1. n. [Formation Evaluation]

The position on a logging tool string that is used as the reference for depth measurements. Each measurement has a different measure point. In normal practice, each measurement is shifted in depth by the distance between the measure point and the reference point. This ensures that all measurements are recorded at the same depth. For a wireline tool, the reference point is normally the bottom of the tool string. For measurements-while-drilling, the reference is the bit (the driller's depth). The term is sometimes used to mean the depth reference.

See: depth wheel,  first reading,  pickup