Charts and Position Fixing

Prerequisites

• You have read the lessons What Is the ICC and Passing the ICC Practical Assessment, so you know that the coastal waters ("C") part of the ICC includes a navigation section done with a chart and plotting instruments.
• You can already handle your type of boat. This lesson covers the chartwork the examiner will ask about; it does not teach boat handling.
• You know the points of the compass and can do simple arithmetic with degrees (adding and subtracting, wrapping past 360).
• Bring, or buy before the test: a Portland-style or Breton-style plotter (or parallel rules), a pair of dividers, a soft pencil (2B), a rubber and a hand-bearing compass if you own one.

Learning Objectives

By the end of this lesson you will be able to:

• Read a navigational chart: latitude and longitude, scale, title block, source data and the most important symbols (ICC coastal syllabus, Navigation 1).
• Explain the difference between a charted depth and a drying height, what chart datum (LAT) is, and work out the depth of water from them (Navigation 1; test guidance, Navigation 1).
• Identify charted hazards: rocks, wrecks, foul ground, shoals and danger lines (Navigation 1).
• Measure distance and direction on the chart, and convert between True, Magnetic and Compass using variation and deviation.
• Take, correct and plot a three-bearing fix, judge the cocked hat and give the position as a latitude and longitude (Navigation 4; test guidance, Navigation 2).
• Explain why a position must be verified from at least two sources, and name practical second sources to check a GPS position (Navigation 4).

What the Examiner Will Ask

The RYA guidance for ICC test centres is short and specific about this part of the coastal test. The examiner is told to:

1. "Use a sample chart and concentrate on the common and important chart symbols, e.g. rocks, wrecks and the difference between a charted depth and a charted drying height."
2. "Prepare a 3-bearing fix and ask the candidate to plot it and give lat/long of the position."

Expect a chart in front of you, a few symbols to explain, then three bearings to plot. The level is RYA Day Skipper; showing the method and the meaning matters more than precision.

The Chart Itself

What a chart is

A nautical chart is a map of the sea and coast drawn on the Mercator projection: a straight line on it is a line of constant direction, so you can lay off a course with a ruler and steer it by compass. The price is that the scale stretches away from the equator, which matters when you measure distance.

The title block

Before you use any chart, read its title block. It tells you:

• The area and the scale, for example 1:75,000. A large-scale chart (1:12,500, say) shows a small area in great detail, such as a harbour. A small-scale chart (1:500,000) covers a wide area with little detail. Use the largest scale available for pilotage and close inshore work.
• The units of depth and height: metres on modern charts, fathoms and feet on some old ones.
• The chart datum for depths (normally LAT; see below).
• The horizontal datum, usually WGS84, the same as your GPS. Older datums come with a correction to apply to GPS positions.
• Source data and date. Some areas, including parts of the Mediterranean, were surveyed long ago. Treat old surveys with caution and leave a bigger margin.

Latitude and longitude

Every position on the chart is defined by two numbers:

• Latitude is the angle north or south of the equator, from 0° to 90° N or S. It is read on the left and right borders of the chart.
• Longitude is the angle east or west of the Greenwich meridian, from 0° to 180° E or W. It is read on the top and bottom borders.

Each degree is divided into 60 minutes ('), and navigators use decimal minutes, not seconds: 50°42.3'N, not 50°42'18"N.

How to read a position from the chart:

1. Put one edge of your plotter (or a pair of dividers) on the point.
2. Run a line horizontally to the side scale and read the latitude where it meets the scale. Count the small divisions carefully; on many charts each minute is split into tenths or fifths.
3. Run a line vertically to the top or bottom scale and read the longitude.
4. Say it in the standard way: latitude first, then longitude, with N/S and E/W. Longitude is given with three figures for the degrees: 001°14.5'W, not 1°14.5'W.

The diagram below shows a fix being read off the border scales of a chart.

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To plot a given lat and long, do the reverse: draw along the latitude, measure the longitude from the nearest meridian with dividers, and mark where they meet.

Measuring distance

One minute of latitude equals one nautical mile (1,852 metres). So you measure distance with dividers on the latitude scale at the side of the chart, at the same height (latitude) as the distance you are measuring.

