GPS stands for “Geometrical Product Specifications”. This family of ISO standards (principles in ISO 8015) is the language of modern drawings: using symbols, values and datums, it expresses not only dimensions but also the form, orientation and position of a part’s features. In the workshop, people speak of GPS dimensioning or geometric tolerancing (often called GD&T); ISO 1101 is its central standard.
Each requirement is read in a tolerance frame divided into boxes: the symbol of the characteristic (flatness, cylindricity, parallelism, perpendicularity, position, run-out…), the value of the tolerance zone, then the datum features (A, B, C) against which it is assessed (ISO 5459). Example: “position Ø 0.05 with respect to A and B” means that the axis of a hole must stay within a cylinder 0.05 mm in diameter, positioned relative to the two datums. This approach expresses the function of the part directly, instead of deducing it from a chain of ± dimensions that add up and leave room for interpretation.
GPS is everywhere in aerospace, automotive, medical and precision mechanics. It is demanding to inspect: measuring a position or a coaxiality relative to datums generally calls for a coordinate measuring machine and a dedicated program, hence time. Geometric tolerances are therefore reserved for functions that justify them. For the rest, the general geometric tolerances of ISO 2768 (classes H, K and L) avoid dimensioning everything; the ISO 2768 tolerances tool summarises them.
In a quote request
Attach a definition drawing dimensioned in GPS, with its datums clearly identified, and flag the critical characteristics so that the workshop prices the corresponding inspection. State the inspection method expected and the number of parts to measure. If the drawing is dimensioned only in plus or minus, specify which surfaces are functional, so the workshop knows where to focus its effort. See also the guide on machining tolerances.