Nautical chart symbols: depths, drying rocks, wrecks and anchorages

Check units and datum before reading a number

Check the title and notes for depth units and chart datum; tide heights must use the same datum before you combine them. Do not assume all charts use metres. In the metric teaching scenes, a large 4 with a smaller, lower 2 reads as 4.2 m, not 42 m.

Water can fall below chart datum. Bridge-clearance and elevation datums may differ from the depth datum. The CHS Chart 1 introduction explains these references. Each exercise states its water level relative to chart datum so you can compare like with like.

• Identify the chart’s units, scale and reference levels.
• Check current chart corrections and local navigational information.
• Keep a feature’s charted value separate from the water level at the time of passage.

Soundings, drying heights and hazards tell different stories

The reference numbers below point to Chart 1 entries. Read the complete symbol and any attached notes, rather than treating every nearby number as the depth of the surrounding water.

A drying rock can disappear under shallow water

In “A rock hidden by the tide”, the drying height is 0.8 m above chart datum and the stated water level is 1.4 m above that same datum. Subtract the drying height: 1.4 − 0.8 = 0.6 m of water over the rock. The rock is covered, but the exercise vessel’s 1.8 m draught cannot clear it.

For a drying feature, water level minus drying height gives water depth over its top. If the result is negative, the top is above the water. For an ordinary sounding below datum, add the signed water level instead. These calculations require consistent units and datum; visibility of the feature does not supply the missing depth.

Depth and under-keel clearance are separate values

In “Choose a passage with enough water”, the western bank is charted at 0.6 m and the eastern passage at 4.2 m. Water level is +1.4 m, draught is 1.8 m, and the authored planning margin is 0.5 m. The west gives 2.0 m of water and only 0.2 m under the keel. The east gives 5.6 m of water and 3.8 m under the keel, leaving 3.3 m beyond the stated margin.

For this exercise: water depth = charted depth + water level; under-keel clearance = water depth − draught. Compare that clearance with the supplied margin. The 0.5 m value belongs to this scene. An actual passage needs an assessment of water-level uncertainty, vessel motion, squat, survey quality and local requirements, rather than borrowing the exercise’s margin.

A wreck symbol’s printed size does not set a passing distance

Chart 1 K 28 identifies a potentially dangerous wreck whose least depth is unknown. Other wreck symbols distinguish different information, including a known least depth. A wreck without the K 28 symbol is therefore not automatically harmless to your vessel.

In “The dotted surround”, the teaching chart supplies no surveyed debris boundary or safe-passing distance. The dots help identify the hazard. Measuring their printed radius cannot establish where debris ends or where your vessel can pass. Explain the missing information before selecting the answer, then compare it with the debrief.

Read anchoring restrictions alongside seabed and depth

A submarine cable route, an anchoring prohibition and an anchorage are separate chart features. Chart 1 section L covers cable symbols, section N covers anchorages and restrictions, and section J identifies seabed material. A sand abbreviation alone does not authorize anchoring, and deep water does not remove a prohibition.

“Where can you anchor?” combines a cable protection area with a designated anchorage outside it. The scene explicitly supplies adequate depth, shelter and swinging room at that anchorage. The decision is to use those facts together. On a real chart, check the restriction’s full extent and establish whether the anchorage suits your vessel and the conditions.