Never use the longitude scale for distance. At 50°N a minute of longitude is only about 0.64 of a nautical mile, so measuring on the top border would make every distance about a third too short.

Speed is in knots (nautical miles per hour): at 6 knots you cover 1 mile in 10 minutes.

Measuring direction

Directions on the chart are measured clockwise from True north, from 000° to 359°, always written with three figures: 045°T, 270°T. Use your plotter:

1. Lay the edge of the plotter along the line, with the arrow pointing in the direction of travel.
2. Rotate the central grid until its north-south lines are parallel with a meridian (or any north-south line) on the chart, with N towards the top.
3. Read the bearing at the index mark.

The commonest slip is reading the reciprocal (225° instead of 045°). Always sense-check against the rough direction.

Depths, Drying Heights and Chart Datum

This is the topic the examiner is told to concentrate on, so make sure you can explain it cleanly.

Chart datum

The sea level is always changing with the tide. A chart needs a fixed reference level for its depths, called chart datum (CD). On Admiralty charts of UK waters, and on most charts of tidal waters in Europe, chart datum is Lowest Astronomical Tide (LAT): the lowest level the tide can be predicted to fall to under average weather conditions. The tide will rarely fall below it; strong offshore winds and high pressure can occasionally push it a little lower.

Because chart datum is so low, the water is almost always deeper than the chart shows. That is the safe way round.

Charted depths (soundings)

A charted depth, or sounding, is the depth of the seabed below chart datum. It is printed as a number, with tenths shown as a smaller figure after it. A sounding printed as 7 with a small 9 means 7.9 metres below chart datum.

Depth of water at any moment = charted depth + height of tide.

Drying heights

Some areas uncover at low tide: sandbanks, mudflats, rocks and harbour bottoms. Their height is given above chart datum, and the number is underlined. An underlined 1 with a small 4 means the seabed there is 1.4 metres above chart datum; it is dry when the tide is lower than 1.4 m.

Depth of water over a drying area = height of tide − drying height.

On the chart, drying areas are usually coloured green (Admiralty charts), with the low-water line as their seaward edge.

Heights on land and clearances

• Heights of lights and land features are given above Mean High Water Springs (MHWS), so that you know how high the light will appear when the tide is high.
• Vertical clearances under bridges and power cables are given above Highest Astronomical Tide (HAT) on modern Admiralty charts, so the real clearance is normally greater. Check the chart key (the "INT 1" or "5011" symbols booklet) for the chart you are using, because older charts and some foreign charts use MHWS for clearances too.

The diagram below puts the three reference levels together.

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Two worked examples

Charted depth. You plan to anchor where the chart shows a sounding of 2.3 m. The height of tide at the time is 1.8 m. Depth of water = 2.3 + 1.8 = 4.1 m. Your boat draws 1.9 m, so you have 2.2 m under the keel at that moment. Now check what happens at the next low water before you settle on it (the lesson Tides and Course to Steer shows how to read heights from a tide table).

Drying height. The approach to a small harbour crosses a bar with a drying height of 1.2 m (underlined 1 with a small 2). The height of tide is 3.0 m. Depth = 3.0 − 1.2 = 1.8 m. With a draught of 1.6 m you would have only 0.2 m under the keel, and less in any swell. That is not enough. Most skippers want at least 0.5 to 1 m of clearance over a bar, more in waves.

Tidal and non-tidal waters

In the Mediterranean and the Baltic the tidal range is usually a few tens of centimetres, so charted depths are close to what you find. But wind and pressure can lower the sea by a few tenths of a metre, and some charts there use a datum near mean sea level, so the water can be slightly shallower than charted. Keep a margin.

Common Chart Symbols and Hazards

Chart symbols are standardised internationally and listed in the booklet INT 1 (Admiralty 5011), so they work on charts from almost any European country.

Rocks

A dotted line (danger line) drawn round a symbol or group of symbols draws attention to a danger. When you see one, give the whole area a wide berth.

