A chart is a survey, a date and a set of conventions. Learn to read all three and you know how far to trust it, on paper or on the plotter.
1 · Position and distance
The grid of a Mercator chart
How do I measure distance in miles on a Mercator chart?
On the latitude scale at the side of the chart, level with the leg: one minute of latitude is one mile. A minute of longitude is shorter, by the cosine of the latitude.
Latitude is read on the side scales, longitude at top and bottom, in minutes and decimals: 45 seconds is 0.75', not 0.45'. Count from the nearest printed line; west longitude grows leftward.
On a Mercator chart a straight line is a rhumb line: one true course throughout. Measure miles on the latitude scale level with the leg: a minute of latitude is a mile. A minute of longitude is shorter, by the cosine of the latitude: half a mile at 60°N.
At 1:200,000 a millimetre of pencil is 200 m. Plan on the passage chart, then change to the largest scale for each harbour; an inset has its own. On a measured distance, average the two runs' speeds, not their times.
2 · Title panel and rose
Small print before soundings
Why is my GPS position about 100 m out on an older paper chart?
The chart is on an older horizontal datum, while GPS works in WGS84. Apply the shift printed in the title panel; with no shift given, carry the position across by bearing and distance from a charted object.
The title panel gives the scale, the units (a fathom is 1.83 m), the levels depths and heights are measured from, and the horizontal datum. GPS works in WGS84: on an older datum apply the printed shift, or the plot can be 100 m out. With no shift given, carry a position across by bearing and distance from a charted object.
Datum shift: worked example
GPS (WGS84): 43°27.30'N 008°16.40'W
Chart note: move 0.06' north, 0.09' east
North adds to N latitude: 27.36'N
East takes from W longitude: 16.31'W
Plot 43°27.36'N 008°16.31'W
The compass rose has an outer ring in true and an inner one in magnetic for the year printed. Update the variation and take it from the nearest rose.
4°30'W 2016 (9'E): ten years × 9' = 1°30' east, so 3°00'W in 2026.
The source diagram dates each survey: old lead-line work missed rocks. In a charted magnetic anomaly fix without the compass.
3 · Depths and heights
Below datum, above datum, above high water
What does an underlined number on a chart mean?
It is a drying height, measured above chart datum. A plain sounding is a depth below chart datum, to which you add the height of tide; a figure in brackets is a height above MHWS that never covers.
A sounding is a depth below chart datum: add the height of tide. An underlined figure is a drying height, above datum. A figure in brackets is a height that never covers, measured above high water (MHWS).
Sums with the tide
Bank drying 2.3, draft 1.8, margin 0.5: you need 2.3 + 1.8 + 0.5 = 4.6 m of tide
Rock (3), MHWS 5.2 m, tide now 0.8 m: it stands 3 + 5.2 − 0.8 = 7.4 m high
Wreck swept 4.2, tide 0.5 m, draft 2.0 m: 4.2 + 0.5 − 2.0 = 2.7 m under the keel
Clearances are measured above HAT, or MHWS on older charts. A rock awash at datum shows only near low water. Keep outside the dotted danger line round a wreck or obstruction. A blank inside a contour is unsurveyed, not deep, and swell breaks on a shoal the sums allow.
4 · Symbols and abbreviations
The seabed, the areas and their notes
What do S, M and R next to a sounding mean when choosing where to anchor?
They give the nature of the seabed: S sand, M mud and Cy clay are good holding; R rock, St stones, Wd weed and Sh shells are poor. Never anchor over a cable, pipeline or foul ground.
Anchor at C, in sand and mud.
On the chart
Meaning
S · M · Cy
Sand, mud, clay: good holding
R · St · Wd · Sh
Rock, stones, weed, shells: poor
PA
Position approximate: a wide berth
ED
Existence doubtful: treat as real
Never anchor over a cable, pipeline or foul ground. A spoil ground may be shallower than charted. Overfalls break at springs with wind against stream. A danger area may be firing: ask range control. In a prohibited area or reserve the chart's note decides; use the visitors' moorings. The tinted band between traffic lanes is the separation zone. Chart 5011 explains every symbol.
