At first glance, a nautical chart looks like an ordinary map with a few extra numbers scattered across it. In reality, it uses its own system of symbols that tells you everything you need for a safe passage: how deep the water is, where the dangers lie, and how to fix your position. For the German boating licence SBF Coastal (SBF See), reading a nautical chart is a fixed part of the exam material. This article explains the basics so you can find your way around any chart.

Why a nautical chart isn’t a road map

A road map shows you where you’re allowed to drive. A nautical chart mainly shows you where you shouldn’t go. Shoals, wrecks, restricted areas and currents are often invisible on the water, but they’re precisely recorded on the chart. That’s why the chart is your most important tool for passage planning and for continuously checking your position on board.

Official German nautical charts are published by the Federal Maritime and Hydrographic Agency (Bundesamt für Seeschifffahrt und Hydrographie, BSH). It issues charts in various editions and updates them regularly so that new construction sites, wrecks, or changed channels are taken into account.

Chart types and scale: from overview to detail

Nautical charts come in different scales, depending on what they’re meant for:

  • Overview charts cover large sea areas and are used for rough passage planning. They show little detail but a wide area.
  • Coastal charts are designed for coastal navigation and show significantly more detail, such as landmarks, sea marks and depth contours.
  • Harbor plans depict individual harbors and approaches at very fine resolution, including quay walls, berths and narrow channels.

The rule of thumb: the larger the scale, the smaller the area shown, but the more detail you see. In practice this means: use the overview chart for rough planning, and switch to the more detailed, larger-scale chart when you’re actually navigating along the coast or entering a harbor.

Every chart shows its scale and chart number in the margin, along with notes on the reference system used and the publisher.

Fixing your position: latitude, longitude and the nautical mile

Two scales run along the edges of every nautical chart:

  • The side edges (east and west margins) show geographic latitude in degrees and minutes.
  • The top and bottom edges show geographic longitude.

For measuring distances, one important rule applies: always measure distances on the latitude scale at the side of the chart, and as close as possible to the level of the leg you’re measuring. The reason: one arc minute of latitude corresponds exactly to one nautical mile (1 nm = 1,852 m). The longitude scale doesn’t work for this, because the meridians converge toward the poles, so a minute of longitude covers a different distance depending on your latitude.

This relationship between the nautical mile and the latitude minute is the basis for distance and speed calculations (knots = nautical miles per hour) and comes up regularly in the navigation task.

The Mercator projection: why course lines are straight

Most charts used for coastal and offshore navigation use the Mercator projection. Meridians and parallels of latitude are shown as straight, mutually parallel lines rather than as the curved lines they’d be on the surface of a sphere. The big advantage: a line of constant compass bearing (called a rhumb line) appears on the chart as a straight line. That makes practical chart work much easier, because you can plot a course with parallel rulers directly as a straight line between your starting point and destination, instead of constantly recalculating it.

The downside of the Mercator projection only shows up at very large distances: areas and distances become increasingly distorted toward the poles. For the areas you’ll sail with the SBF Coastal, though, this practically never matters.

Reading depth figures: chart datum and sounding values

The numbers shown in the water on the chart are soundings (depth figures), usually in meters. They don’t refer to the current water level, but to a fixed reference plane called chart datum (CD). This level is deliberately set low, so that under normal conditions the actual depth doesn’t fall short of the figure shown. The actual water depth at a given spot is the charted depth plus the current tidal height. How to calculate that tidal height is explained in detail in the article on the rule of twelfths.

Drying areas are especially important: areas that emerge from the water at low tide, such as tidal flats. On the chart they’re shown with different coloring or shading than permanently submerged areas, and the associated height figures are usually underlined to distinguish them from ordinary depth figures.

Depth contours connect points of equal depth and give you an instant sense of how shallow or steep the water is at a given spot. Closely spaced contour lines mean a steep drop-off, while widely spaced lines mean a shallow, gently sloping bottom.