Wrecks

Other symbols you must know

• Foul ground (#): wreckage, old moorings or other obstructions. Fine to sail over, bad to anchor in.
• Depth contours (lines joining equal depths, often 2, 5, 10, 20 m) with the shallow areas tinted blue. Use them to keep in deep water and as a check on your position with the echo sounder.
• Seabed type: S sand, M mud, Sh shells, G gravel, R rock, Wd weed. Sand and mud make good anchorage; rock and weed do not.
• Anchorage (anchor symbol) and prohibited anchorage (anchor with a cross or inside a magenta dashed boundary).
• Lights: a magenta flare with the characteristic, e.g. Fl(2)10s15m12M (two flashes every 10 seconds, 15 m above MHWS, nominal range 12 miles). Buoyage and lights are covered fully in the lesson Buoyage and Harbour Pilotage.
• Tidal diamonds: a magenta diamond with a letter, linked to a tidal stream table on the chart (see the lesson Tides and Course to Steer).
• Abbreviations you will meet: PA (position approximate), ED (existence doubtful), Obstn (obstruction), Dries, conspic (conspicuous, useful for bearings).

How to answer "What is this?"

Answer in two parts: what it is, then what it means for you. For example: "That is a rock which covers and uncovers, with a drying height of 2.1 metres. It will be under water at high tide, so I would plan to stay well outside the danger line." That second sentence is what shows the examiner you are a skipper and not just reciting a symbol list.

True, Magnetic and Compass

The chart is drawn to True north. Your compass points to magnetic north. To use one with the other you must allow for two errors.

Variation

Variation is the angle between True north and magnetic north. It depends on where you are and changes slowly over the years. You read it from the compass rose on the chart nearest to your position. The rose shows something like:

"Var 2°15'W (2015) decreasing 9' annually"

To bring it up to date for 2026: 11 years × 9' = 99' ≈ 1°40'. So 2°15'W − 1°40' = about 0°35'W. For small-boat navigation, round to the nearest degree: 1°W.

Variation varies a lot across Europe. It is close to zero in southern England, a few degrees west in the west of Ireland, and a few degrees east in the Mediterranean (around 4 to 5°E in Croatia and Greece in recent years). Charterers moving from UK waters to the Med must remember that the sign flips.

Deviation

Deviation is the error in a particular boat's compass caused by the boat's own magnetic fields: the engine, steel fittings, loudspeakers, electronics. It changes with the boat's heading and is shown on a deviation card if the compass has been "swung" (checked on different headings). Most charter boats have no deviation card: keep phones and other magnetic items away from the steering compass and check it against a charted transit when you can.

A hand-bearing compass, used on deck clear of metal, has no fixed deviation, so its bearings are treated as magnetic.

Converting

The rule for each step is the same:

• True → Magnetic → Compass: add West, subtract East.
• Compass → Magnetic → True: add East, subtract West.

Memory aids: "Error West, compass best; Error East, compass least" (the compass reading is bigger when the error is West). Or CADET: Compass ADd East to get True.

The diagram below shows the chain and three worked conversions.

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Worked example 1 (course). You plot a course of 250°T in the west of Ireland. Variation is 3°W, deviation on this heading 2°E. True to Magnetic: add West, 250 + 3 = 253°M. Magnetic to Compass: subtract East, 253 − 2 = 251°C to steer.

Worked example 2 (bearing). In Croatia (variation 5°E) you take a hand-bearing of 031°M on a lighthouse. To plot it, Magnetic to True: add East, 031 + 5 = 036°T.

Do a sense check every time: in the Med the true figure is bigger than the magnetic one; in western UK and Irish waters it is a little smaller.

Fixing Your Position

Why fix at all when you have GPS?

The ICC syllabus says you must be able to "fix a position and understand the importance of verification of position from at least two sources". GPS is excellent but it is still a single source, and it fails in ways that are easy to miss:

• Power or aerial failure.
• Signals are jammed or spoofed. GNSS interference has been reported widely in the Baltic and the eastern Mediterranean in recent years.
• The chart or plotter uses a different horizontal datum, so the position is offset.
• An electronic chart, viewed zoomed out, hides a rock that appears only at a larger scale.
• The navigator types a waypoint in wrongly and follows it faithfully onto a reef.

