Charts, Chartwork & Basic Navigation
The GPS will tell you where you are — until the day it doesn't. This module covers the full IYT Module 19 syllabus at Working depth: chart publishers and small-craft charts, the plotting instruments, reading the chart's colours, scales and cautions, the navigator's vocabulary (CTS, leeway, set, drift, COG, SOG), dead reckoning, estimated positions, and the whole family of fixes — with an interactive plotter where you take the bearings and watch the cocked hat appear.
Charts & Who Publishes Them
A chart is the mariner's map of the sea: coastline and its landmarks, depths, objects on and under the water, hazards, aids to navigation, channels, anchorages, harbours, tide and current information, and the local magnetic variation — everything route planning and pilotage lean on.
Published by the UK Hydrographic Office; sold through approved agents, with the Home Waters Catalogue (NP 109) free and the full catalogue (NP 131) on view at every agent — both annual. Agents also carry tidal stream atlases and pilot books.
Ottawa-published charts — about 946 of them — covering all Canadian coasts plus the big navigable lakes, rivers and waterways, with digital charts and publications alongside. Browse at charts.gc.ca.
Washington-published, in four regional catalogues: 1 — Atlantic & Gulf (incl. Puerto Rico/USVI), 2 — Pacific (incl. Hawaii, Guam, Samoa), 3 — Alaska, 4 — Great Lakes. See nauticalcharts.noaa.gov.
Chartwork Instruments
Good navigation needs surprisingly little kit — and the right pencil matters more than students expect:
Soft enough not to score the chart and to erase cleanly; mechanical ones skip the sharpening. Add a notebook, sharpener and a good eraser.
Measure distance against the latitude scale. School dividers work; the single-handed brass bow type is easier to use in a seaway.
Walk or roll a course or bearing across to the printed compass rose. Honest limitation: on a rolling boat in bad weather they slip — not recommended at sea.
A circular protractor with an alignment grid on a rectangular ruler. Needs no compass rose, works on rough surfaces and any size of vessel, and is the most accurate of the plotters (correction factor about 1°).
Reading the Chart — Scale, Colours, Depths & the Rose
Large scale = small area, big detail (harbour charts); small scale = big area, less detail (passage charts). Use the largest scale available — and beware switching charts mid-plot: a scale change is the classic way to mark off the wrong distance.
Measured on the latitude scale: one minute of latitude = one nautical mile (1 NM ≈ 1.1 statute miles). Never measure distance on the longitude scale at the top or bottom — those minutes shrink toward the poles.
Soundings come in feet, fathoms or metres (1 fathom = 6 ft) — the unit is declared in the title block and margins. US charts tend imperial, European metric. Read it before you trust a single number.
Printed at several places on the chart: outer ring true 000°–359°, inner ring magnetic, with the variation and its annual change noted in the centre. True north is the top of the chart (strip charts excepted).
The chart colours tell you at a glance what is water and what is not:
Land that never floods. Any heights printed on it are measured above MHWS (mean high water springs — roughly the average level of the highest tides).
The zone that is under water at high tide and dry at low tide. Its drying heights (how far it dries out — shown underlined) are measured up from chart datum, the low reference level that depths are given from.
Never dries. The depths (soundings) are measured down from chart datum — so with any tide in, there's nearly always more water than the chart shows.
The grid: parallels of latitude run east–west, 0° at the equator to 90° at each pole; meridians of longitude run pole-to-pole, measured 0°–180° east or west of the Prime Meridian at Greenwich, London. Together they name any position on Earth.
