How to read what the chart is actually telling you — and why misunderstanding charts sinks ships
Contents
Use the links below to jump to any section:
- Why Charts Are Different from Maps
- What a Nautical Chart Is Really For
- Scale: Why Detail Lies to You
- Depths, Soundings, and the Illusion of Safety
- Charted Dangers and What They Actually Mean
- Drying Heights and Tidal Reality
- Colours, Shading, and Human Misinterpretation
- Buoys, Beacons, and What the Chart Assumes You Know
- Datums and Why Positions Sometimes “Don’t Agree”
- Notes, Abbreviations, and Warnings People Ignore
- Updating Charts and the Myth of “Still Valid”
- Professional Chart Reading Habits
1. Why Charts Are Different from Maps
Most civilians approach nautical charts as if they were road maps or hiking maps.
This is a fundamental mistake.
A map tells you where you can go.
A chart tells you where you must not go.
Charts are defensive documents. They exist to warn you about danger, not to guide you gently through safe areas.
If a chart looks empty, it usually means no one promises it is safe.
2. What a Nautical Chart Is Really For
A nautical chart is a legal navigation document.
It is designed to allow a mariner to:
- identify dangers
- understand depth limitations
- assess safe water
- plan routes with margins
Charts do not guarantee safety.
They describe known information with defined accuracy limits.
Everything else is your responsibility.
3. Scale: Why Detail Lies to You
Chart scale determines how much detail can be shown.
Large-scale charts show small areas in high detail.
Small-scale charts show large areas with limited detail.
The danger is assuming detail exists where scale does not allow it.
If a danger is not shown on a small-scale chart, that does not mean it does not exist. It means you are not zoomed in enough to be warned.
This is why professional navigation always shifts to the largest appropriate scale near land.
4. Depths, Soundings, and the Illusion of Safety
Soundings on charts are measured relative to chart datum, not the sea surface.
This means:
- the number shown is not “current depth”
- it represents depth at a defined low-water reference
A sounding of “10” does not mean ten metres beneath you right now. It means ten metres at chart datum, assuming nothing else.
Depth is dynamic. Charts are static.
Confusing the two causes groundings.
5. Charted Dangers and What They Actually Mean
Symbols for rocks, wrecks, obstructions, and foul ground are warnings, not decorations.
They mean:
“Something dangerous is known to exist here.”
They do not mean:
- the danger is isolated
- the danger is small
- the danger is accurately positioned
Many groundings occur beside charted dangers, not on them.
The danger is often bigger than the symbol.
6. Drying Heights and Tidal Reality
Drying heights indicate how far features rise above chart datum when exposed at low water.
This information is critical for coastal navigation.
A drying height does not tell you when the feature dries. It tells you how high it is above datum.
Whether it is safe depends on tide height at that time.
Ignoring drying information is one of the fastest ways to put a ship aground.
7. Colours, Shading, and Human Misinterpretation
Charts use colour to convey risk, but humans interpret colour emotionally.
Blue areas feel safe.
White areas feel neutral.
Green and brown feel land-like.
This is dangerous thinking.
Colour is symbolic, not advisory.
Safe water is not defined by colour. It is defined by depth relative to your ship.
8. Buoys, Beacons, and What the Chart Assumes You Know
Charts assume you understand buoyage systems.
They do not explain them.
A buoy symbol is not guidance. It is information. It tells you what exists, not how to use it.
Many civilian errors come from treating buoys as “road signs” rather than floating hazard markers.
The safe water is where the chart and buoyage agree — not where the buoy alone suggests.
9. Datums and Why Positions Sometimes “Don’t Agree”
Charts use defined horizontal and vertical datums.
If your position source uses a different datum, plotted positions may not align exactly with charted features.
This does not mean the chart is wrong.
It means references are mismatched.
Understanding datums prevents the dangerous assumption that “the ship must be off position.”
10. Notes, Abbreviations, and Warnings People Ignore
Charts contain notes, cautions, and abbreviations that explain limitations and hazards.
These are often ignored because they look secondary.
They are not.
Many charts explicitly warn that areas are unsurveyed, unreliable, or subject to change. Navigators who ignore these notes navigate on assumption, not information.
11. Updating Charts and the Myth of “Still Valid”
Charts are living documents.
Buoys move. Wrecks shift. Depths change. Surveys improve.
An uncorrected chart is not “mostly right.”
It is partially wrong, and you don’t know which part.
Professional navigation includes constant awareness of chart currency.
12. Professional Chart Reading Habits
Experienced navigators read charts actively.
They look for:
- where danger could extend
- where information is missing
- where margins are thin
- where assumptions could creep in
They do not look for reassurance.
They look for risk.
Closing Perspective
A chart does not tell you where you are safe.
It tells you where someone has already found danger.
Understanding this transforms chart reading from drawing lines into thinking defensively — which is the core of coastal navigation.
Tags
chart interpretation · coastal navigation · nautical charts · chartwork · bridge watchkeeping · maritime safety