When two independent methods agree, trust the fix. When they disagree, you have caught a problem early.

Ways to check a GPS position

• A compass bearing on a charted object, ideally crossed with a second or third bearing (a full fix, below).
• A transit: two charted objects in line give you a position line with no compass error at all.
• A depth contour: if the plotter says you are on the 10 m line and the echo sounder (corrected for height of tide) reads 4 m, something is wrong.
• A radar range off a charted headland, if the boat has radar.
• Passing close to a buoy you have positively identified.
• Your DR or EP: where you expect to be from the course steered, speed and time since the last fix. A dead reckoning (DR) position uses course and distance through the water; an estimated position (EP) also allows for tidal stream and leeway. If the GPS position is miles from your EP, find out why.

The three-bearing fix

This is the classic visual fix and the one the examiner will ask you to plot.

Choosing the objects. Pick three objects that are:

• Charted and positively identified: a lighthouse, a church spire, the end of a breakwater, a conspicuous tower or headland edge. Avoid buoys if you can: they move on their moorings and can be out of position.
• Well spread in angle: ideally about 60° apart for three objects (or 90° for two). Bearings that cut at shallow angles give a long thin triangle that hides error.
• Reasonably close: an error of one degree moves a position line about 1/60 of the distance to the object, so a bearing on a headland 2 miles away is better than one on a mountain 20 miles away.

Taking the bearings.

1. Stand somewhere clear of metal with the hand-bearing compass, braced against the boat's motion.
2. Take the bearing of the object nearest the bow or stern first (its bearing changes slowly) and the object nearest the beam last (its bearing changes fastest).
3. Take all three quickly, write them down, and note the time and log reading straight away.

Plotting.

1. Convert each bearing to True with variation (no deviation for a hand-bearing compass).
2. Put the plotter on the chart at the bearing, with its edge passing through the charted object.
3. Draw a line out to sea from the object, towards where you are. You only need the part near your position.
4. Repeat for the other two. Mark the end of each line with a single arrowhead pointing away from the object.
5. The three lines will rarely meet at a point. They form a small triangle called the cocked hat.
6. Mark the fix with a dot in a circle and write the time beside it, e.g. 1030.

The diagram below shows a typical three-bearing fix and its cocked hat.

Judging the cocked hat

• Small triangle (a few cables across or less): a good fix. Take the centre, or if there is danger nearby, assume you are at the corner closest to the danger and navigate from there.
• Large triangle: something is wrong. Check, in this order: did you identify the objects correctly; did you apply variation the right way (the commonest error: a 10° triangle often means variation applied in the wrong direction twice); did you plot the right number; did you take the bearings slowly while the boat moved quickly; is there a magnetic item near you or the compass. Take a fresh set of bearings rather than trusting a bad fix.

Worked example: plotting the examiner's fix

The examiner gives you this, with variation from the chart rose 2°W:

Magnetic to True, subtract West, so each True bearing is 2° less. Plot the three lines from the three objects. The three directions are spread nicely (NE, NW and S), so the cocked hat should be small. Mark the fix, then read off the position: suppose it reads 50°42.3'N 001°14.5'W. Say it aloud exactly like that: "Fifty degrees, forty-two point three minutes north; zero zero one degrees, fourteen point five minutes west." Then compare it with the GPS. If they agree within a cable or two, you have verified your position from two sources.

Worked Example: Making Sure in Croatia

Sophie has chartered a yacht in Croatia and is approaching a narrow passage between two islands. The plotter shows her in mid-channel. She checks with three hand-bearings: lighthouse 352°M, chapel 081°M, headland astern 205°M. With variation 5°E (Magnetic to True, add East) she plots 357°T, 086°T and 210°T. The cocked hat is small, but 0.3 miles west of the GPS position, closer to a rocky shoal marked with a plain cross inside a danger line. The echo sounder reads 6 m: the plotter position shows 18 m, her fix shows the 5 to 10 m zone. The bearings and the depth agree with each other, not with the plotter.

She slows down, steers to keep clear of the shoal, and later finds that a software update had set the plotter to the wrong datum. Two independent sources told the same story, and she believed them.

