NAVIGATION · Lesson 1 of 6
Understanding Nautical Charts
30 min · Beginner friendly
You’ll learn toExplain Mercator projection and why courses plot as straight lines · Choose appropriate chart scales for passage and pilotage · Understand chart datum, WGS84 and keeping charts corrected
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A nautical chart is a map of the sea: where the water is deep, where it is dangerously shallow, where the hazards lie, and how the coastline is arranged. Every technique in this module happens on one — so the first job is to understand what the sheet is telling you, and what it quietly assumes you know.
Take a guess
Decide before you read: the chart shows a sounding of 2.0 m and your boat draws 1.5 m. It is mid-tide on an ordinary day. Is the real depth under you right now more or less than the printed 2.0 m?
Commit to an answer before reading on — a wrong guess here helps the right idea stick.
The Mercator Projection
A chart flattens the curved Earth, and the flattening almost every chart uses is the Mercator projection: the meridians (north–south lines) are drawn straight, vertical and to each other, and the parallels (east–west lines) straight and horizontal. The prize is this: a boat steering a steady course crosses every at the same angle, so her track plots as a straight line — a rhumb line. Lay a ruler from where you are to where you want to go, read the angle of the line, and steer it.
Knowledge check
Why does a boat steering a constant compass course plot as a straight line on a Mercator chart?
Choosing the Right Scale
Knowledge check
Two charts cover the same harbour: one at 1:25,000 and one at 1:150,000. Which shows the rock that matters in the entrance?
| Purpose | Typical scale | What it shows |
|---|---|---|
| Passage planning | 1:150,000 or smaller | A whole stretch of coast; headlands, major lights, offshore dangers |
| Coastal navigation | around 1:75,000 | The coast you are sailing along, with enough detail to your position |
| Approach and pilotage | around 1:25,000 | Harbour approaches, buoyage, channels and close-in dangers |
| Harbour plans | 1:10,000 or larger | Individual berths, marina entrances, every pontoon and pile |
A well-planned passage uses several charts: a small scale chart to see the whole , and large scale charts for the departure, the destination and any tricky pinch points along the way. Entering an unfamiliar harbour on a passage chart is a classic beginner error.
Chart Datum: Where Depths Start
Over the sounding: charted depth + height of tide = 2.0 + 3.5 = 5.5 m of water.
Over the bank: height of tide − drying height = 3.5 − 1.2 = 2.3 m of water.
Every depth printed on a chart is measured down from chart datum, on modern UK and European charts at approximately the (LAT) — the lowest level the tide is ever predicted to fall. It is a deliberately pessimistic choice: on almost every day of the year the real sea level is higher, so the actual depth is the charted depth plus the (calculated in the Tides module). Heights work the other way: a lighthouse's elevation or a bridge is measured above Mean Springs (MHWS) — again the cautious choice. Features that cover and uncover carry a above chart datum; the next lesson deals with those.
Knowledge check
Chart datum on a modern UK chart is best described as…
Trusting the Sheet: WGS84 and Corrections
Positions need a reference too — a horizontal datum. The worldwide standard is (World Geodetic System 1984): it is what your receiver uses natively and what modern charts are drawn to, so a satellite position plots straight onto the chart. Some older charts use local datums, and an uncorrected position transferred between datums can be out by a couple of hundred metres — enough to put you on the rocks in a narrow entrance. The chart title block states the datum and any shift to apply.
The chart also goes stale: the seabed shifts, buoys are moved, new wrecks appear, lights change character. Hydrographic offices publish a continuous stream of updates called Notices to Mariners, and the margin of every official chart records its edition and correction state so you can see at a glance how current it is.
- New editions replace the chart entirely when changes are too extensive to patch.
- Notices to Mariners list small corrections — a moved buoy, a new obstruction — applied by hand or by your electronic chart provider's updates.
- Temporary and Preliminary notices cover short-lived changes such as dredging operations or a light temporarily extinguished.
Your turn
You are planning a 35-mile coastal passage from Westhaven to Salt Island, finishing with entry into the narrow, rock-lined Salt Island harbour. Which charts would you gather, and what two things would you check on each before trusting it?
Work it through on paper first — then check yourself against the solution. Reading a worked answer feels like learning; producing one is learning.
Key points
- Charts are drawn on the Mercator projection: meridians are parallel, so a steady course plots as a straight rhumb line.
- Large scale = small area, big detail (1:10,000 harbour plan); small scale = big area, little detail (1:150,000 passage chart).
- Depths are measured below chart datum (approximately LAT); heights of lights and bridges are measured above MHWS.
- Actual depth = charted depth + height of tide.
- WGS84 is the horizontal datum used by GNSS and modern charts — always check the title block on older charts.
- Charts must be kept corrected via Notices to Mariners; an uncorrected chart slowly becomes fiction.
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