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The ISO GPS standard system based on DIN EN ISO 8015 regulates the language with which requirements for the geometry of a product are communicated. With an entry – constructed from “specification elements” – on the technical drawing, a specification operator in the standards system is invoked. A specification operator collates rules (“operations”) that are used to allow all the contract parties to understand the requirements of a geometric element or the relationship between geometric elements.

The principle of standard setting applies here, namely the “default principle”. Unless indicated through the addition of supplementary information in the contract agreement, the specification operator only includes default operations in a fixed order according to ISO standards. The default is chosen so that the operations approximately correspond to the most commonly used procedure and conventional understanding in practice.

In the following image, the concept is illustrated using the shaft geometric element with a nominal diameter of 20 mm as an example. At first only the defaults specification is considered, as shown on the left side. The drawing entry specifies a “linear size element” according to DIN EN ISO 14405-1 of the type “cylinder”. The default specification operator demands consideration of the “local two-point size”: All independently determined individual distances from opposite lying points must lie within the tolerance range, i.e. according to tolerance code “h9” of DIN EN ISO 286-1 at an interval of between 19.948 mm and 20 mm. Five arbitrary two-point sizes on the non-ideal surface model are represented as a geometric equivalent.* One suitable means of verification (“verification operator”) for this specification would be to carry out a check using a go/no go gauge: 1) Pass check (go): Is the maximum material size exceeded?; 2) Fail check (no-go): Is the minimum material size undershot?

The deviation from the default setting can be identified using suitable specification modifiers (and/or abbreviations). In this way a “special specification operator” is obtained. In this example, the “E in circle” modifier symbol sets the envelope condition. The envelope condition requires that the evaluated geometry lies in an enveloping cylinder 20 mm in diameter and that all two-point sizes are greater than or equal to 19.948 mm. A suitable verification would be to test using a ring gauge or to carry out a no-go test using a go/no go gauge.

Below are examples of modifier text symbols with their meanings, such as those that can be assigned to a characteristic as property “modifiers” in infra CONVERT.