Approximate flatness error from the highest and lowest measurement points on a surface (minimum zone method) for machined parts and surface plates.
平面度 = 最高测点 − 最低测点（最小区域法近似）。
| Item | Formula / Note | Source / Standard |
|---|---|---|
| Flatness deviation (minimum zone) | Approximate flatness by the difference between the highest and lowest sampled points | ISO 12781-1:2011, Geometrical product specifications (GPS) - Flatness - Part 1: Vocabulary and parameters |
| Least-squares reference plane | A least-squares reference plane is a common alternative to the minimum zone | ISO 12781-2:2011, Geometrical product specifications (GPS) - Flatness - Part 2: Specification operators |
| Tolerance zone | The flatness tolerance zone is the space between two parallel planes separated by the tolerance value | ISO 1101:2017, Geometrical product specifications (GPS) - Geometrical tolerancing |
| Filtering and sampling | Follow the specified sampling and filtering; point distribution affects the result | ISO 12781-2:2011, Geometrical product specifications (GPS) - Flatness - Part 2 |