Tidal Levels as Defined in UK showing Legal Definition of Foreshore
Tidal Levels as Defined in USA showing Private Ownership Boundary by State
Tidal Levels are standard heights which are determined from the shape of the graph of tide height over a fixed duration at a specific location.
Many different tidal levels are in use throughout the world, some being more appropriate for diurnal or mixed tides, and some more relevant for semi-diurnal tides.
Tidal Levels are frequently used as vertical datums by hydrographers and surveyors in cartography, in establishing legal boundaries and territorial rights, and in planning flood defences.
Note that througout this article, the tide is the "Astronomical Tide" - which is that part of the tide caused by the Moon and the Sun.
The other part of the tide is caused by the weather (e.g. wind and pressure) and is known as the "Meteorological Tide". In most places the astronomical tide dominates.
Because of this meteorological variation, HAT is not the highest possible tide and LAT is not the lowest possible tide.
Tidal Levels are often defined as being an average over a time period of one metonic cycle of 19 years or over a lunar nodal cycle of 18.61years.
HAT / LAT: The highest/lowest level that can be expected to occur under average meteorological conditions and under any combination of astronomical conditions.Note 1,2
MHWS / MHWN: The average of the two successive high waters during those periods of 24 hours when the range of the tide is at its greatest/least.Note 3,4
MLWS / MLWN: The average of the two successive low waters during those periods of 24 hours when the range of the tide is at its greatest/least.Note 3,4
MHW / MLW: The average of all high/low water levels over a given period.Note 5,6,7
MHHW / MLHW: The average of the higher/lower high water level of each tidal day over a given period.Note 8
MHLW / MLLW: The average of the higher/lower low water level of each tidal day over a given period.Note 8
DTL: The arithmetic mean of mean higher high water and mean lower low water.Note 8
MTL: The arithmetic mean of mean high water and mean low water.Note 8
MSL: The average observed height of the surface of the sea relative to a stated vertical datum.Note 9,10
ISLW: Indian Spring Low Water. MSL - (M2 + S2 + K1 + O1)Note 11,12
GeoTide and Tidal Levels
In determining tidal levels, GeoTide can improve upon the accuracy which would be obtained by a simple examination of past tidal records because i) the harmonic analysis process automatically disregards the meteorological and other noise in the tide and ii) its analysis can extend over the required 18.61 year time period - even though the source tidal data may be considerably shorter than this. Of course this does not obviate the need for as much accurate historical data as possible. In order to gain a meaningful accuracy for these figures at least one month of very high quality data is required and preferably more.
Historical Tidal Levels
The definition of tidal levels has changed considerable over the past 200 years. For example LAT and HAT are relatively recent concepts, with the first official usage being found in 1960. The tables below show some historic tidal levels and their dates of usage.
UK Ordnance SurveyRef 2
HWMOT: High Water Mark of Ordinary Tides: 1868 to 1935
LWMOT: Low Water Mark of Ordinary Tides: 1868 to 1935
HWMMT: High Water Mark of Medium Tides:1935 to 1965
LWMMT: Low Water Mark of Medium Tides 1935 to 1965
MHW: Mean High Water: 1965 to present
LHW: Mean Low Water: 1965 to present
LWM Low Water Mark
HWM High Water Mark
HWMOST High Water Mark of Ordinary Spring Tides
LWMOST Low Water Mark of Ordinary Spring Tides
OHWS Ordinary High Water Springs
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