Figure1. Effect of implied datum

In coordinate tolerancing, datums are implied and thus depend on the interpretation of the inspector. Depending upon how a part is placed on the gauging surface may produce different measurements by the different inspectors. Hence a good part may be rejected or a bad part may be accepted.

An example is given in Figure 1. It is clear that the measurements for the hole location in setup-A will be quite different from that in setup-B. To clear such confusions, GD&T specifies the datums explicitly on the part drawing, wherever it is required. In GD&T language, datums are surfaces (sometimes axis and centerlines) from which the measurements should be made to determine the geometric tolerance zones of a feature.

Datums are used primarily to locate a part, in a repeatable manner, for checking geometric tolerances related to the datum features.