Reading Nautical Chart Symbols (Without Squinting)

Depth, soundings, and contours

Soundings are the small numbers scattered across the water area, showing depth at mean lower low water (the average of the lower of the two daily low tides). On US charts, soundings are in feet by default; metric charts use meters and a sounding datum noted in the title block.

Blue contour lines (isobaths) connect points of equal depth at standardized intervals: 6 ft, 12 ft, 18 ft, 30 ft, 60 ft, 120 ft, then by 60 ft to 600 ft. The closer the lines, the steeper the underwater terrain.

Drying heights — areas exposed at low tide — are shown with a different convention: a number underlined or in italics with units of feet above the chart datum. Always check whether you're reading depth-below or height-above.

Aids to navigation (ATON)

The IALA-B system (Americas, Korea, Japan, Philippines): red on the right returning from sea ('red right returning'). Charts show buoys as small diamonds with the buoy color and number.

Red triangles = nun buoys (pointed top), even-numbered, mark the right side of a channel returning from sea.

Green squares = can buoys (flat top), odd-numbered, mark the left side returning from sea.

Yellow buoys mark special areas (cable crossings, fish farms, restricted areas).

Daymarks on poles use the same color and shape rules — red triangle on a pole = right-returning, green square = left-returning.

Lights and light characteristics

Light symbols use abbreviations for the flash pattern, color, and period. Common codes: Fl = flashing, Q = quick (60 flashes/min), Iso = isophase (equal on/off), Oc = occulting (mostly on, brief off), Mo(A) = Morse code letter A (...).

A typical entry: 'Fl R 4s 22ft 4M' = flashing red light, 4-second period, 22 feet above water, 4 nautical mile range.

Match the chart symbol to the actual light's characteristics by counting seconds with a watch — this is how you positively identify a light at night when there are several within sight.

Hazards: wrecks, rocks, and obstructions

Wrecks: an asterisk (sunken wreck, depth unknown), a hull symbol (visible wreck), or a number with the wreck symbol indicates depth over the wreck.

Rocks: '+' = rock awash at chart datum (low tide). A small circle with a cross = rock with depth less than chart datum (always submerged, but shallow). A bare '+' inside a circle = obstruction with cleared depth shown.

Foul areas: a hatched zone with 'foul' or '#' marks — anchoring or grounding here may snag on debris.

Magenta markings call attention to hazards that change frequently or require special attention (cable crossings, restricted areas, advisory zones).

Bottom composition

Two-letter codes near soundings tell you what's on the bottom. Critical for choosing an anchorage and for predicting where to set an anchor cleanly.

Common: S = sand, M = mud, Cl = clay, R = rock, G = gravel, Co = coral, Sh = shells, Wd = weed (kelp/eelgrass).

Combinations indicate mixed bottoms: 'S M' = sand and mud (excellent holding); 'R Co' = rock and coral (terrible holding, also illegal in many anchorages).

Where to get the official reference

NOAA Chart No. 1 is the free PDF reference for every symbol on every US chart. Download it from nauticalcharts.noaa.gov.

Print a paper copy and stash it on the boat. Paging through it once at the dock will teach you ten symbols you'd never decode in the moment.

International (non-US) charts use INT 1, the IHO's equivalent reference — also free, also worth downloading if you cruise internationally.