Height of Tide

The vertical distance between the water’s surface at any given time and the established chart datum, typically the lowest astronomical tide.

The height of tide is a critical measurement in maritime navigation, as it directly affects the depth of water available for vessels to safely pass through or anchor in a given area. Tides are caused by the gravitational forces exerted by the moon and the sun, combined with the Earth’s rotation, and they follow a predictable pattern of rising (flood tide) and falling (ebb tide). The height of tide at any given location and time can be predicted using tide tables, which are based on long-term observations and calculations of tidal behaviour.

The chart datum, usually the lowest astronomical tide (LAT), is used as a reference point for measuring the height of tide. This ensures that mariners have a consistent and reliable baseline from which to calculate the depth of water. When the tide is higher than the chart datum, the water level will be deeper; when it is lower, the water will be shallower. Understanding the height of tide is essential for safe navigation, particularly in areas with significant tidal ranges, shallow waters, or when approaching harbours, docks, or navigating through channels. Mariners must also consider the height of tide when planning to pass under bridges or other structures, as it affects the clearance available.