What is the relationship between datums and measurement methods?

• A. The datums define the measurement frame; measurement must be performed with reference to the datum frame.
• B. Datums are optional; measurement uses global coordinates.
• C. Measurement methods define datums.
• D. Datums are only for design, not measurement.

Answer: (A)

Datums establish the reference frame for measurement. In precision measurement, you create a datum reference frame from specific features on the part (primary, secondary, tertiary). This frame defines the origin and the orientation of the measurement axes. All measurements are then performed and reported relative to that same datum frame, so you get consistent, repeatable results regardless of how the part is oriented in the setup.

This is why datums aren’t optional—they’re the framework that guides how you measure. Global coordinates aren’t used because, without a defined datum frame, measurements could vary with how the part sits or how the instrument is placed. The datum frame is defined by the part, not by the measurement method itself, and the method must reference that frame to determine features’ sizes, locations, and orientations.

So, the best approach is to measure with reference to the datum frame because it provides a consistent, unambiguous basis for all tolerance evaluations.