NAVIGATION · Lesson 2 of 6
Chart Symbols and Abbreviations
25 min · Beginner friendly
You’ll learn toRead depth soundings, contours and drying heights · Recognise symbols for rocks, wrecks and other dangers · Identify seabed quality abbreviations and their use for anchoring
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A chart is written in a dense visual language: a tiny asterisk, an underlined figure, a dotted circle — each a precise statement about what lies beneath you, and misreading one can be the difference between a peaceful anchorage and a sickening crunch. A working of a few dozen symbols covers almost everything a Day Skipper meets; keep the official symbol booklet (Chart 5011 in the UK) aboard as the dictionary for the rest.
Take a guess
Decide before you read: a green-tinted bank on the chart carries the underlined figure 1.8. Tonight's height of tide at low water is 1.5 m and your boat draws 1.2 m. Do you cross the bank?
Commit to an answer before reading on — a wrong guess here helps the right idea stick.
Soundings and Contours
The scattered numbers across the water areas are soundings — spot depths below , in metres, with decimals printed as a smaller digit tucked low beside the main figure. Joining places of equal depth are depth contours at standard depths such as 2 m, 5 m, 10 m and 20 m, and between them colour tints grade the danger: land buff, drying areas green, shallow water dark blue, deeper water pale blue, deep water white. With practice you stop reading individual numbers and start seeing the shape of the seabed, the way you read hills from contour lines on a land map.
Knowledge check
On the approach you see a 4 with a small 5 tucked low beside it. What is it saying?
Drying Heights
Knowledge check
A bank on the chart carries the underlined figure 1.8. The height of tide right now is 2.5 m. How much water covers the bank?
Over the sounding: charted depth + height of tide = 4.5 + 2.5 = 7.0 m of water.
Over the bank: height of tide − drying height = 2.5 − 1.8 = 0.7 m of water.
Rocks, Wrecks and Obstructions
Knowledge check
From the diagram: which symbol marks a rock lurking right at the surface at the lowest tides — awash at chart datum?
Knowledge check
A hull-and-masts silhouette sits inside a dotted circle. What is the dotted line telling you?
Wrecks deep enough to be safe are marked Wk instead, with a swept depth where one has been measured. An Obstn is a hazard of uncertain nature — a lost container, debris, the remains of a structure — worth avoiding even where the depth looks ample, especially when anchoring.
The Seabed: Will the Anchor Hold?
Small letter abbreviations scattered among the soundings name the bottom itself — and the bottom decides whether your anchor holds tonight or drags at 3 a.m. Before the table settles it, commit to a guess below.
Knowledge check
You are choosing a spot to anchor for the night. Which charted seabed abbreviation would you most want to see?
| Abbreviation | Seabed | Holding for an anchor |
|---|---|---|
| S | Sand | Good — most anchors dig in well |
| M | Mud | Generally good, especially firm mud |
| Cy | Clay | Good once the anchor is dug in |
| G | Gravel | Moderate — can be loose |
| Sh | Shells | Moderate to poor |
| R | Rock | Poor — anchors skid or jam |
| Wd | Weed | Poor — the anchor may never reach the bottom |
Reading a Real Approach
Work it out
On the approach to Marram Bank you see: a sounding of 9 with a small 4; a green area carrying an underlined 2 with a small 1; a star-shaped rock symbol just outside the green; and the letter M nearby. Decode all four, then decide: with a predicted height of tide of 1.5 m at low water tonight, is the green area safe to cross in a yacht drawing 1.6 m?
Within 0.05 m is fine.
Within 0.05 m is fine.
A negative answer would mean the bank stays covered.
| Symbol | Meaning | Your response |
|---|---|---|
| Star (asterisk) symbol | Rock awash at chart datum | Assume it is at or near the surface at ; keep well clear |
| Cross with four dots | Underwater rock, depth unknown | Treat as very shallow; give it a wide berth at all tides |
| Underlined figure on green | Drying feature, height above chart datum | Compare drying height with height of tide before crossing |
| Hull-and-masts silhouette | Wreck dangerous to surface craft | Stay outside the dotted danger line around it |
| Obstn | Obstruction of uncertain nature | Avoid, especially when anchoring or in shallow water |
| Dotted circle or line | Danger boundary | Keep outside the dotted line |
Key points
- Soundings are metres below chart datum; a small subscript digit is the decimal (4 with small 5 = 4.5 m).
- Underlined figures on green areas are drying heights, measured above chart datum.
- Water over a drying feature = height of tide − drying height.
- Learn the danger set: rock awash (star), underwater rock (cross with dots), dangerous wreck (hull silhouette), Obstn, and the dotted danger line meaning keep out.
- Seabed letters guide anchoring: S and M good, G and Sh moderate, R and Wd poor.
- Keep the official symbol reference aboard for anything you do not recognise.
Finished this lesson?
Working through the sections marks it complete — or mark it yourself.