Highest Astronomical Tide

Highest Astronomical Tide
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Fig 5.9 – HAT is measured from CD. CD can also be considered the same level as LAT and the lowest the tide will ever recede under astronomical conditions.

So under certain astronomical events we can experience extraordinary tides. The highest limit of this is called HAT and the lowest is LAT (Fig 5.9). HAT is the highest level that the tide is ever predicted to rise through astronomical events and LAT the lowest the tide is ever predicted to fall. We measure the heights of most charted objects from MHWS but clearances under bridges etc. are measured from HAT. During extreme astronomical events the tide may go out and fall past MLWS and reach the level of LAT (Lowest Astronomical Tide) which is also used as our Chart Datum level (CD).