A reference plane chosen when a water level station is installed, placed low enough that all readings stay positive. Gauge and staff readings are logged relative to it, and its height is fixed by levelling to the bench marks. Once enough data exist, tidal datums are computed as heights above station datum, and users convert observations to MLLW or other datums with the published offsets.
Raw series is on station datum, subtract 7.85 to get it on MLLW before you plot it.
Also heard as
- gauge zero
- staff zero
- STND
Related terms
Bench mark
A permanent, stable mark of known height near a tide station that preserves the station's datums.
Tide staff
A graduated board fixed vertically in the water and read by eye to check a gauge against datum.
Tidal datum
A local zero for heights, computed by averaging one stage of the tide, such as high or low water, across many years.
Tide gauge
An instrument that records water level continuously at a fixed point, the basic tool of tidal measurement.