Charts look dense because they answer hundreds of questions at once. They become readable when you know the system: everything is either a depth (measured down from chart datum), a height (measured up from a stated datum), a symbol (from a standard dictionary — U.S. Chart No. 1 or Admiralty NP5011), or metadata in the title block telling you how to read the rest. This guide works through a deliberately synthetic example chart — marked as such, because you should never navigate on a picture from a website.

An educational chart extract, not for navigation. Land is shown in beige with a lighthouse whose light is described as flashing twice every ten seconds, visible 15 miles. Soundings in metres and decimetres are scattered over the sea, a dashed 5-metre contour runs across, a rock drying 1.2 metres is marked with an underlined figure and cross, a red can port-hand buoy and a green conical starboard-hand buoy mark a channel, a compass rose shows true north and the magnetic variation, and a scale bar shows one nautical mile.Fl(2) 10s 20m 15M5 m contour8₂6₅4₈3₂5₁9₄121511₃181₂dries 1.2 mFl R 4sFl G 4sNVar 3°10′W 2026 (8′E)1 nautical mileEDUCATIONAL EXAMPLE — NOT FOR NAVIGATION
What to notice: Our educational extract. Find each feature as the sections below cover it: soundings, the underlined drying height, the dashed depth contour, two lateral buoys with light descriptions, a lighthouse with its full light characteristic, the compass rose with variation, and the scale bar.

A labelled synthetic chart extract with soundings, drying rock, contour, buoys, lighthouse, compass rose and scale bar, marked educational example not for navigation.

Start with the title block

Somewhere on every chart is its self-description: name and number, scale, units of depth and height, the sounding datum (“depths in metres, reduced to Chart Datum, which is approximately Lowest Astronomical Tide” on Admiralty charts; Mean Lower Low Water on U.S. charts), the height datum for clearances, the horizontal datum (WGS84 on modern charts, which is what GPS speaks), and the survey information. Thirty seconds here prevents the classic errors: reading feet as metres, or trusting soundings surveyed in 1893.

Scale, and the two rulers around the edge

Two chart frames covering the same coast. The small-scale chart, 1 to 150,000, shows a wide area with little detail. The large-scale chart, 1 to 25,000, shows a small area with soundings, buoys and the harbour entrance in detail. Large scale means large detail.1 : 150 000 — small scale, big area5₂3₁72₅1 : 25 000 — large scale, small area, detail
What to notice: Same coastline, two charts. The passage chart locates the harbour; the harbour chart gets you in. Moving to the largest available scale as you approach is a habit, not a preference — smaller scales omit detail.

Two chart frames showing a small-scale wide view and a large-scale detailed harbour view of the same coast.

The borders are your rulers: latitude up the sides, longitude along top and bottom, and one minute of latitude is one nautical mile — measure distance only on the latitude scale, level with your working. Our latitude/longitude guide covers plotting positions; on small-craft charts a printed scale bar is also provided, like the one in the example.

Depths, drying heights, contours and the blue

  • Soundings (4₈ = 4.8 m): depth below chart datum. Real depth = sounding + height of tide; the chart-datum guide does these sums.
  • Drying heights (underlined, 1₂): the top of a feature that covers and uncovers, measured above datum. NOAA's rule: elevations of features that cover and uncover are underlined and referenced to the sounding datum.
  • Depth contours join equal depths, and the colour tints (dark blue shallowest, white deep) let you read safety at a glance. Pick your personal no-go contour for the day — say 5 m — and treat its line as a fence.
  • Rocks, wrecks and obstructions each have precise symbols (a cross for a rock, categories of wreck) whose exact form encodes how dangerous they are; when a symbol matters to your route, look it up in Chart No. 1 / NP5011 rather than guessing.
  • Seabed letters (S sand, M mud, R rock, Wd weed, Sh shells) matter twice: anchoring, and knowing what your echo sounder's bottom will look like.

Buoys, beacons and light descriptions

Buoy symbols show shape and colour (see the buoyage guide for the IALA system), and lit marks carry a compressed light description. The lighthouse in our example reads Fl(2) 10s 20m 15M: group-flashing two flashes every 10 seconds, 20 metres above the height datum, nominal range 15 nautical miles. The buoys read Fl R 4s and Fl G 4s. The grammar is universal: rhythm (F fixed, Fl flashing, Q quick, Oc occulting, Iso isophase, Mo(A) Morse), colour (default white; R red, G green, Y yellow), period, height, range. Learn to read it fluently — at night the light description is the buoy.

The compass rose

The outer ring is true, the inner ring magnetic, and between them is printed the variation with its date and annual change — NOAA's Coast Pilot: each rose shows the date, magnetic variation and annual change. Update the variation to this year before using it, and see the variation-and-deviation guide for the arithmetic. Charts of areas with local magnetic anomalies say so in a note; notes in general are part of the chart — read the boxed ones near your route.

What changes on a plotter

  • Same data, same datums, same symbols (vector charts redraw them, raster charts photograph the paper). The sums and the symbol dictionary still apply.
  • Zoom is the new scale — and the new trap: zoomed out, ENCs suppress detail exactly like a small-scale chart, and some plotters flag it subtly (“overzoom” warnings do the reverse). Approach at the electronic equivalent of the largest scale: zoomed in with full detail layers on.
  • The boat symbol seduces you into pilotage-by-cursor. Cross-check with the depth sounder and your eyes; GPS position on an old survey can be more precise than the chart is accurate.
  • Keep an independent backup: a second device, and for coastal passages a paper chart or printed chartlet with your plan on it. The GPS-fails question on every forum has one good answer: you noted your position recently, so you start dead reckoning from it (see the course-to-steer guide).
What would you do?The shortcut over the shallowsMotoring home at half tide on a falling 2.0 m tide (height of tide now 1.1 m), you consider a shortcut across an area your plotter shows as pale blue at the current zoom. Zooming in reveals scattered soundings of 0₈ and 1₂, and an underlined 0₅ in the middle. Your draught is 1.5 m.Decide, then reveal the reasoning

Do the datum sums before trusting the tint. Water over the 0.8 m sounding now: 0.8 + 1.1 = 1.9 m — 0.4 m under your keel and falling. Over the underlined 0₅ (a feature drying 0.5 m): 1.1 − 0.5 = 0.6 m — you would ground. The pale tint at the first zoom hid exactly the detail that matters, which is why the largest scale (deepest zoom, full layers) is a rule and not a style. Take the channel; the shortcut saves eight minutes and risks the keel and the tide-table's whole afternoon.

Check yourselfOn the example chart, the light description at the lighthouse is Fl(2) 10s 20m 15M. Tonight, from about 12 miles away, what exactly should you expect to see, and what does the 20m tell you?Show answer

Two white flashes, then darkness, with the whole cycle repeating every 10 seconds — count it with a watch to identify the light. White, because no colour letter is given. 15M is its nominal range, so at 12 miles it should be visible in clear weather if your height of eye lets you see over the horizon; 20m is the height of the light above the high-water datum (used with height of eye to work out that geographic visibility), not a range and not a depth.

Key takeaways

  • Read the title block first: units, datums, survey date.
  • Numbers: plain = depth below datum; underlined = dries above datum; bridge/cable figures = clearance above a high-water datum.
  • Distance from the latitude scale only; use the largest-scale chart (or deepest sensible zoom) for close work.
  • Light descriptions are a grammar — rhythm, colour, period, height, range. At night they are how you tell marks apart.
  • The rose gives true and magnetic with the variation's date; update it. Plotters show the same chart — audit them, and keep a backup.