Datum Reference
A datum reference identifies the datum used to locate, orient, or constrain a measured feature.
A datum reference is an indication of which datum, or combination of datums, is used to establish the location, orientation, or constraint for a feature being measured or specified. In manufacturing and inspection, it commonly refers to the datum letters or datum sequence shown in a drawing, feature control frame, inspection plan, or CMM program.
Datum references are used in GD&T, CMM inspection, first article inspection, and characteristic mapping to ensure that measurement results are evaluated against the same reference structure intended by the engineering definition. For example, a position tolerance may be checked relative to datums A, B, and C in a defined order.
A datum reference should not be confused with the physical datum feature itself. The datum feature is the actual surface, hole, slot, or other part feature used to establish the reference. The datum reference is the drawing or data-model instruction that identifies how that reference is applied. In CMM data imports or AS9102 Form 3 preparation, datum references are reviewed to confirm that the measurement setup, coordinate alignment, and reported characteristic match the drawing requirements.
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