This module guides you through how tides work and how to carry out tidal calculations using the Tide Tables for the purposes of achieving AMSA Master qualifications. This module uses the 2025 Queensland Tide Table as a reference for exercises, references and notes acknowledging the copyright, acknowledgements and disclaimers found on the inside cover of the MSQ Queensland Tide Tables.
What you will learn in this module
Understanding tidal definitions, reading standard port tables, calculating tides at secondary ports and applying UKC calculations.
Introduction
Tides are the periodic rise and fall of water levels in oceans, seas, bays, and other bodies of water, primarily caused by the gravitational pull of the moon and, to a lesser extent, the sun. This gravitational interaction creates a bulge of water on both the side of the Earth facing the moon and the opposite side, leading to high tides at these locations. As the Earth rotates, different areas experience high and low tides as they pass through these tidal bulges.
When the sun, Earth, and moon align (during full and new moon phases), the gravitational forces combine, leading to higher high tides and lower low tides, known as spring tides. When the sun and moon are at right angles to each other (during the first and third quarter phases), the gravitational forces partially cancel each other out, resulting in smaller tidal ranges, called neap tides.
Tide tables can be used for any given locale to find the predicted times and tidal range. They are however only predictions, the actual time and height of the tide is affected by wind and atmospheric pressure. Most coastal areas experience semi-diurnal tides which are two high tides and two low tides in a day, due to the moon’s orbital period and the Earth’s rotation. Other locations have diurnal tides being one high and low tide each day.
The Tide Tables provide predictions for Standard ports and Secondary Places as well as tools for calculating tide levels at anytime during the day. Additionally, it provides tables for Sun and Moon rise and set.
More scientific specifics for Tides on Wikipedia
Tidal Definitions and Notes
Following are some of the most common definitions in the Queensland Tide Table publication. More definitions and notes available in the publication itself.
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.
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 utilising a navigational chart, tidal height should be added to chart depth. If preceded by a minus sign, it should be subtracted.
Secondary places – are those for which daily predictions are not provided in the Queensland Tide Tables. 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.
Tidal Levels – A list of tidal levels referred to Queensland Port Datum for standard ports and selected secondary places is given in the following tables:
- Semidiurnal Tidal Planes
- Diurnal Tidal Planes
In addition, the tables for semidiurnal and diurnal tidal planes provide the factors necessary to calculate tidal predictions at the selected secondary places (referred to Queensland Port Datum at each secondary place) from the tidal predictions of 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 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 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.
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 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.
Always check the latest marine weather forecast and add a safety margin when tide predictions are critical to passage planning.