Reading the table

Four numbers,
two assumptions.

A tide table row is simple to read and easy to misread. The mistakes are predictable, and two of them are specific to Australia.

01 · The row

The four things a row tells you

Every tide table row carries exactly four pieces of information:

The date.
The time of the turning point, in a stated time zone and time basis.
Whether it is a high water or a low water. Some tables show this as a label, others by the position of the column.
The predicted height of the water at that moment, measured above a stated datum.

And two things it does not tell you:

How deep the water will be where you are. A tide height is not a depth. See below.
What the water will actually do on the day. A prediction is astronomy. Weather moves real water away from it, sometimes by a large margin. Why, and by how much.

02 · The number

The height is not the depth under your boat

This is the single most consequential misreading, and it is worth being blunt about.

A tide height of 1.8 metres does not mean 1.8 metres of water. It means the sea surface is predicted to sit 1.8 metres above the chart datum for that port. To get an actual depth you add the tide height to the charted depth ("sounding") at your position, because charted depths are measured down from the same datum.

Depth of water ≈ charted depth + tide height

So a spot charted at 0.5 metres, at a moment when the tide is 1.8 metres, has roughly 2.3 metres over it. And at a low water of 0.1 metres, that same spot has roughly 0.6 metres, which is the number that matters.

Chart datum in Australia is set near the lowest tide that astronomy is expected to produce, so tide heights are almost always positive. Occasionally they are negative, which means the water is predicted to fall below the datum and there is less water than the chart shows. The full datum explanation is here, and it is the page to read before you trust any height for anything that matters.

03 · Shape

Reading the curve versus reading the table

The table gives you turning points only: the moments of high and low water. Between them the water is moving continuously, and most of the time you actually care about a moment between the rows, not the rows themselves.

A tide curve shows the whole shape and is the better tool when it is available. When you only have the table, you can estimate the in-between heights, and the standard method is the rule of twelfths.

04 · Estimating

The rule of twelfths, worked

The rule assumes the tide takes about six hours to go from low to high, and that it moves in this proportion of the total range each hour:

1/12 · 2/12 · 3/12 · 3/12 · 2/12 · 1/12

In other words it moves slowly at the turn, fastest in the middle two hours, and slowly again at the next turn. Half the total movement happens in the middle third of the cycle.

A worked example

Illustrative figures, not a real prediction for any location.

Low water 0.4 m at 06:00. High water 2.2 m at 12:00. The range is 2.2 − 0.4 = 1.8 m, so one twelfth is 0.15 m.

By 07:00, up 1/12 (0.15 m) → 0.55 m
By 08:00, up 2/12 (0.30 m) → 0.85 m
By 09:00, up 3/12 (0.45 m) → 1.30 m
By 10:00, up 3/12 (0.45 m) → 1.75 m
By 11:00, up 2/12 (0.30 m) → 2.05 m
By 12:00, up 1/12 (0.15 m) → 2.20 m

The same proportions run in reverse on a falling tide. The arithmetic is deliberately crude so it can be done in your head on the water, and that is its whole value.

05 · Limits

Where the rule of twelfths stops working

The rule is an approximation to a sine curve. It is reasonable where the real tide curve is close to a sine curve, and it degrades wherever it is not. In Australia there are three common situations where it should not be relied on.

At a diurnal port

Where there is only one high and one low a day, the water has roughly twelve hours to rise, not six. Applying a six hour rule to a twelve hour rise will put your estimate hours out. Parts of northern Australia, most notably the Gulf of Carpentaria, are diurnal. How to tell which type your port is.

Where the rise and fall are not equal

In many estuaries and rivers the flood runs faster than the ebb, or the reverse, so the curve is asymmetric and the six equal hours the rule assumes do not exist. The further from the open coast, the more distorted the curve tends to be.

Where there is a stand of tide

Some places hold near the top or bottom of the tide for an extended period rather than turning cleanly. The rule will understate how long you have at the turn, which sounds harmless but is exactly the error that strands a vehicle or a boat.

The safe reading: use the rule of twelfths for planning margin, not for clearance. If the answer is within a few tens of centimetres of something that matters, get a real tide curve or the official table for the port rather than an estimate between two rows.

06 · The common error

Local time, standard time and daylight saving

More people get the wrong tide time from a correct table than get a wrong height. The cause is almost always the time basis.

Tide tables state the time basis they are published in, and it is not always the clock time you are living in. A table may be published in local standard time all year, in which case you must add an hour yourself during daylight saving. Others are published in local clock time with the change already applied. Check the note at the top of the table. Do not assume.

Australia makes this worse than most countries:

Queensland does not observe daylight saving. New South Wales, Victoria, Tasmania, South Australia and the ACT do. The Northern Territory and Western Australia do not.
South Australia and the Northern Territory run half hour offsets, so an hour of daylight saving on top of a half hour offset catches people out.
On the day the clocks change, the interval between two consecutive tide rows will look an hour wrong. It is not; the clock moved underneath them.

If you are crossing a state border for a trip, or reading a national table from a state that keeps different time to the one you are standing in, resolve the time basis before anything else.