What is a 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. On the map, the Ritz Hotel is a datum. So are streets, the train station, the museum and the restaurant. Thus, by using an origin, datums, directions and distances people have all the information they need to get from one location to another.
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 (Figure 7).