5 · Lights and signals
The legend beside a light
What does Fl(3) WR 15s 21m 18/15M mean beside a light on the chart?
Three flashes every 15 seconds, with white and red sectors, 21 m above MHWS, nominal range 18 miles for the white and 15 for the red. The legend always runs character, colour, period, elevation, range.
The legend runs: character, colour, period, elevation, range. Fl(3) WR 15s 21m 18/15M is three flashes every 15 seconds, white and red sectors, 21 m above MHWS, nominal range 18 miles for white, 15 for red.
You see a light at its nominal range or its horizon, whichever is less: a 3M pierhead light will not show at 12 miles. Sector limits and obscured arcs are true bearings from seaward. Horn(2) 30s is two blasts every 30 seconds; a buoy's bell needs waves and may be silent in a calm. Racon (T) paints a Morse T on the radar beyond the mark. A virtual AIS mark exists only on screen.
6 · Corrections and publications
The newest information wins
How do I correct a paper chart with Notices to Mariners?
Find the last notice number in the chart's bottom-left margin, then apply every later Notice to Mariners for that chart in ink and write its number in the margin; temporary notices go in pencil.
Notices to Mariners carry the changes: a new breakwater, a moved buoy, a wreck. Each names the previous notice for that chart, so a gap shows. A new edition holds everything issued before its date.
Correcting a chart
Bottom-left margin: last notice 2025-3120
2025-2988: older, already in
2025-3305, then 2026-0412: apply in ink
Write both numbers in the margin
Temporary notices: pencil, rubbed out later
Publication
What it gives
This year's almanac
Tides, streams, lights, radio, ports
Pilot book
Approaches, berths; plans are sketches
Tidal stream atlas
Streams hour by hour
List of Lights
Each light in full
List of Radio Signals
VTS and port channels
Carry paper charts of the route and every likely port. Read the port page before arriving: whom to call, opening hours, water over the sill. When sources disagree, the newer wins: a corrected chart over an old pilot book, a fresh warning over the chart.
7 · Electronic charts
The same data, with traps of its own
What should I set the safety contour to on my chart plotter?
To your draft plus a margin, along with the depth alarm. If the chart has no safety contour at that value, the plotter takes the next deeper one, and its route check ignores the tide.
A raster chart is a picture of the paper chart. A vector chart is a database: tap an object for its details, switch to night colours. It hides detail when zoomed out, so check each leg zoomed in; over-zooming enlarges errors, not accuracy.
Symbol
ZOC
Position within
6 · 5 · 4 stars
A1 · A2 · B
5 · 20 · 50 m
3 stars
C
500 m
2 stars
D
Worse than C
U
U
Not assessed
Set the safety contour and depth alarm to draft plus a margin; with no contour at that value the plotter takes the next deeper one. Its route check ignores the tide: do that sum yourself. Mark a paper position every hour, so a dead plotter costs one hour of dead reckoning.
Practice questions
Questions from this topic, as the app asks them. Open one to see the answer.
Passage chart
Mercator, 1:500,000
Covers 47°00'N to 50°00'N
Your leg: 25 M, near 49°40'N
You are measuring a leg near the top of this small-scale passage chart. Where do you read the miles?
On the longitude scale along the top edge
On the latitude scale level with the leg
On the latitude scale at the bottom edge
On any part of the latitude scale
Show the answer
B. On the latitude scale level with the leg
On a Mercator chart the latitude scale stretches towards the pole; read the miles level with the leg's own latitude.
All 100 questions of this topic are in the app: practice by topic with the reason after every answer, your mistakes brought back, and a timed mock exam. Free, no sign-up, in the browser or in Telegram.
