Tidal Definitions and Datum

Chart Datum (CD) is the reference level from which depths on charts and tidal heights are measured. It is set at approximately the Lowest Astronomical Tide (LAT) — the lowest tide that can be predicted under normal conditions. Depths on the chart are below CD, heights of tide are above CD.

Key tidal levels: MHWS (Mean High Water Springs), MHWN (Mean High Water Neaps), MLWN (Mean Low Water Neaps), MLWS (Mean Low Water Springs). The charted drying height of a rock or sandbank is measured above CD — subtract the drying height from the height of tide to find the depth of water over it.

Mean Sea Level (MSL) is the average level of the sea over a long period and sits roughly mid-way between MHWS and MLWS. Overhead clearance uses a different high-water datum from charted depths: current UKHO/IHO charting practice leads with Highest Astronomical Tide (HAT) for bridge and cable clearances where tide is appreciable. Older UK chart examples may state MHWS, so always read the chart title block or chart note before calculating clearance.