Height of tide

(also Rise and Range) The Height of the tide is the difference of the water level at that moment above Chart datum, which is effectively the lowest level the sea reaches. A term not much used nowadays is the Rise which is properly the Height at High Water specifically; the figure shown under Height in many tide tables should properly be Rise, because it relates to High Water, but the word Height seems more common nowadays. The Range of the tide is the difference between successive Low and High Waters. This figure is the same as Height only at Lowest Astronomical Spring tides, because at other times the Low-Water tide level is not so low as Chart datum. It is important to remember this, because it is a common error to use the height shown in a tide table as if it were the amount by which the level will rise and fall on that day – in other words to confuse it with Range. If you really need to know the range it can be deduced from the height of Mean level: subtract the Height of Mean Level from the Height of HW on that day, and double the result. (Because the tide rises as far above the Mean Level as it falls below, you see.) Now all you need is the Mean Level, which is rarely shown on the tide tables I filch from chandlers’ counters. But it can be found by looking for the maximum Springs height and halving it. What you are looking for is the Highest Astronomical Tide which is the opposite of Chart datum, so to speak. Half-way between those high and low points is the Mean level. (Please see Tides, where Neaps and Springs are discussed as well for con¬venience.)