Direction, Time, Speed — and the Navigator's Vocabulary
Direction is an angle from 000° (true north), clockwise through 360°. Time is the 24-hour clock, always — "oh five twenty", "seventeen twenty" (and never "…hours"). Speed is in knots: one knot is one nautical mile per hour, so "one knot per hour" is a tautology that marks the speaker instantly.
| Term | Meaning |
|---|---|
| Course to Steer (CTS) | The direction to be maintained to the destination — the intended course line drawn on the chart. |
| Heading | Where the bow is actually pointing (ship's compass). Differs from the course when leeway, tide or current are being countered. |
| Leeway | How far the wind pushes the boat off her intended track — significant in a sailboat hard on the wind. |
| Speed (S) | Speed through the water — not necessarily over the ground. |
| Set / Drift | The direction the current or tide is pushing you (set) and how fast (drift). |
| COG / SOG | Course and speed actually made over the seabed — water motion and leeway folded in. |
Dead Reckoning & the Estimated Position
You can't always see charted landmarks to fix your position — it may be dark or foggy, or you may be well offshore. In between fixes you work out roughly where you are from where you last were, plus how far and in what direction you've travelled since. There are two levels of this. The difference between them is how much they allow for the water and wind pushing the boat off the line she was pointed along.
The simple estimate. Start from your last known position, plot the course you steered (converted to true), and mark off along it the distance run — how far you've travelled, read from the log (the instrument that measures distance through the water). That point is your DR position: a dot on the course line with a half-circle round it, labelled with the time and log reading. A DR assumes the boat went exactly where she was pointed, so it takes no account of tide, current or wind — it's only as good as an honest log and a steady helm, and the error grows the longer you run it. Restart it from each new fix.
A DR improved with what you know about the water and wind, done in two steps. First allow for leeway: swing the track downwind by the angle the wind has been blowing the boat sideways, to get the water track — the path the boat actually cut through the water. Then allow for the current: from the end of that track draw an arrow in the direction the water is flowing (set), as long as the distance it would carry you in the time elapsed (drift). Where the arrow ends is the EP — a dot inside a square. More accurate than a DR, but still an estimate, not a fix. A current from dead ahead or astern only changes your speed over the ground; a current across your course pushes you bodily sideways.
Bearings, Fixes & the Cocked Hat
The most common position line (LOP) is a hand-bearing-compass bearing of a charted object — lighthouse, buoy, beacon, water tower, chimney, headland — taken away from the boat's magnetism, written down with time and log, converted to true, and drawn on the chart. One LOP says "you're somewhere on this line" (still useful: it can prove you're clear of a danger). Two or more crossing LOPs make a fix. Take the bearings on the plotter below — click each landmark:
Confirm each object on land is the object on the chart. Write bearings as taken, fast; beam bearings last (they change quickest). Cut angles: at least 40°, under 120° — ~90° ideal for two objects, ~60° for three.
Three LOPs almost never meet in a point. Small triangle: use the centre. Large: assume the corner closest to danger. Enormous: re-take the bearings.
Transits (two charted objects in line) are bearing-error-proof. Leading lines: steer on a mark ahead (or a back bearing astern) — if the bearing changes you're off track. Clearing lines box a hazard: outside the stated bearing = standing into danger. Round points wide — cutting corners is how groundings happen.
The Running Fix & the Log Book
Only one charted object in sight? Fix off it anyway — with time and motion as your second line. Take a bearing (record bearing, log, time), sail a steady course until the bearing has changed significantly, and take it again:
- Plot LOP 1 through the light: 245°T at 0900
- Plot LOP 2 through the light: 300°T at 1000
- From anywhere on LOP 1, lay off the course and distance run 0900→1000: 185°T, 5 NM
- Add the current vector for the hour: 215°T, 1 NM
- Through the end of that vector, draw a line parallel to LOP 1 — the transferred position line (double arrowheads at each end)
- Where it cuts LOP 2 is the 1000 fix — label time and log
The log book is the navigational record everything above hangs from — the reference for working up every DR, EP and fix. Minimum columns: time, log reading, course steered, barometric pressure, wind speed/direction, position (lat & long), skipper, helmsman, comments — plus weather, sea state and engine hours as kept. IYT also produces an MCA-approved personal seatime logbook (vessels, tonnage, distances, night watch hours, role aboard) — the paper trail behind every certification; see IYT Worldwide.
Charts & Navigation — Knowledge Check
12 questions at IYT exam standard (75% to pass), drawn from the IYT Module 19 knowledge review.