Port Reference Tide Gauge

A port reference tide gauge is the primary, permanently leveled gauge that defines the tidal datums and chart datum for a port and its approaches. Its long record fixes the local Chart Datum (normally LAT in IHO practice), Mean Sea Level, and the high and low water datums against which Admiralty Tide Tables and the port’s predictions are computed. Every secondary or temporary gauge in the harbor is reduced to this reference through simultaneous observation or co-tidal differences. The gauge is connected to a network of land benchmarks by precise leveling, so chart depths, dredging tolerances, and under-keel clearance all trace back to its zero.