Chart symbols and depths

Soundings: the depths on the chart

In Lesson 4 you learned that the depths on a chart are measured from chart datum: a low reference level, approximately the lowest astronomical tide (LAT) — the lowest level the tide can be predicted to fall to in average weather. Because chart datum is so low, the sea is nearly always above it, so the depths on the chart are close to the least water you will find.

The small figures scattered over the sea are soundings. Each one is the depth at that spot, in metres, below chart datum. In shallower water the chart also gives the decimetres (tenths of a metre), printed as a smaller figure set slightly lower down: 7₄ means 7.4 m and 13₉ means 13.9 m. In deeper water a few tenths no longer matter, so only whole metres are printed — on the practice chart, anything 21 m or more.

Drying heights

Close to the shore you will find figures that are underlined. An underlined figure is a drying height: the height of ground that uncovers at low water, measured upwards from chart datum. 0₆ means the ground there dries 0.6 m — at a chart-datum low tide it stands 0.6 m clear of the water.

Drying areas are tinted green. They lie between the low-water line (where the depth is zero at chart datum) and the coastline, which is drawn at the high-water line (MHWS). On the practice chart look along the coast north of the old tower: the foreshore is marked 0₆. Rocks can dry too: Mew Rock, south-west of Port Kerran, dries 1.2 m.

A plain figure is a depth below chart datum. An underlined figure is a height above it.

The depth you actually have

A chart can’t know the state of the tide when you arrive, so it gives every depth at chart datum and leaves the tide to you. The height of tide is how far the sea surface is above chart datum at a given moment. Tide tables give it, and Module 4 shows you how to work it out for any time. Once you have it, the arithmetic is simple:

If the second answer comes out negative, the ground is uncovered: it stands that far above the water.

Depth contours and tints

A depth contour is a line joining points of equal depth, just like the height contours on a land map. The practice chart has contours at 2, 5, 10 and 20 m, each labelled with its depth. Between them, colour tints make shallow water stand out at a glance:

Contours also show the shape of the seabed. Where they are close together the bottom shelves steeply; where they are far apart it shoals gradually. Out to the south-east, Brennan Bank rises from deep water inside rings of 20, 10 and 5 m contours to a least depth of 2₁ — 2.1 m at chart datum, in the middle of open water.

Rocks

Rocks are drawn four ways, depending on whether — and when — they show above water:

The dotted ring is a danger line: it is drawn round a hazard to make it stand out from the soundings around it. You will see it round The Teeth, round the wreck south-west of the island, and on real charts round dangerous rocks and obstructions.

Wrecks and obstructions

A wreck whose depth is unknown, but which is considered dangerous to surface navigation, is drawn as a long line crossed by three short strokes inside a dotted danger line. When the least depth over a wreck is known, the chart prints that depth with the abbreviation Wk instead. South-west of the island the practice chart shows 4₂ Wk in a dotted line: 4.2 m over the wreck at chart datum. Even at chart datum that is plenty for most yachts in normal conditions — but a wreck can foul an anchor, so don’t anchor on one.

An obstruction is anything else on the seabed that could damage or foul a boat — debris or an old mooring block, for example. It is labelled Obstn, inside a danger line where it is a hazard, with its least depth when that is known.

Heights and landmarks

Heights on land, and the heights (elevations) of lights, are measured above mean high water springs (MHWS) — the average height of high water at spring tides, about the highest the sea normally reaches. So for most of the time a landmark stands a little higher above the water than the chart says. Altogether, the figures on a chart are measured from just two levels:

Inland, the chart shows only what helps you from seaward: the coastline, hilltops, and conspicuous landmarks such as the church spire, the radio mast and the old tower on the Isle of Kerran. These matter because you can take a compass bearing of them: two or three bearings of well-chosen landmarks give you a fix — your position — which you will learn in Module 2. On Admiralty charts a landmark that is especially easy to see from seaward is usually labelled in capital letters or marked “conspic”. The practice chart keeps its labels simple, but its spire, mast and tower are all conspicuous.

At sea, a magenta anchor marks an anchorage. On the practice chart, Kerran Roads, north of the island, is a recommended anchorage sheltered from southerly winds.

Lights and buoys at a glance

A light is marked with a magenta flare, and a lighthouse is drawn as a star with the flare beside it. Beside it is its characteristic — Gull Point is Fl(3)10s28m15M. The “28m” is the light’s elevation above MHWS; the rest tells you how it flashes and how far it can be seen. Buoys are drawn by shape and colour — the red cans and green cones either side of the channel into Port Kerran, for example — and a small magenta flare means the buoy is lit. Module 3 decodes lights, buoys and their colours in full.

Every symbol and abbreviation used on Admiralty charts is listed in NP5011, the Admiralty’s guide to chart symbols and abbreviations. Nobody memorises all of it, but you should recognise the common ones on sight. Explore them here: