Definition
The counterpart of LAT, derived over the same 19-year span. Vertical clearances under bridges and overhead power cables on Admiralty charts are measured from HAT, giving the worst-case least clearance; US charts generally use Mean High Water. Elevations of lights are measured from MHWS on Admiralty charts. Using the wrong reference when calculating air draught clearance produces an error of the order of the spring range.
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