What is Geometrical Product Specifications (GPS)?

I offer a free "Introduction to GPS" tutorial. It is about 90 minutes spread over 14 lectures and cover many of the topics explained here in more details: Introduction to GPS.

Geometrical Product Specifications(GPS) is the international symbol language used to express tolerances in technical drawings. It defines the nominal geometry of the components that make up the product, and the tolerances that quantify the allowable deviations from this nominal geometry.

The ISO-GPS language is defined in a number of ISO standards developed by ISO Technical Committee 213 "Dimensional and geometrical product specifications and verification". I have been the chairman of ISO TC 213 since 2008.

Any time a design has to be captured and communicated, it is done using ISO-GPS. This global specification language enables a drawing of a component developed in one country to be sent to another country, where it can be understood and the component manufactured. ISO-GPS is an effective communications link, even when the designer and supplier do not have a common language. It helps products to be made more easily and efficiently, using fewer resources in the process.

Clearly defined specifications

Globalization has greatly increased the need for ISO-GPS standards that can express complex geometrical requirements and relationships unambiguously. Tribal knowledge within a company, and lunchtime discussions between design, manufacturing and quality staff, can no longer be used to fill in the product information that is not captured in technical drawings.

Modern manufacturing requires a robust specification language in which tolerances are well defined and not open to interpretation. Unambiguous specifications allow the tolerance values to be larger, while still ensuring product function. This reduces the cost of the product and gives technically sophisticated companies a competitive advantage.

In response to these market factors, the GPS language, as defined in various ISO International Standards, has developed over the past couple of decades. It has evolved from a few selfcontained documents written for craftsmen, to a large set of complex and interrelated documents written for, and by, engineers and mathematicians.

Rule-based standards

The format has changed from example-based standards to rule-based standards. Example-based standards are appealing and appear easy to read and understand, because they often omit or hide complexities. The shortcomings of the examples only become evident when they have to be applied to specify real products that are typically much more complex. At first glance, modern ISO-GPS standards may therefore appear unnecessarily complex and difficult to read. However, this complexity is necessary to provide a sufficiently rigorous theoretical basis for the specifications to be unambiguous, even when components have significant geometrical deviations.

It is quite difficult to start learning ISO-GPS without any help and guidance because the information and rules are spread out in many standards. This means there may be fundamental rules written in one standard that have to be known in order to apply another standard correctly. In many ways we can compare ISO-GPS standards to laws. Both are written in a language and a style that are very precise and unambiguous to the trained reader, but which novices may find difficult or impossible to understand.

Tolerancing Styles

Tolerancing can broadly be classified as following one of three styles:

The links take you to pages that briefly discuss the three types of tolerancing.

Henrik S. Nielsen