Drying Height

The vertical distance between the seabed or a drying feature, such as a rock or sandbank, and chart datum, which is usually lowest astronomical tide.

The drying height is a critical concept in marine navigation, particularly in tidal areas, as it indicates the elevation of a feature that becomes exposed during low tide. This measurement is typically shown on nautical charts and is essential for mariners to avoid grounding their vessels. The chart datum, used as the reference point for the drying height, is the lowest level to which the tide is expected to fall under normal astronomical conditions. When the tide is below this level, the drying height represents the portion of the seabed or feature that remains above the waterline.

For example, if a rock has a drying height of 2 metres, it means that when the tide falls to the chart datum, it will be exposed and stand 2 metres above the water level. Conversely, the rock will be submerged when the tide rises 2 metres above chart datum (Height ot Tide of 2 metres). Understanding drying heights is vital for safe navigation, especially in shallow waters or areas with significant tidal ranges, as it helps mariners plan their routes, anchoring spots, and port entries. Misjudging drying heights can lead to vessel grounding, damage, or delays. Drying heights are often marked with special symbols on charts, and mariners must interpret them in conjunction with tidal predictions to ensure safe passage.