Reading a chart

A tour of the chart

In Lesson 3 you used the scales around the edge of a chart to read and plot a position. Now we look at everything else printed on it. A chart can look crowded at first, but every chart follows the same conventions, so once you know where to look, any chart — from a harbour plan to an ocean chart — starts to make sense.

In UK waters the standard paper charts are Admiralty charts, published by the UK Hydrographic Office (UKHO), and our practice chart follows their style. Every symbol and abbreviation they use is explained in a booklet called NP5011, Symbols and Abbreviations used on Admiralty Paper Charts. You don’t need it yet: Lesson 8 introduces the symbols that matter most.

The title block

The title block is the chart’s label, placed where it hides nothing important. Read it whenever you pick up a chart you don’t know, because it answers questions you must never guess:

• The title — which stretch of coast or sea the chart covers.
• The units — modern Admiralty charts give depths and heights in metres. Some older charts used fathoms and feet, and the title block tells you which.
• The levels that depths and heights are measured from (chart datum and MHWS, explained below).
• The projection and the scale — usually Mercator, and a scale such as 1:50 000.

The practice chart’s title block says: Depths in metres below chart datum · Heights in metres above MHWS · Mercator projection · Scale 1:50 000 (approx.) — and, in red, Not for navigation, because the Isle of Kerran is fictional.

Before using any chart, read its title block: what area, what units, measured from where, and at what scale.

Chart scale: large and small

The scale compares a distance on the chart with the same distance in reality. On a 1:50 000 chart, one unit on the paper — a centimetre, an inch, anything — stands for 50 000 of the same units on the sea.

“Large scale” and “small scale” sound back to front at first. A large-scale chart has a small number after the colon, such as 1:10 000: everything is drawn large, so it covers a small area in great detail. A small-scale chart has a big number, such as 1:500 000: everything is drawn small, so it covers a large area with much less detail. A good memory aid: large scale, large detail.

Charts are published at many different scales, and there are no fixed boundaries between the kinds. The table is a rough guide, and its scales are only examples.

The Mercator projection

The Earth is round and a chart is flat, so every chart has to distort something. Most charts you will use are drawn on the Mercator projection, which makes two choices:

• Meridians are drawn as parallel straight lines. On the globe they meet at the poles, so the chart stretches everything east–west, more and more the further you are from the equator.
• North–south is stretched by the same amount, so that shapes stay correct. That means the latitude scale gets longer towards the poles too.

The pay-off is that a straight line on a Mercator chart crosses every meridian at the same angle. It is a line of constant direction, called a rhumb line, so a course drawn with a ruler is one you can steer.

Colours, contours and soundings

The colours give you an instant picture of where you can and can’t take a boat:

The depths at which the blue tints stop vary from chart to chart. On the practice chart, darker blue is less than 5 m deep at chart datum, pale blue is 5 to 10 m, and white is deeper than 10 m.

A depth contour is a line joining points of the same 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. Contours close together mean the seabed shelves steeply; widely spaced contours mean a gentle slope.

The small numbers scattered over the sea are soundings: individual measured depths in metres below chart datum. In shallower water the tenths are shown as a smaller figure, so 2₁ on Brennan Bank means 2.1 m. Lesson 8 covers soundings, rocks and wrecks in detail.

Chart datum, depths and heights

A depth is only useful if you know what it is measured from. Because the tide rises and falls, charts measure every depth from a fixed low level called chart datum (CD). In UK waters chart datum is approximately Lowest Astronomical Tide (LAT): the lowest level the tide can be predicted to fall to in average weather. So there is rarely less water than the chart shows, and most of the time there is more.

• Depths (soundings) are measured down from chart datum.
• Drying heights are measured up from chart datum and are underlined: an underlined 1₂ means the ground dries 1.2 m.
• Heights of land, landmarks and lights are measured above mean high water springs (MHWS), the average height of high water at spring tides. Gull Point light’s Fl(3)10s28m15M includes its height: 28 m above MHWS.

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, lined up with the meridians.
• The inner ring is magnetic: it is turned by the local variation — the angle between true and magnetic north — so its 000° points to magnetic north, where a compass points.

Directions read from the true ring are written with a T, such as 045°T; directions read from the magnetic ring are written with an M, such as 045°M.

In the middle is the variation statement. On the practice chart it reads VAR 3°00′W (2025) (8′E): in 2025 the variation was 3°00′ west, and it changes by 8′ east each year. A change in the same direction as the variation makes it bigger; a change the other way makes it smaller. Here the change is east and the variation west, so the variation gets 8′ smaller every year. That is why the year matters.

The border scales

Around the edge of the chart are the border scales: latitude up the left and right sides, longitude along the top and bottom. Each is divided into minutes, shown by the alternating black and white bands, and on a chart of this scale each minute is divided into tenths by small ticks.

In Lesson 3 you used them to read and plot positions. They have a second job: because one minute of latitude is one nautical mile, the latitude scale is also the chart’s scale of distance.

Notes, cautions and source data

Real charts carry printed notes and cautions for things a symbol can’t say: the rules of a restricted area, a warning that sandbanks shift and depths may have changed, or a table of tidal streams. Read every note that applies to the area you will sail in.

Admiralty charts also show a source data diagram: when and how each part of the chart was surveyed. Some areas are still charted from old or partial surveys, which can miss small dangers, so give yourself a bigger margin there. The practice chart keeps things simple and has neither.

Keeping charts up to date

The sea keeps changing: buoys are moved, lights are altered, wrecks appear and sandbanks shift. The UKHO publishes corrections every week in Admiralty Notices to Mariners (NMs). A paper chart is corrected by hand, and each correction is recorded by its year and number in the bottom left-hand margin, so you can see at a glance how up to date the chart is.

When a chart has collected too many changes, a new edition is published with a new edition date printed in the margin, and the old edition is cancelled.

Paper and electronic charts

Electronic charts come in two kinds. Everything in this lesson — datums, colours, symbols — applies to both.

Module 5 covers chart plotters, and how to use them safely alongside paper.