Why Nautical Charts Matter

A nautical chart is far more than a map of the sea. It's a detailed document that tells you about water depths, underwater hazards, navigation aids, tidal information, and the nature of the seabed. Knowing how to read one can mean the difference between a safe passage and a grounding. Even if you rely on electronic chartplotters, understanding the underlying chart data is an essential seamanship skill.

Chart Scale: Choosing the Right Chart

Charts come in different scales, and using the right one for your situation is critical:

  • Large-scale charts (e.g., 1:10,000 or 1:25,000) show small areas in great detail — perfect for harbors and anchorages.
  • Small-scale charts (e.g., 1:500,000 or smaller) show large ocean areas — useful for passage planning but lacking detail for coastal navigation.

Always use the largest scale chart available for the waters you're actually sailing through.

Depth Soundings

The numbers scattered across the water areas of a chart are depth soundings. In most modern charts, these are given in metres, though older US charts may use fathoms (1 fathom = 6 feet). Soundings are measured from a reference level called Chart Datum, which is typically the lowest astronomical tide — meaning actual depths are usually greater than shown.

Underwater Hazards and Seabed

Charts use specific symbols to warn you of what lies beneath:

  • Rocks awash (asterisk symbol): Rocks that appear at the waterline around low tide — a serious hazard.
  • Submerged rocks (+ symbol with dotted circle): Rocks below the surface. The depth figure nearby tells you how much water is over them.
  • Wrecks: Shown with a specific symbol, sometimes with depth noted. Some are navigational hazards.
  • Shoals and sand bars: Indicated by closely spaced depth figures that suddenly become shallow.

Seabed composition is abbreviated on charts: S = sand, M = mud, R = rock, Sh = shells. This matters for anchoring.

Navigation Aids: Buoys and Lights

Buoys and lighthouse symbols on charts correspond directly to real-world aids to navigation. They follow the IALA buoyage system:

  • Red buoys (port-hand marks): In IALA Region A (Europe, Africa, Asia, Australia), red marks are kept to port when entering harbor.
  • Green buoys (starboard-hand marks): Kept to starboard when entering. Region B (Americas, Japan, Philippines) reverses these colors.
  • Cardinal marks: Black and yellow buoys that indicate the safe side to pass a hazard — North, South, East, or West cardinal marks.
  • Isolated danger marks: Black and red, placed over a hazard with deep water all around.

Contour Lines and Blue Shading

Like topographic maps, nautical charts use depth contour lines to connect points of equal depth. Darker blue shading typically indicates shallower water — a quick visual cue for risk. Pale or white water areas are generally deeper and safer.

Chart Datum and Tidal Heights

Always remember that chart depths are a minimum based on low tide. To find the actual depth at any time, you add the height of tide (from tide tables) to the charted depth. This is especially important when crossing sandbars or entering shallow anchorages near low water.

Practical Tips for Chart Reading

  1. Always identify the chart datum unit (metres or fathoms) before using depth figures.
  2. Check the chart's publication date — charts must be kept up to date with official notices to mariners.
  3. Use the chart's legend/title block to understand what all abbreviations mean.
  4. Cross-reference your paper chart with your chartplotter — don't rely on only one source.
  5. When in doubt about a passage, look for the deeper water route even if it's longer.