Tidal Levels

GeoTide Tidal Levels: UK (UKHO) & USA (NOAA)

Tidal Levels are depth or height contours defined by the rise and fall of the astronomical tide. Throughout the world the definitions adopted by either the UK or the USA are in general use.

 

 

UNITED KINGDOM / SCOTLAND

 

UNITED STATES OF AMERICA (USA)

Private Ownership Boundary by State

 

Tidal Levels are :-

• standard contour heights
• defined using the Astronomical Tide over a fixed duration at a specific location
• vertical datums used by hydrographers and surveyors in cartography
• legally boundaries used to establish territorial, ownership, access and fishing rights
• used in setting the height of flood defences.

Some tidal levels are more appropriate for diurnal tides, and some are more relevant for semi-diurnal tides.

 

Note that throughout this article, the tide to which we refer is the "Astronomical Tide" - this 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 necessarily the highest possible tide and LAT is not necessarily 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.

 

Definitions

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}

Notes

1. As defined by United Kingdom National Tidal and Sea Level Facility (NTSLF)^{Ref 1}
2. For HAT NOAA^{Ref 3} have "The elevation of the highest predicted astronomical tide expected to occur at a specific tide station over the National Tidal Datum Epoch." with a corresponding change to LAT.
3. For MHWS / MLWS / MHWN / MLWN the NTLSF^{Ref 1} add " throughout the year (when the average maximum declination of the moon is 23.5°)".
4. For MHWS / MLWS / MHWN / MLWN CoastalWiki^{Ref 4} add "throughout a year" and "(approximately once a fortnight)".
5. For MHW and MLW the IHO^{Ref 2} definition specifies a period of 19 years.
6. For MHW / MLW Coastal Wicki^{Ref 4} have "The arithmetic mean of the published values of mean high water springs and mean high water neaps." with corresponding definitions for MLW.
7. For MHW NOOA^{Ref 3} have "The average of all the high water heights observed over the National Tidal Datum Epoch."
8. The definitions of MHHW, MLLW NOAA add "observed". MTL and DTL ^{Ref 3}.
9. This definition of MSL is as defined by IHO^{Ref 2} .
10. For MSL NOOA^{Ref 3} have "The arithmetic mean of hourly heights observed over the National Tidal Datum Epoch."
11. For ISLW - the American Meteorological Society^{Ref 7} have " is a tidal low water datum, designed for regions of mixed tides, that is depressed below mean sea level by the sum of the amplitudes of the principal semidiurnal lunar and solar tides and the principal diurnal tides i.e. by (M2 + S2 + K1 + O1); originally developed for parts of the Indian Ocean"
12. For ISLW European Commission / Inspire^{Ref 8} have "A tidal surface datum approximating the level of the mean of the lower low water at spring tides".

GeoTide improves upon the accuracy compared with what 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.

GeoTide Analyzer determines the following tidal levels.

HAT: Highest Astronomical Tide

MHWS: Mean High Water Springs

MHHW: Mean Higher High Water

MHW: Mean High Water

MLHW: Mean Lower High Water

MHWN: Mean High Water Neaps

MSL/ML : Mean Sea Level

MLWN: Mean Low Water Neaps

MHLW:Mean Higher Low Water

MLW: Mean Low Water

MLLW: Mean Lower Low Water

MLWS: Mean Low Water Springs

ISLW: Indian Spring Low Water

LAT: Lowest Astronomical Tide.

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 Survey^{Ref 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

UK Admiralty

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

 

References

1. United Kingdom National Tidal and Sea Level Facility (NTSLF)
   https://www.ntslf.org/tgi/definitions
2. Hydrographic Dictionary, Part I, Volume I, English, Special Publication No. 32, Fifth Edition, International Hydrographic Organization Monaco, 1994
   See https://www.iho.int/iho_pubs/standard/S-32/S-32-eng.pdf
3. NOAA Tidal Datums
   https://tidesandcurrents.noaa.gov/datum_options.html
4. CoastalWiki.
   www.coastalwiki.org
5. Permanent Service for Mean Sea Level (PSMSL)
   https://www.psmsl.org/train_and_info/training/manuals/glossary.html
6. From Ordnance Survey Tidal level Nomenclature.“Ordnance Survey data collection and mapping of tidal features” Brian Baily The journal of The Charles Close Society, Sheetlines, 90 (April 2011), pp.4-17.
   See https://www.charlesclosesociety.org/files/Issue90page4.pdf
7. American Meteorological Society
   http://glossary.ametsoc.org/wiki/Indian_spring_low_water
8. European Commission / Inspire
   See http://inspire.ec.europa.eu/codelist/WaterLevelValue/indianSpringLowWater