Module 04 · Definitions
Reading notes · Module 04 · Tides
Chart datum and the seven words hung from it: every figure on a chart has an address.
Open the interactive chapter →The same material with the chart, the drawings and the exercises.
The line
Two figures, and neither one is a depth
This is the chart itself — about 2.1 miles of it, with the coast on one side, the five- and two-metre contours out in the bay, and the green of everything that dries at low water.
Two of the figures are boxed, 380 metres apart: 0.7 out in the channel, and 1.8 with a line under it, up on the green.
Both are metres. Both are printed in the same ink, in the same size, on the same paper. And they mean opposite things — one is water you can float in and the other is ground that stands up out of it.
Look at which one is bigger. The larger number is the shallower place — in fact it is not a place you can float over at all for part of the day. If you read a chart expecting big numbers to mean deep water, that is the figure that puts you aground.
A number on a chart says nothing until you know what it is measured from. That is what this part is about, and by the end of it you will know where every figure the chart prints is measured from.
First, the line they are both measured from.
The line everything is measured from
Water, and the ground under it. Nothing else yet — and notice that there is nothing here you could print on a chart, because the water will not stay where it is.
Chart datum. The level the tide is not expected to fall below. It is drawn in magenta because that is the ink the Admiralty uses for tidal information, and it is the only thing in this picture that never moves.
A sounding is measured down from the line to the seabed. That is the figure the chart prints out in the fairway. The tide adds to it: whatever the sounding says, there is that much water there even at datum, and more whenever the tide is up.
And where the ground stands above the line, the chart prints the figure underlined: a drying height, measured up from the datum. Same ink, same size, opposite side. The underline is the whole difference.
Now drag the water up and down. Watch the two figures: they do not move. That is why they can be printed on a piece of paper — they are measured from the line, and the line does not care what the tide is doing.
Three distances
The three words that put boats aground
Height. Sounding. Depth. In ordinary speech they are near enough the same word. On a chart they are three different distances, and one of them is not even on the chart.
You already have two of them. The sounding is the figure you read in the last screen, measured down from the datum. The height of tide is how far the water stands above that same line — and it is the only one that changes while you watch.
The third is the one you actually float in: the depth of water, the whole column standing over the seabed. It is not printed anywhere, because it depends on the day.
And a fourth thing that is none of the three: what your echo sounder reads. It measures from its own transducer, somewhere under your hull — so it is not the depth of water either. That one is for the block on what is under the keel, and it is not this one.
Three distances on one picture
The height of tide, on the left: from the datum up to the surface. It is what the almanac gives you for a time and a place, and it is the same figure everywhere in the harbour at any one moment.
The sounding, in the middle: from the datum down to the seabed. That is the figure printed on the paper, and it belongs to that one spot.
And the depth of water, on the right: from the seabed all the way up to the surface. That is the water you are floating in.
Look at the three bars. The depth of water is the other two stacked: depth = sounding + height of tide. Nothing else comes into it — no draught, no keel, no boat at all. Just the water and the ground.
Now drag the water. Two of the three bars move and one does not. The sounding is the paper's and stays put; the height of tide is the day's; and the depth of water is both of them together, so it moves too.
Whose is each one. The sounding belongs to the chart. The height of tide belongs to the day. The depth of water belongs to the two of them, and it is the only one of the three you can actually run aground in.
Which means you can name any of the three from a sentence, without a single figure. Try it: “it does not change when the tide comes in” — that can only be the sounding, because it is the one printed on the paper. And “the same everywhere in the harbour right now” is the height of tide: one figure for the whole port at one moment. That is the next screen.
And it is still not what is under your keel. That subtraction needs your draught, and that is the block on what is under the keel, not this one.
The four levels
Four numbers the almanac prints once
Everything you have read so far changes: the height of tide changes through the day, the depth of water changes with it. Now four numbers that do not. The almanac prints them once for the port, at the head of its page, and they are the same in March as in November.
They are not predictions. They are averages — what high water and low water come to, on average, on the days of the biggest range and on the days of the smallest. Range is the distance between one high water and the low water beside it: the whole swing, not a height above anything.
And those two kinds of day have names, because every tide table you ever open is arranged around them. The days of the biggest range are springs; the days of the smallest are neaps. At the end of this part you will watch a fortnight turn from one into the other and back.
And an average happens to nobody. Next Tuesday high water will not be the average of anything: it will be whatever it is. What these four give you is the shape of the tide at this port — how far up it usually comes, how far down it usually goes, and how much difference there is between a big day and a quiet one.
