How do you read a nautical chart?

Reading a chart properly means treating it as a working document, not just consulting it for a quick look at the coastline. Depths are given as soundings, reduced to a chart datum that represents a low-water reference so that the charted figure is normally the shallowest the navigator should expect, though the actual depth at any moment also depends on the state of the tide, which must be added separately. Depth contours joining points of equal depth let the eye pick out the shape of the seabed at a glance, and a navigator planning a passage in shallow water pays close attention to which contour the ship's draught, plus squat and safety margin, will actually clear.

Dangers are marked with standardised symbols: a plus sign or star for a rock awash or covered, a wreck symbol showing whether it is dangerous or has adequate clearance over it, and shading or a dotted line for foul ground. Aids to navigation, lighthouses, lit and unlit buoys, and beacons, are shown with symbols that also encode their characteristics: the abbreviation and figures beside a light describe its colour, the rhythm it flashes in, the period in seconds, and its nominal range, all of which the navigator reads to identify a light positively at night before trusting a fix off it.

Position on the chart is read from the latitude scale up the sides, always used for measuring distance since one minute of latitude equals one nautical mile, and the longitude scale along the top and bottom, which is not used for distance because it compresses toward the poles. Courses and bearings are measured against the compass rose, printed at intervals across the chart, which gives both true and magnetic north along with the local variation and its annual rate of change, so the navigator can convert between true, magnetic and compass courses.

Beyond these basics, the chart carries traffic separation schemes and their direction of flow, anchorage areas, restricted and prohibited zones, submarine cables and pipelines that must not be anchored over, and a mass of further detail conveyed only through the standard symbols and abbreviations published in the chart symbols catalogue that every deck cadet learns to use. Whether on paper or as an ENC on ECDIS, the underlying language of the chart is the same, and a navigator who cannot read the paper chart symbols fluently will also struggle to interpret what ECDIS is displaying when something looks unusual.