Common Mistakes

• Reading depth as if it were the depth now. A sounding is below chart datum; add the height of tide. A drying height is above chart datum; subtract it from the height of tide.
• Missing the underline. An underlined figure is a drying height, not a depth. Confusing them can put you aground on a falling tide.
• Measuring distance on the longitude scale. Always use the latitude scale at the side, level with your work.
• Applying variation the wrong way. Write the rule on the chart table: True to Magnetic, add West. Then sense-check the answer.
• Taking bearings next to the engine, a speaker or the guardrail stanchion with a phone in your pocket. Move clear and stow magnetic items.
• Trusting a big cocked hat. Find the error or retake the bearings.

Summary

• Latitude is read on the side borders, longitude on the top and bottom. Give positions latitude first, in degrees and decimal minutes, with N/S and E/W.
• One minute of latitude is one nautical mile. Measure distance on the side scale only.
• Chart datum is usually LAT. Charted depths are below it; drying heights (underlined) are above it. Light heights are above MHWS; clearances usually above HAT.
• Depth now = charted depth + height of tide; over a drying area, depth = height of tide − drying height.
• Know the rock and wreck symbols, danger lines and foul ground, and say what each means for your boat.
• True to Magnetic to Compass: add West, subtract East. Reverse the rule going back to True.
• A three-bearing fix uses three charted, well-spread, nearby objects; beam bearing last; a small cocked hat is a good fix.
• Always verify a position from at least two independent sources: GPS plus a bearing, transit, depth or radar range.

Check Your Understanding

1. A chart shows a sounding of 3 with a small 4 at your planned anchorage. The height of tide is 2.1 m. What is the depth of water?

Answer: 3.4 + 2.1 = 5.5 m. Charted depths are below chart datum, so you add the height of tide.

1. A sandbank shows an underlined 1 with a small 8. What does this mean, and how much water is over it when the height of tide is 2.5 m?

Answer: It is a drying height: the bank is 1.8 m above chart datum. Depth = 2.5 − 1.8 = 0.7 m, not enough for most yachts.

1. What is chart datum on most charts of UK and tidal European waters, and why is it chosen?

Answer: Lowest Astronomical Tide (LAT), the lowest predictable tide. Because the sea rarely falls below it, the actual depth is almost always at least the charted depth, which errs on the safe side.

1. The examiner points to a plain cross (+) inside a dotted line. What is it?

Answer: An underwater rock of unknown depth, dangerous to surface navigation, with a danger line drawn round it. Keep well clear of the whole area inside the line.

1. Why should you never measure distance on the top or bottom border of the chart?

Answer: Those borders show longitude. A minute of longitude is shorter than a nautical mile everywhere except the equator (about 0.64 M at 50°N). Only a minute of latitude equals one nautical mile, so use the side scale level with your work.

1. Variation is 4°E. You take a hand-bearing of 125°M on a lighthouse. What do you plot?

Answer: Magnetic to True, add East: 125 + 4 = 129°T. No deviation is applied for a hand-bearing compass used clear of metal.

1. You plot a course of 300°T. Variation is 2°W and there is no deviation. What do you steer?

Answer: True to Magnetic, add West: 302°M (and 302° on the compass, with no deviation).

1. In a three-bearing fix, which bearing should you take last, and why?

Answer: The object nearest the beam, because its bearing changes fastest as the boat moves. Objects ahead or astern change slowly and can be taken first.

1. Your three bearings produce a cocked hat over half a mile across. What could be wrong?

Answer: A misidentified object, variation applied the wrong way, a plotting error (wrong figure or wrong object), bearings taken too slowly, or a magnetic object near the compass. Check each and retake the bearings rather than trusting the fix.

1. Why does the ICC syllabus stress verifying your position from at least two sources, and what would you use to check a GPS position?

Answer: GPS can fail, be jammed, use the wrong datum, or be misread (wrong waypoint, zoomed-out chart). An independent check, such as a compass bearing or three-bearing fix, a transit, the echo sounder against charted contours, or a radar range, catches those errors before they put you aground.