Four of them, and you will meet their initials in a moment. The initials come last, on purpose: a set of four capital letters learned before the idea is four capital letters.
Where the four sit on the line
The two at the top: the average high waters. One for the days of the biggest range, one for the days of the smallest. The big one is higher — that is what a big range means at the top of the tide.
And the two at the bottom, the average low waters, the same way round: on the days of the biggest range the water goes furthest down. So the big pair sits outside the quiet pair, top and bottom.
Those are the four figures the almanac prints for this port. Notice they are all above the datum — even the lowest of them. The datum is set at the lowest astronomical tide, and an average low water is not that low.
The distance between a pair is its range: the outer pair gives the big one, the inner pair the small one. That is the whole difference between a lively day and a quiet one , and it is what the next topic but one is about.
Now the initials. M for mean, then HW or LW , then S or N . Read them in that order and they say themselves — and their place on the line says the rest: high up top, low below, springs outside neaps.
The two extremes
The two that never appear in a daily table
Two more levels, and they are of a different kind. The four you have just learned are averages. These two are limits: the highest the tide is ever expected to reach, and the lowest it is ever expected to fall.
They sit outside the four, top and bottom, and no daily table prints them. You will not find them in the columns for Tuesday: they are properties of the port, not of a day.
Astronomical is the word that matters in both their names. It means the tide as the sun and moon would make it , with the weather left out. A hard gale or a deep low can put the water above the highest, and a big high-pressure system can hold it below the lowest.
And there is a third reference on the same chart, measured from neither of these. The elevation of a lighthouse — the height of its light above the sea — is measured up from MHWS. Each of the three answers a different question: the datum for the water under you, HAT for the air over you, and MHWS for how high a light stands.
So a prediction is a prediction. That is worth carrying: every figure you work out in this module is what the sky says, and the sky is not the only thing pushing the water about. What to do about that is the block on what is under the keel, and it is not this one.
Why a bridge hangs from the other end
The lowest astronomical tide: the bottom of the bottom, the level the tide is not expected to go below however long you wait. Look where it falls — it is exactly on the datum line.
And that is not a coincidence in the drawing. The datum is put there on purpose. Set it any higher and the chart would promise water that is not always there; set it lower and every sounding would read deeper than it needs to. So the two are the same line, and the chart writes LAT beside it.
Now the highest astronomical tide, at the top: the most water the sky alone will ever put over the datum here. It is the other limit, and the same idea the other way up.
And here is what it is for. A bridge is charted with a vertical clearance, and that figure is measured up from HAT — not from the datum, and not from today's water. It is the air you have on the worst day of the year , so the chart never promises more headroom than there is. Any other day you have more.
Which is the mirror of the sounding: the chart is pessimistic at both ends. Least water below, least air above.
Watch what more water does to your mast
Nothing here is scored. Drag the water up and down and watch the gap between her masthead and the underside of the bridge.
Every other figure in this part gets better as the tide rises. This one is the exception, and it is the one that takes rigs off boats: more water means less room, because the water lifts her.
The clearance on the chart is measured up from HAT, so it is the worst case — the least air there will ever be. Almost always there is more. Working out how much is a job for later in the module.
Range and the fortnight
Range: the distance between two turns
The picture turns. This is no longer a slice through the water at one moment: it is one day, running left to right, with the datum along the bottom. The water comes up to high water and starts back down.
And down to low water, and up again. Twice each in a day, near enough — which is why the almanac prints four times for most days.
The range is what separates them: high water minus low water. Not a level, not a depth — a difference . The arrow has a foot on each turn and neither of them is the datum.
And here is what it is not. That second arrow goes from the datum up to high water: that is the height of high water, and it is a different number. Range is measured between the two turns; height is measured up from the line. Mixing them is the commonest way to read a tide table wrong.
The fortnight
Fourteen days of Aldern Quay, one after another. Drag along them and watch the range: it grows for about a week, then shrinks for about a week, and then does it again.
The big ones are springs: the biggest tides of the fortnight, around new and full moon. High water is higher and low water is lower than usual.
The small ones are neaps: the smallest of the fortnight, around the quarter moons. Less range, so less water at high tide and more at low.
And notice the times sliding. High water is about fifty minutes later each day — which is why yesterday's tide table is no use to you today. Working out the height at a particular hour is the block on standard port heights; this one only had to teach you what the words mean.
Open the interactive chapter →The same material with the chart, the drawings and the exercises.