Definitions for

**"Gage Datum"**is the elevation of the zero point of the reference gage from which gage height is determined as compared to sea level (see " Datum"). This elevation is established by a system of levels from known benchmarks, by approximation from topographic maps, or by geographical positioning system.

The arbitrary zero datum elevation which all stage measurements are made from.

The arbitrary "zero plane" from which all stage measurements are taken from. Usually set below the natural bottom of the channel so all stage height readings will be greater than zero.