The first time I laid a parallel rule across a SHOM chart of the Golfe du Morbihan I felt slightly foolish. Everything I needed was there, just arranged a touch differently from the Admiralty charts I had learned on, and the soundings were in metres rather than the fathoms my old eyes still translated by instinct. Plotting a course is the same job the world over: you draw a line from where you are to where you want to be, then you turn that line into something your steering compass can hold. It is the turning-it-into-a-compass-figure part that trips beginners, so that is where I will spend most of the words.
If you have never built a passage plan from scratch, read your first passage plan template before you sit down with the chart, because the course you plot only makes sense inside a plan.
What a French chart gives you
A SHOM chart (Service Hydrographique et Oceanographique de la Marine) is the official charting authority for French waters, and it carries everything an Admiralty chart does. Depths are in metres, reduced to the lowest astronomical tide. Heights of lights and bridges are in metres above the highest tide. Distances you measure yourself off the latitude scale at the side, where one minute of latitude equals one nautical mile. That last rule never changes whoever drew the chart, so always step your dividers up the side margin level with your working area, never along the top or bottom.
Printed somewhere on the chart is at least one compass rose. The outer ring points to true north. The inner ring, offset by a few degrees, points to magnetic north. The gap between them is the magnetic variation, and it is the single number that turns a true bearing into a magnetic one. For more on which charts to actually buy and carry, see my notes on charts for French waters.
Step one: draw the ground track
Lay your line from your departure fix to your destination or your next turning point. Measure its direction against the nearest compass rose using parallel rules, or read it off a chart plotter overlay if you prefer, and write it down. This is your true course, the heading you would steer if the Earth had no magnetism and the sea never moved. It is a fiction, but it is the honest starting point.
Measure the distance with the dividers off the side scale. A line that runs 18 nautical miles at 5 knots over the ground will take you three hours and 36 minutes, and knowing that lets you work out when you reach the next hazard.
Step two: true to magnetic, the bit people fear
Here is the rule I was taught as a teenager and have never improved on. From true to magnetic, you correct for variation. Variation in the English Channel and along the French Atlantic coast sits at only a degree or two west in 2026, small enough that some skippers ignore it for short hops, but get into the habit anyway.
The chart rose was printed in a given year and the variation drifts a little annually. A rose might read, for example, 4 degrees 30 minutes west dated 2020 with an annual decrease of 8 minutes. Six years on you would subtract six times eight, that is 48 minutes, leaving roughly 3 degrees 42 minutes west for 2026. West variation gets added to a true course to give magnetic; east variation gets subtracted. The old hands chant "variation west, compass best" to remember that a westerly variation makes the magnetic figure larger than the true one.
Step three: deviation, your boat's own error
Deviation is the error your own steel, engine and electronics induce in the steering compass, and it is unique to your boat and changes with the heading you are on. A swung compass comes with a deviation card listing the error on each heading. You apply it after variation, working from magnetic to compass. If you have no card, get the compass swung, because a 5 degree deviation over a 30 mile leg drops you well over two miles off track.
Stack the corrections in order and you go true, variation, magnetic, deviation, compass. To reverse it, reading a compass heading back to a true bearing, you unwind the same chain.
Step four: let the tide push back
A plotted course assumes you travel in a straight line, and at sea you rarely do, because the tidal stream sets you sideways. On a French chart the tidal information sits in lettered diamonds keyed to a table, giving the stream's direction and rate for each hour relative to high water at a standard port. The rates are quoted as two figures, neap and spring. To hold your ground track you offset your steered heading up-tide by the angle the stream would otherwise push you. Working that triangle is its own skill, and I have set it out step by step in your first time reading tidal streams.
The size of the stream depends on the day. Check the French tidal coefficient before you sail: a coefficient near 100 means strong springs and a big sideways shove, a coefficient around 45 means gentle neaps and a course you barely need to adjust.
French chart symbols are mostly the same, with a few twists
If you are coming from Admiralty charts you will recognise almost everything, because the international hydrographic conventions are shared. Depths in figures, drying heights underlined, a wreck shown by its little hull-and-mast symbol, the dotted line of a depth contour: all familiar. What catches some visitors is the abbreviations, which on a SHOM chart follow French words. The seabed quality near an anchorage might read "S.vase" for sandy mud, or "roche" for rock, where an English chart would say "S.M" or "rk". A lighthouse character written in French uses the same flash counts and periods, so a light shown as "Fl(2)10s" reads identically, but the colour words and the harbour notes are in French. Keep a small glossary by the chart table for your first season and the gaps close fast.
Buoyage is identical, because France and the UK both sit in IALA Region A, so red cans mark the port side coming into harbour and green cones the starboard side, the opposite of the United States. That one is worth burning into memory before you plot a pilotage line through a buoyed channel.
A worked feel for it
Say my ground track measures 250 degrees true. Variation is 2 degrees west, so magnetic becomes 252. My deviation card shows 3 degrees east on that heading, so compass becomes 249. The tide for the hour sets 340 at 1.5 knots, pushing my bow off to starboard, so I head up perhaps 10 degrees into it and steer roughly 239 on the compass. None of those numbers is hard on its own. The discipline is doing them in the same order every single time so you never wonder whether you added when you should have subtracted.
The mistakes I see beginners make
- Measuring distance off the top scale instead of the side. The longitude scale is not a mile scale and it lies to you.
- Forgetting that French soundings are metres, then reading a 3 metre patch as if it were three fathoms and grounding on a falling tide.
- Applying variation the wrong way. Write the chant on a sticky note by the chart table until it is automatic.
- Plotting a perfect course and then never checking it against the GPS track once underway. Plot, steer, then verify.
Build the habit on flat, forgiving water before you take it to a tidal gate, and the day a French chart lands on your table you will reach for the parallel rules without a second thought.
