Navigation
A working chart you can practise on. Switch tools below, then read off a position, measure a distance with the ruler, or fix where you are by taking bearings to charted marks. This is a teaching chart – not for real navigation.
Where am I?
At sea you usually know your position from GPS, but a sailor still keeps a plot by hand – instruments fail, and a chart with a pencil never runs out of battery. The skill is knowing several independent ways to answer one question: where am I right now?
Dead reckoning
Start from a known position (a “fix”). From there, advance your position using the course you steered, your speed, and the time elapsed – course and distance run give a new estimated position. Allow for the tide and current setting you sideways, and for leeway, the small sideways slip caused by the wind. The result is your estimated position, the backbone of the plot between fixes.
A three-bearing fix
When you can see charted landmarks – a lighthouse, a headland, a tower – take a compass bearing to each with a hand bearing compass. Each bearing is a line of position: you must be somewhere along the reciprocal of that line, drawn outward from the mark. Plot two such lines and they cross at your position; plot a third as a check. Because no bearing is perfect, three rarely meet in a point – they form a small triangle called a cocked hat. Take your position as the corner nearest any danger, to stay on the safe side.
Other lines of position
A bearing is not the only line you can plot. A transit – two charted objects seen exactly in line – is the most accurate line of all, and needs no compass. A depth contour matched to your echo sounder is a line of position too. Even a single line, crossed with your dead-reckoning track, gives a useful running fix.
* Place names and marks on the chart are illustrative and placed for practice only.
Which way is north?
A fixed reference compass: true north (cyan, ★) is the chart’s up; the needle (red) points to magnetic north, offset from true by the variation – 5° East here (angle enlarged for legibility). A real chart prints this offset, the year it was measured and its small annual change, on the nearest compass rose.
There is more than one “north”. The chart is drawn to true north (the geographic pole), but your compass points to the magnetic pole – somewhere else entirely, and slowly wandering. The angle between the two is the variation, and every chart prints it on a compass rose. To read variation on a real chart, use the rose nearest your position.
Variation – true vs magnetic
Variation (also called magnetic declination) is the angle between true and magnetic north at your position. It can be east or west of true, it differs from one part of the world to another, and it drifts a little each year – which is why the rose notes both the year and the annual change. For the waters on this chart (northern Greece) it is roughly 5° East today. You read it straight off the nearest compass rose and apply it to every bearing you take from, or plot on, the chart.
Deviation – your boat’s own error
Even after variation, the compass in front of you can still be wrong, because the boat’s own iron, engine and electronics tug the needle away from magnetic north. That error is deviation, and unlike variation it changes with the boat’s heading. A compass adjuster “swings” the boat to measure it and writes out a deviation card listing the deviation on each heading. Keep handheld compasses and phones well clear of speakers, engines and steel.
True → magnetic → compass
To turn a course off the chart (true) into the number to steer (compass), peel off the variation, then the deviation. Worked example – true course 090°T, variation 5°E, deviation 2°W:
090°T − 5°E (variation) = 085°M · 085°M − 2°W (deviation) = 087°C – steer this
Error West – compass best (the compass reads higher than true); error East – compass least (it reads lower). Going from true down to compass you add westerly errors; coming back up you add easterly ones. The sailor’s mnemonic for the order is “True Virgins Make Dull Companions” – True, Variation, Magnetic, Deviation, Compass.
Depths & symbols
A chart is a page of symbols before it is a page of geography. This is an analytical guide to the figures and marks you’ll meet on a real chart – depths and drying heights, rocks and wrecks, and the other symbols that warn you off trouble. All heights, depths and ranges below are given in metres unless stated otherwise.
Depths & the chart datum
Every depth figure (a sounding) on the chart is measured downward from a fixed reference plane, the chart datum – normally close to the lowest tide the area ever sees, so that on paper you are almost always shown less water than is really there. In tideless or nearly-tideless waters, like the Mediterranean, datum sits close to mean sea level. Soundings are printed in metres on modern charts (older or foreign charts may use fathoms or feet – always check the chart’s title block first).
The lights, lighthouses and buoys printed on a chart are explained on the Rules of the Road sheet, next to the lights other vessels show – everything you see on the water, in one place.
Other chart symbols
A handful of further symbols round out the picture: a fine dotted line with a small anchor marks a recommended anchorage; a fine broken line traces a submarine cable or pipeline (keep your own anchor well clear of it); a magenta dashed boundary marks a restricted, prohibited or exercise area; and a short arrow labelled in knots gives the rate and set of a tidal stream at that point. Coastline itself carries a code too – a firm line is a stable, surveyed shore; a broken or dotted line means the coast is unsurveyed, or the drying line uncertain, and calls for extra caution.