Reading a Nautical Chart

A nautical chart is a map built for the water, and its whole job is to answer one question: is it safe here? The first thing to check is the scale, printed as a ratio like 1:50,000 — one unit on paper equals 50,000 in the real world. A large-scale chart (like 1:20,000) zooms into a harbour with fine detail; a small-scale chart (like 1:200,000) covers a whole coastline with less. Bigger denominator = bigger area, less detail.

The numbers scattered across the water are soundings — water depths at a reference level called the chart datum. Most charts use Lowest Astronomical Tide (LAT), the lowest tide predictable under normal conditions. Because real tides almost always sit above datum, a printed depth is a conservative worst-case: a sounding of 3 means at least 3 metres of water even at an extreme low tide, and usually more once you add the actual tidal height for the moment. (Recall that height of tide adds to charted depth.)

Symbols pack the rest of the picture: buoys and beacons, a tiny anchor for anchorages, asterisks or crosses for rocks, and colours for zones — deep water white, shallows blue, and green for drying areas that uncover at low tide. Drying heights are shown as underlined figures, meaning that much height above datum. The official key to every mark is Chart No. 1, the symbol dictionary every skipper keeps aboard.

One last habit: always confirm the chart's depth units (metres, feet, or fathoms) and its datum in the title block before trusting a single number.