Module 01 · Reading the seabed
Reading notes · Module 01 · The chart
Soundings and chart datum, contours and colour, what dries — and the dangers.
Open the interactive chapter →The same material with the chart, the drawings and the exercises.
The sounding
Is there water enough to get in?
You can say where Stennick Haven is, how far it is and which way to steer. That was the last part.
What you cannot say is whether your boat will float there.
That answer is on the chart already, written in blue. This chapter teaches you to read it.
A sounding is a minimum, not a depth
Every blue figure on the water is a sounding: how much water there is at that spot.
But not how deep it is now. It is what there is with the tide at the lowest it ever goes. It is the least you will find, and almost always there is more.
That is why it is safe to plan with. If your boat fits over the sounding, it fits at any state of the tide.
How it is written, and where it is measured from
The small figure is the decimal, not a footnote. 23 is two metres thirty, not two and then three.
All of it is measured down from one line called chart datum, chosen so low that the tide hardly ever falls below it. That is what makes a sounding a minimum.
How much the tide adds above that line, and when, is module 04. Here you only read what the chart already says.
Contours and colour
The lines join equal depths
Two thin lines run through the water: the 2 metre contour and the 5 metre one.
Every point along a contour is the same depth. Cross one and you have gone from deeper to shallower, or the other way.
A contour is not a wall. It is a boundary, and it tells you which side of it you are on.
Dark blue is shallow. White is deep.
The contours divide the water into bands, and the bands are tinted. The darker the blue, the less water.
This is what the colour is for: you can see the shape of the seabed without reading a single number. One look tells you where the deep water runs and where it shoals.
Numbers are for when you need the exact figure. Colour is for the glance you take every few minutes.
What dries
Green is neither land nor water
The green is ground that covers and uncovers. Water over it at high tide, dry at low tide.
It is not land: you cannot walk it at any state of the tide. It is not water: you cannot float there at any state either.
It is the part of the chart that catches people out, and it does so with a number.
The underlined figure goes UP
On the green you will find figures written the same way — big number, small decimal — but underlined.
That underline reverses the direction. It is not depth below datum: it is height above it. How far the ground stands up out of the water at the lowest tide, not how far down it lies.
Same format, opposite sense. It is the single most misread thing on a chart.
What that means for you
An underlined 73 is a bank standing 7.3 metres proud of datum. At low water it is a hill of mud. At high water there may be enough over it to float.
Whether there is, and when, is module 04. What this chapter gives you is the number the tide has to beat.
On this chart the drying heights run from 0.7 m to 7.3 m.
Dangers
Every danger is filed by where it sits
Not by how big it is, and not by how bad it is. By where it sits relative to chart datum — the same line every sounding is measured from.
Five rungs, top to bottom. That ladder is the whole classification; everything else is detail of the drawing.
The danger line measures nothing
The ring of dots round a danger is not its size, and it is not that its position is doubtful.
It is a shout: this would not stand out on its own, or there is more than one of them in here. Do not go in.
That is why not everything has one. A wreck drawn as its own sounding stands out by itself. A bare cross does not — so the cross gets the ring.
The number is the decision
Two wrecks on this chart, and they are not even drawn the same. One is printed as its own sounding, 34 marked Wk, with a bracket under the figure that means it was checked with a wire drag: 3.4 metres over it at the lowest tide.
The other is the hull-and-masts symbol, which is the one used when the least depth is not known. No figure at all, and a danger line round it.
With a number, you know how much water you have and you decide. Without a number, there is no decision to make: you do not pass.
And when nobody is sure where it is
Sometimes there are two letters beside the symbol.
That — not the ring — is what says the position is not to be trusted.
A danger marked PA may not be exactly there, so you give it more room, not less.
The obstruction on this chart is marked PA.
Open the interactive chapter →The same material with the chart, the drawings and the exercises.