Charted depth and datum: why depth figures need a reference value

A depth figure on a nautical chart does not automatically indicate the current water level. It is referenced to a specified vertical datum, known as the chart datum or depth datum. Only once this datum is known can charted depth be related to a local water-level report.

What a chart datum means

Hydrographic surveys measure water depth at the time of the survey. For charting, depths are referenced to a local datum. For example, NOAA uses Mean Lower Low Water (MLLW) for its nautical charts in tidal waters; this is the local datum used by NOAA, not a universal standard for all countries and charts. Tide gauges and reference marks help relate water-level observations to the applicable datum. The local tide describes how the water level changes; it is not the chart datum itself.

Charted depth and current water level

The current water level may differ from the chart datum. In a harbor, water levels are influenced not only by the tide but also by weather conditions and changes in river discharge. NOAA explains that nautical charts show depth relative to a local standard datum, while current gauge reports are also referenced to a local datum. A water-level reading is therefore meaningful only when considered together with the appropriate local reference datum. Strong winds can also cause a weather-related rise in water level.

For shallow areas in a navigable channel, available depth and under-keel clearance are assessed alongside the vessel’s actual draft and local conditions; route selection is the responsibility of the vessel’s command.

Reference, not an isolated depth figure

The reference datum and location matter just as much as the depth figure. The datum is stated in the nautical chart notes; only with a water-level reading that matches the location and time can the difference between charted depth and the current water depth be understood.