 0, 1, and 2, basically consistent with the national standard
   German DIN 876:
   Grade 00 (Ultra-precision), Grade 0 (Precision), Grade 1 (Normal)
   US ASME B89.3.7: Grades AA, A, B, and C, where AA ≈ Grade 000, A ≈ Grade 00
   III. Reference for Selecting Granite Gauge Grades
   Grade 000 / 00: CMMs, vision system bases, metrology institute standards, ultra-precision laboratories
   Grade 0: Precision machining workshop inspection, gauge verification, high-precision tooling
   Grade 1: Regular precision inspection, scribing, mold inspection
   Grade 2: Ordinary assembly, fitting, welding tooling
   IV. Key Supplements
   The larger the accuracy grade number, the larger the tolerance and the lower the accuracy; Grade 000 is the highest, and Grade 2 is the lowest.
   Granite materials such as Jinan Green, Zhangqiu Black, and Indian stone are preferred. Only materials with uniform density and good thermal stability can stably achieve Grade 000/00.
   In addition to flatness, high-precision gauges will also specify: parallelism, perpendicularity, surface roughness, and linear expansion coefficient.