How to read a nautical chart

A nautical chart is a scale representation of a sea area showing depths, the shape of the seabed, hazards, navigation marks and the coastline, drawn so a navigator can measure distance and direction directly off it.

Latitude runs up the side and longitude across the top. Distance is measured on the latitude scale at the side of the chart, never the longitude scale, because one minute of latitude equals one nautical mile. The compass rose gives directions and, on most charts, the local magnetic variation.

The scattered numbers are charted depths in metres below chart datum, a reference level set at approximately the lowest astronomical tide. Because the sea almost never falls below it, the actual depth under your keel is normally the charted depth plus the height of tide at the time.

An underlined number is a drying height: ground that uncovers at low water, expressed as a height above datum. Blue and green tints shade the shallows and drying areas so the eye picks them up quickly, and the contour lines join equal depths — the tighter they bunch, the more sharply the bottom shelves.

Heights of lights, bridges and headlands are measured from a different reference, usually mean high water springs, so vertical clearance under a bridge shrinks as the tide rises.

Chart symbols are standardised and catalogued; in UK waters the reference is Admiralty publication 5011. Seabed type is abbreviated — S for sand, M for mud, R for rock — which matters when choosing where to anchor. Wrecks, rocks and obstructions each have their own symbols, with the dangerous ones ringed by a dotted danger line.

Buoyage follows the IALA system. In Region A, which covers Europe, red can-shaped marks are left to port and green conical marks to starboard when entering harbour. Cardinal marks indicate the safe side of a danger by compass quadrant, and their topmarks and light rhythms are worth learning early.

Lights are printed with their characteristics, for example Fl(3)G.10s8m6M: three green flashes every ten seconds, eight metres above sea level, visible six miles. Read left to right and it is entirely logical.

To fix your position, take compass bearings of two or three clearly identifiable charted objects, correct them for variation, and plot them from those objects using a chart plotter or parallel rule. Where the lines cross is your position; a third bearing produces a small triangle that confirms it and shows how good the fix is.

To plan a course, draw the intended track, measure its direction and length, then allow for tidal stream and leeway to work out the heading to steer. That calculation — the difference between where you point and where you actually go — is the heart of Day Skipper and Coastal Skipper theory.

Chartplotters are excellent and nobody sensible ignores them. But screens fail, and a plotter only ever shows a fragment of the picture at the zoom level you have chosen. The navigator who understands the chart underneath can sanity-check the electronics, and carry on without them.