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Notes and definitions

Definitions

Tidal datum epoch

The tidal datum epoch is the recommended interval for the calculation of tidal datums. Australian tidal authorities have adopted the 20 year Tidal Datum Epoch 1992 to 2011 (inclusive) as the basis for determining Lowest Astronomical Tide (LAT) datum. This Tidal Datum Epoch is known as LAT(1992). Queensland has moved to a Tidal Datum Epoch 2010 to 2029 (inclusive) for tidal plane determination excluding LAT.

In the 2022 edition of the Queensland Tide Tables, the semidiurnal and diurnal tidal planes of the standard ports' were updated to incorporate the latest available tidal observations, prediction information and allowance for sea level rise, i.e. they have been determined using Tidal Datum Epoch 2010 to 2029 and given as heights above the Queensland Port Datum (LAT(1992)). It is intended that the 2022 tidal plane values will now remain fixed until the tidal datum epoch is reviewed or significant changes occur.

The mean sea levels listed in the table mean sea level used for the tidal predictions will change over the course of the tidal datum epoch as they include the most recent observations and an allowance for sea level rise.

Datum of tidal heights

The height of the tide (expressed as metres and decimals) is referred to the port datum (LAT datum). When a low water falls below datum, it is marked with a minus sign (-).

When using a navigational chart, tidal height should be added to chart depth. If preceded by a minus sign, it should be subtracted.

Standard port

Standard ports are those provided as daily tables of the predicted times and heights of high and low waters. The tide times are referred to Australian Eastern Standard Time and the tide heights are referred to LAT datum. Semidiurnal and diurnal tidal planesLocation of standard ports.

Secondary place

These locations are grouped and associated to the adjacent standard port with a similar tidal pattern. Data sufficient for calculating their times and heights is supplied following the standard port prediction tables.

Daylight saving

During any time of daylight saving, one hour must be added to the predicted times shown.

Tidal levels

A list of tidal levels referred to LAT datum for standard ports and selected secondary places is given in the Semidiurnal and diurnal tidal planes.

In addition, the semidiurnal tidal planes and diurnal tidal planes provide the factors necessary to calculate tidal predictions for the selected secondary places (referred to LAT datum at each secondary place) from the tidal predictions for the standard ports.

Rise

The rise of the tide is the height of the high water above port datum.

Range

The range of the tide is the difference between the height of high water and the next succeeding or last preceding low water.

Semidiurnal

Semidiurnal tide refers to a tide which has a period or cycle of approximately half of one tidal day (about 12.5 hours). Semidiurnal tides usually have two high and two low tides each day. The tides at Brisbane Bar are a typical example of semidiurnal tides.

Diurnal

Diurnal tide refers to a tide which has a period or cycle of approximately one tidal day (about 25 hours). Diurnal tides usually have one high and one low tide each day. The tides at Karumba are a typical example of diurnal tides.

Highest tides for year

King tide is a non-scientific term, but the popular concept is that it is the higher high waters which occur around Christmas. Equally high tides occur in the winter months during the night.

Meteorological effects on tides

Meteorological conditions which differ significantly from the seasonal averages, will cause corresponding differences between the predicted and the actual tide.

Variations in tidal heights are mainly caused by strong or prolonged winds and by unusually high or low barometric pressure. Tidal predictions are computed for average barometric pressure.

Low pressure systems tend to raise sea levels and high pressure systems tend to lower them. However the water does not adjust itself immediately to a change of pressure but it responds to the average change in pressure over a considerable area.

The effect of wind on sea level, and therefore on tidal heights and times, is variable and depends on the topography of the area in question. In general, it can be said that wind will raise the sea level in the direction towards which it is blowing.

A strong wind blowing straight onshore will cause the water to 'pile up' resulting in high waters to be higher than predicted. Winds blowing off the land will have the reverse effect.

Guide to abbreviations

LAT (lowest astronomical tide) and HAT (highest astronomical tide)

These are the lowest and highest levels which can be predicted to occur under average meteorological conditions and any combination of astronomical conditions.

These levels will not be reached every year. LAT and HAT are not the extreme levels which can be reached, as storm surges may cause considerably higher and lower levels to occur.

LAT has been used as port and chart datum since 1994.

MSL (mean sea level)

The mean level of the sea over a long period (preferably 18.6 years) or the mean level which would exist in the absence of tides.

AHD (Australia height datum)

This datum has been adopted by the National Mapping Council as the datum to which all vertical control for land based mapping is to be referred.

MHWS (mean high water springs)

The long-term mean of the heights of 2 successive high waters during those periods of 24 hours (approximately once a fortnight) when the range of tide is greatest, during full and new moon.

MLWS (mean low water springs)

The long-term mean of the heights of two successive low waters over the same periods as defined for MHWS.

MHWN (mean high water neaps)

The long-term mean of the heights of two successive high waters when the range of tide is the least at the time of first and last quarter of the moon.

MLWN (mean low water neaps)

The long-term mean of the heights of two successive low waters over the same periods as defined for MHWN.

MHHW (mean higher high water)

The mean of the higher of the two daily high waters over a long period of time. When only one high water occurs on a day, this is taken as the higher high water.

MLHW (mean lower high water)

The mean of the lower of the two daily high waters over a long period of time. When only one high water occurs on most days, no value is printed in the MLHW column, indicating that the tide is usually diurnal.

MHLW (mean higher low water)

The mean of the higher of the two daily low waters over a long period of time. When only one low water occurs on most days, no value is printed in the MHLW column, indicating that the tide is usually diurnal.

MLLW (mean lower low water)

The mean of the lower of the daily low waters over a long period of time. When only one low water occurs a day, this is taken as the lower low water.

MHW (mean high water)

The mean of all high waters observed over a sufficiently long period (preferably over the national tidal datum epoch). For those stations with shorter series, simultaneous observational comparisons are made with a control tide station in order to derive the equivalent datum.

MLW (mean low water)

The average of all low waters observed over a sufficiently long period (preferably over the national tidal datum epoch). For stations with shorter series, simultaneous observational comparisons are made with a control tide station in order to derive the equivalent datum.

LWD (low water datum)

The mean height of the lower low waters at springs. This is a local plane which usually satisfies the criterion that the tide seldom fell below it. In the past, this was generally chosen for port and chart datum in Queensland waters, however it was superseded by LAT datum in 1994.

Last updated
03 December 2021