# Geometric Dimensions & CMM Terminologies

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Also learn GD & T used in CMM software and datasheet:

## What is Alignment and Datum in CMM?

With today’s CMM software, the CMM measures the workpiece’s datums establish the Part Coordinate System, and mathematically relates it to the Machine Coordinate System.

The process of relating the two coordinate systems is called alignment. With a street map, we do this automatically by turning the map so that it is parallel to the street (datum) or to a compass direction (i.e., north). When we do this, we’re actually locating ourselves into the “world’s coordinate system.”

What is Datum?
A datum is a location. We use datums as guides to tell others where we are or as directions on how to get to places.

For example, to get from the train station (origin) to the restaurant, you walk 2 blocks north on Elm Street (datum), take a right, and walk 2 blocks east on Maple (datum).

In metrology, a datum is a feature on a workpiece such as a hole, surface or slot. We measure a workpiece to determine the distance from one feature to another.

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