Reading a nautical chart: latitude, longitude, symbols and depths

Decide

Question 1/1

What is INT 1?

The lesson in plain words

One key for the world
- Chart symbols follow a standard set by the IHO (International Hydrographic Organization).
- The key is INT 1, published by each national office: NP5011 for Admiralty charts, US Chart No. 1 for US charts.
- Most symbols are the same worldwide, so learning INT 1 once lets you read almost any official chart. Keep a copy aboard.
Colours
- On paper charts land is usually buff or yellow, areas that dry are green, shallow water is blue and deeper water is white.
- Magenta marks lights, many notes, and restricted areas, cables and pipelines.
- How deep the blue goes varies between charts, so read the soundings and contours, not the colour alone.
Dangers
- A rock awash at chart datum is a cross with a dot in each corner.
- A rock that covers and uncovers is drawn as a small star with its drying height.
- An underwater rock dangerous to surface navigation is a cross.
- Wreck symbols vary with exposure and known depth: a submerged wreck may be labelled Wk, with a depth or danger line; check INT 1. A danger line is a dotted boundary marking danger to surface navigation.
Seabed and areas
- Letters describe the seabed: S sand, M mud, G gravel, R rock, Sh shells, Wd weed.
- That helps you choose where to anchor.
- An anchor symbol marks an anchorage.
- Magenta lines with notes show restricted areas, and submarine cables and pipelines, where anchoring is often prohibited.
Chart corrections
- Check that you have the current paper-chart edition, then apply outstanding Notices to Mariners after its last recorded correction.
- Also check Temporary and Preliminary (T&P) Notices still in force.
- These notices report new wrecks, moved buoys and changed lights.
- The IALA module covers buoy and light identification.

Key data

Symbol key: INT 1: NP5011 (Admiralty), US Chart No. 1
Seabed: S sand, M mud, G gravel, R rock, Sh shells, Wd weed
Magenta: Lights, notes, restricted areas, cables, pipelines
Corrections: Current edition; outstanding Notices after last correction

Key points

Symbols follow the IHO key INT 1 (NP5011, US Chart No. 1).
Buff land, green drying areas, blue shallows, white deep water, magenta lights and notes.
A dotted danger line round a rock or wreck means it matters to surface vessels.
Seabed letters (S, M, G, R, Sh, Wd) help you choose an anchorage.
Use the current edition; apply outstanding Notices to Mariners after its last correction; check T&P Notices in force.

Check what you have read

Question 1/3

What does a dotted line drawn round a rock or wreck mean?

True or false?

Decide each statement before you see the answer.

Seabed letters (S, M, G, R, Sh, Wd) help you choose an anchorage.
A dotted danger line round a rock or wreck means it matters to surface vessels.
A new-looking chart needs no corrections.
Use the current edition; apply outstanding Notices to Mariners after its last correction; check T&P Notices in force.
Anchor anywhere the depth suits you.
White water on the chart is always safe.

Practice case & exam keys

Situation

For a lunch stop you compare two bays. Bay A: 4 m, S, an anchor symbol. Bay B: 5 m, R, inside a magenta cable area. Which do you choose and why?

Show the answer

Bay A. Sand gives good holding and the anchor symbol marks an anchorage. In Bay B, rock gives poor holding and can trap the anchor, and anchoring in a cable area is usually prohibited and can damage the cable. Still check the depth against the tide and the shelter from the wind.

Exam keys

INT 1 is the international key to chart symbols (NP5011 for Admiralty charts).
Rock awash: a cross with dots in the corners; a danger line is dotted.
Seabed letters: S sand, M mud, G gravel, R rock, Sh shells, Wd weed.
Use the current chart edition and apply outstanding Notices to Mariners after its last recorded correction.