For passage planning, it helps to mark an imaginary safety contour in advance: a depth contour you don’t want to cross given your draft, with a reasonable margin. That way you can see at a glance which areas to avoid on your planned course, instead of comparing every single depth figure against your draft one by one.

Spotting shoals, wrecks and other hazards

Beyond the plain depth figures, the chart marks individual hazards with their own symbols:

  • Individual shoals and rocks are marked with specific symbols, sometimes noting whether they’re always visible or only at low water.
  • Wrecks have their own symbol and are often additionally marked with an isolated danger mark as a sea mark.
  • Restricted areas (such as military exercise areas, nature reserves or cable routes) are set off with color or dashed boundary lines and explanatory text.

If you’re unsure how to read a symbol, the chart legend, included with every chart set, explains every symbol used.

Landmarks and bearing objects for chart work

To fix your position by cross bearing, you need clearly identifiable landmarks that are unmistakable both in reality and on the chart: lighthouses, church towers, prominent buildings or distinctive terrain features. Such bearing objects are highlighted on the chart, often with their exact position and additional information such as height or, for lights, their light characteristic.

For a reliable cross bearing, ideally take three bearings on different landmarks. If the three bearing lines don’t intersect at a single point on the chart but instead form a small triangle, that’s called the cocked hat. Your most likely position lies in or near that triangle, and a small cocked hat tells you your bearings were taken cleanly.

Sea marks and the compass rose on the chart

Floating and fixed sea marks, meaning buoys, beacons and lights, are also entered on the chart at their exact position with the matching symbol. How to correctly interpret these marks while underway, for example whether a buoy marks the side of a channel or a danger, is explained in detail in the article Sea Marks & the Lateral System Explained.

Larger coastal charts also show one or more compass roses. They show you the true north arrow as well as the variation that applies to that chart section, which you need for course conversion. The full calculation, including the sign rule, is covered in the article Course Conversion for the SBF Coastal.

Practical chart work: plotting course and distance

For the navigation task you need more than just reading skills, you also need to actively work with the chart. Two tools are essential:

  • Dividers: used to pick up a distance on the chart and transfer it to the latitude scale at the side margin, at the same level as the leg you measured.
  • Parallel ruler (or plotting triangle): used to draw a course line between two points and “walk” it, parallel, to the nearest compass rose to read off the true course.

You plot your own position on the chart with the date, time and, ideally, your dead-reckoning position (derived from course, speed and elapsed time). That way you, or anyone checking afterward, can always trace where you were and where you’re heading next. This interplay of reading the chart and actively working on it is exactly what the navigation task tests in the SBF Coastal exam.

Keeping charts up to date

Nautical charts aren’t static documents. New wrecks, changed channel depths or newly placed sea marks are continuously published through Notices to Mariners, which chart owners use to correct their charts by hand or digitally. Anyone navigating with outdated charts risks missing important hazards or finding that sea marks are no longer where the chart shows them.

Paper chart and electronic chart

Many boats today also carry a chartplotter or a navigation app that displays the same information digitally. The symbols and principles stay the same, only the display is electronic instead of printed. The SBF Coastal exam still requires working with a paper chart, though, because once you understand the underlying system by hand, you can transfer it effortlessly to any plotter, regardless of manufacturer. The paper chart also remains a reliable backup if the onboard electronics ever fail.

How to prepare

Chartwork questions in the theory part of the SBF Coastal exam test exactly this basic understanding: recognizing symbols, correctly interpreting depth figures, and measuring distances correctly on the latitude scale. In the Boatpass app you can train the relevant questions from the official ELWIS question catalog, including the typical chart excerpts that also appear in the exam.

Conclusion

Being able to read a nautical chart means understanding its language of symbols, depth figures and scales: the scale determines the level of detail and coverage, the latitude scale at the chart’s edge gives you the correct distance, and chart datum is the reference for every depth figure. Master these basics and you’ll not only pick up points in the exam, but also gain the confidence to orient yourself on the water at any time.