Reading the chart

Depths: the numbers in the sea

The small figures scattered over the sea are soundings. Each one is the depth at that spot, in metres, measured from a low reference level called chart datum. On most modern charts chart datum is about the lowest the tide ever falls, so a sounding is close to the least water you will find there; most of the time there is a little more.

In shallower water the chart also gives the tenths, printed as a smaller figure set slightly lower down. Read the two as one number: 7₄ is 7.4 m and 13₉ is 13.9 m. In deep water only whole metres are printed, because a few tenths no longer matter.

How much the tide adds depends on where you are. In most of the Mediterranean the tide rises and falls by only a few tens of centimetres, and in the Caribbean it is small too, so the charted depth is close to what the depth sounder will show. On tidal coasts such as the Atlantic shores of Europe the tide can add several metres, and working it out becomes part of the plan.

Contours and colours

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

Contours also show the shape of the seabed. Where they crowd together the bottom shelves steeply; where they are far apart it shoals gently. That matters when you anchor: volcanic islands such as the Aeolians, and the inner bay of Kotor in Montenegro, are deep almost to the shore, so you need plenty of chain and a spot where the contours open out. Out to the south-east of Kerran, Brennan Bank rises from deep water inside rings of contours to just 2₁: 2.1 m, in the middle of open sea.

Ground that dries

Close to the shore you may find figures that are underlined. An underlined figure is a drying height: the height of ground that is uncovered at low tide, measured up from chart datum. 0₆ means the ground there stands 0.6 m clear of the water at the lowest tide. Drying areas are tinted green. You will see few of them in the Mediterranean, where the tide is small, but on tidal coasts whole bays can empty at low water.

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

Rocks, wrecks and the danger line

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

The dotted ring is a danger line, drawn round a hazard to make it stand out. A wreck is a long line crossed by short strokes; when its depth is known it is printed with Wk instead. South-west of Kerran the chart shows 4₂ Wk: 4.2 m over the wreck at chart datum. That is enough water to sail over, but a wreck can foul an anchor, so never anchor on one.

On land the chart shows only what helps you from the sea: the coastline, hilltops such as Beacon Hill at 87 m, and landmarks you can see from offshore, such as the church spire, the radio mast and the old tower. Lighthouses are marked with a magenta flare and their light described beside them: Gull Point light stands 28 m high. Lesson 7 decodes the lights.

What the seabed is made of

Small italic letters on the chart describe the seabed. They matter more to a charter skipper than almost anything else on the chart, because they tell you whether the anchor will hold.

In the Mediterranean the weed is usually Posidonia seagrass, which grows in dark meadows between pale patches of sand. It holds an anchor badly, and it is protected in many places. Across the Balearics, Mallorca included, anchoring on it is banned and patrol boats check anchored yachts in summer; many Corsican bays and parts of the Maddalena national park in Sardinia have similar rules. The answer is the same everywhere: drop the anchor on a pale sand patch, or use a buoy.

In the Caribbean and the Indian Ocean the same goes for coral. In the British Virgin Islands and the Seychelles you anchor only on clear sand, which shows as pale turquoise water, and never on coral or seagrass. Snorkel over the anchor to check it has dug in.

The compass rose

The magenta compass rose lets you measure directions on the chart. It has two rings:

• The outer ring is true: 000° points to true north, along the meridians to the top of the chart.
• The inner ring is magnetic: it is turned by the local variation, the angle between true north and where a compass points.

In the middle is a note giving the variation, the year it applies to and how much it changes each year. On the practice chart it reads 3°00′W (2025) (8′E): 3° west in 2025, changing by 8′ east a year. In the Mediterranean variation is only a few degrees and changes slowly; in the Caribbean it is more than ten degrees west, enough to put you well off course if you ignore it. Lesson 4 shows how to use it with the hand-bearing compass.

Latitude, longitude and reading a position

Every place on Earth has an address made of two numbers. Latitude is how far north or south of the equator you are, from 0° at the equator to 90° at the poles. Longitude is how far east or west of the meridian through Greenwich you are, from 0° to 180°. Each degree is divided into 60 minutes, and on a chart the minutes are divided again into tenths.

A position is always written latitude first, with three-figure degrees for longitude. The Mediterranean bases are north and east, the Caribbean is north and west, and the Seychelles are south of the equator:

On the chart, latitude is marked up both sides and longitude along the top and bottom. To read a position, go straight across from the point to the side scale and read the latitude, then straight up or down to the top or bottom scale and read the longitude. West of Greenwich, longitude increases to the left; east of it, to the right.