refzone-datum reference paper

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Brief Introduction to Vertical Datums and Tidal Levels

SeaZone Solutions Ltd 1 Oct 2006


Chart DatumChart Datum is the plane below which all depths are published on a navigationalchart. It is also the plane to which all tidal heights are referred, so by adding the tidal

height to the charted depth, the true depth of water is determined. By internationalagreement, Chart Datum is defined as a level so low that the tide will not frequentlyfall below it. In the United Kingdom, this level is normally approximately the level ofLowest Astronomical Tide. Chart Datum is shown on charts as the zero metrecontour.

As Chart Datum is related to the range of the tide in any given area, its level variesfrom place to place and there is a varying relationship between it and OrdnanceDatum (Newlyn) the Ordnance Surveys vertical datum to which all land heights arereferred. For each tidal station around the UK coastline, there will generally be acalculated difference between Chart Datum and OD(N). These values can be foundin the Admiralty Tide Tables United Kingdom and Ireland publication, so for small

areas the relationship can be fairly easily determined. However, for large areasadditional complex calculations are required to calculate a smooth Difference Modelbased on the tidal range. Presently, the difference model used by SeaZone isaccurate to about 0.8 m. The UKHO is currently undertaking a project known asVertical Offshore Reference Framework (VORF) which will create a more accuratedifference model for UK waters.

The diagram below (Figure 1) shows the level of Chart Datum and other tidal levelsand how they relate to each other and the coastline. The acronyms used in thediagram are explained below.

HAT (Highest Astronomical Tide) & LAT (Lowest Astronomical Tide).The highest and lowest levels respectively which can be predicted to occur underaverage meteorological conditions and under any combination of astronomical

conditions; these levels will not be reached every year. HAT and LAT are not the

Figure 1: Diagram showing Tidal Definitions Source: Chart 5011, UKHO


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extreme levels which can be reached as storm surges and other meteorologicalconditions may cause considerably higher and lower levels to occur.

Spring TidesDuring the full moon and new moon phases of a Lunar cycle (approximately afortnightly occurrence), the gravitational forces of the moon and the sun act toreinforce one another. The tides experience an increased range during thesephases. Since the combined tidal force is increased the high tides are higher and thelow tides are lower than average. Spring tide is a term which implies a welling upofthe water and bears no relationship to the season of the year.

Neap TidesDuring the first and third quarter phases of a Lunar cycle (again approximately afortnightly occurrence), the gravitational forces of the moon and the sun counteractone another. The tides experience a decreased range during these phases. Sincethe combined tidal force is decreased the high tides are lower and the low tides arehigher than average. The word Neapis of Greek origin, meaning scanty.

MHWS (Mean High Water Springs) & MLWS (Mean Low Water Springs)The height of mean high water springs is the average of the heights of twosuccessive high waters during those periods of 24 hrs (approximately once afortnight) when the range of the tide is greatest. The height of mean low watersprings is the average height obtained by the two successive low waters during thesame period, i.e. -

MHWS The average HEIGHT of the HIGH WATERS of SPRING TIDES above ChartDatum.

MLWS The average HEIGHT of all LOW WATERS of SPRING TIDES above Chart

Datum.

MHWN (Mean High Water Neaps) & MLWN (Mean Low Water Neaps)The height of mean high water neaps is the average, throughout a year as definedabove, 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 ofmean low water neaps is the average height obtained from the two successive lowwaters during the same periods, i.e.

MHWN The average HEIGHT of the HIGH WATERS of NEAP TIDES above ChartDatum.

MLWN The average HEIGHT of the LOW WATERS of NEAP TIDES above ChartDatum.

The values of MHWS, MHWN, MLWN and MLWS vary from year to year in a cycle ofapproximately 18.6 years. In general the levels are computed from at least a yearspredictions and are adjusted for the long period variations to give values which arethe average over the whole cycle. The values of Lowest Astronomical Tide (LAT)and Highest Astronomical Tide (HAT) are determined by inspection over a span ofyears.

MSL (Mean Sea Level)Mean Sea Level is the average level of the sea surface over a long period, normally19 years, or the average level which would exist in the absence of tides, i.e. The


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average HEIGHT of the surface of the SEA at a TIDE STATION for all stages of theTIDE over a 19 year period, usually determined from hourly height readingsmeasured from a fixed predetermined reference level (Chart Datum).

SeaZone Technical Support TeamOctober 2006

refzone-datum reference paper

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