The height of mean high water neaps is the average, throughout a year as defined above, of the heights of two successive high waters during those periods (approximately once a fortnight) when the range of the tide is least. The height of mean low water neaps is the average height obtained from the two successive low waters during the same periods, i.e.
MHWN (Mean High Water Neaps) :- The average HEIGHT of the HIGH WATERS of NEAP TIDES above Chart Datum.
MLWN (Mean low Water Neaps) :- The average HEIGHT of the LOW WATERS of NEAP TIDES above Chart Datum.
The values of MHWS, MHWN, MLWN and MLWS vary from year to year in a cycle of approximately 18.6 years. In general the levels are computed from at least a year’s predictions and are adjusted for the long period variations to give values which are the average over the whole cycle. The values of Lowest Astronomical Tide (LAT) and Highest Astronomical Tide (HAT) are determined by inspection over a span of years.