ISO 25378:2011

Overview

ISO 25378:2011 - Geometrical product specifications (GPS) - Characteristics and conditions - Definitions - provides a consistent, mathematically based vocabulary and conceptual framework for geometrical specifications, characteristics and conditions. It formalizes terms (for example, geometrical specification, condition, individual characteristic, population characteristic, local vs global) using concepts from ISO 17450‑1 and ISO 22432. ISO 25378 is intended as a roadmap for standards developers and software makers rather than as a prescriptive design or production standard.

Key Topics

• Definitions and scope: precise meanings for GPS terms used across the ISO GPS matrix (workpiece, assembly, population of workpieces/assemblies).
• Condition: a combination of a limit value and a binary relational mathematical operator (upper/lower limits, combined limits, or function-defined limits).
• Characteristics:
  • Individual characteristic - property measured on a single workpiece (subtypes: local individual and global individual).
  • Population characteristic - statistical function derived from a population of individual values (used for SPC and process-level assessments).
• Local vs Global: distinction between characteristics yielding multiple local values (e.g., two-point diameters along a cylinder) and those yielding a single unique value (e.g., minimum circumscribed cylinder diameter).
• Operators and duality principle: specification operators can conceptually act as specification or verification operators; definitions align with the operator-based model in ISO 17450‑1 and ISO/TS 17450‑2.
• Mathematical descriptions: terms are given using the formal mathematical approach referenced in Annex B of ISO 17450‑1.
• Limitations: ISO 25378 does not define GPS symbology, specific tolerance values, or industrial design instructions - it standardizes terminology and conceptual mapping.

Applications / Who uses it

• Standards developers: harmonize terminology across GPS standards and create consistent chains of standards.
• Metrologists and measurement labs: adopt precise definitions for verification procedures and reporting.
• Software vendors (CAD/CAM/inspection/SPC): implement consistent data models, operators and verification algorithms.
• Quality engineers and manufacturing researchers: design statistical process control (SPC) strategies using population characteristics.
• Academics and trainers: teach GPS concepts with standardized vocabulary.

Related Standards

• ISO 17450‑1: General concepts - model for geometrical specification and verification
• ISO/TS 17450‑2: Basic tenets, operators and uncertainties
• ISO 22432: Features utilized in specification and verification
• ISO 3534 series: Statistical vocabulary and symbols

By using ISO 25378 vocabulary and operator concepts, organizations ensure clearer communication of geometrical requirements, more consistent verification workflows, and improved interoperability for measurement and CAD/inspection software.