Navigation

Courses: true, magnetic, compass

From compass to true course, via deviation and variation.

A course can be given against three reference directions:

Compass course (C) — what the magnetic compass on board reads.
Magnetic course (M) — referred to magnetic north, i.e. compass course plus deviation.
True course (T) — referred to geographic north, i.e. magnetic course plus variation.

Conversion

True = compass + deviation + variation. Easterly values count positive, westerly negative. Going back from true to compass course, the amounts are subtracted again.

See alsoVariation · Deviation · Dead reckoning

Variation

Angle between geographic and magnetic north — depends on location and changes over time.

Variation describes how far the earth's magnetic field at a given place deviates from geographic north. It is printed on the chart (compass rose) with its year and annual change, and must be carried forward to the current year. Easterly variation is positive, westerly negative.

Deviation

The boat's own magnetism deflects the compass — depending on the course steered.

Engine, fittings and electronics distort the compass. This deviation differs from boat to boat and changes with the course steered. It is recorded in a deviation table and applied when converting between compass and magnetic course.

Dead reckoning

Advancing the position from course, speed and time — plus set from current and wind.

In dead reckoning the position is advanced from the course steered, the speed through the water and the time elapsed. Current and wind set the boat off in addition.

Current triangle

In the current triangle, course and speed through the water are added graphically to the current vector. The result is course and speed over ground — or the drift angle needed to hold an intended track over ground.

In the appThis term comes with a drawing in the Keelwake app.

Bearings & fixing your position

Determining the ship's position from bearings on known objects.

A bearing gives the direction to an object. Two simultaneous bearings on different objects (a cross bearing) intersect at the ship's position; a third bearing line serves as a check. Compass bearings must be converted to true, just like courses.

Chart & chart datum

Mercator projection, depth reference and scale.

Nautical charts are usually drawn in Mercator projection: courses appear as straight lines of constant angle. The latitude scale at the chart edge serves as the scale — one minute of latitude equals one nautical mile.

Chart datum

Depths refer to chart datum (usually the lowest astronomical tide, LAT). The actual depth of water = charted depth + current height of tide.

Lateral buoyage (IALA A)

Red to port, green to starboard — when entering the channel.

In IALA system A (Europe), lateral marks show the sides of the channel in the direction of buoyage, i.e. coming in from seaward:

Port: red, blunt (can-shaped) buoys, red light where lit.
Starboard: green, pointed (conical) buoys, green light where lit.

The direction of buoyage is laid down officially; when in doubt the chart shows which way it runs.

Cardinal marks

North, east, south and west marks point to the safe side of a danger.

Cardinal marks stand in relation to a danger: you pass north of the north mark, south of the south mark, and so on. They are recognised by their topmarks (two cones) and by a white quick-flashing light:

North: both cones pointing up, continuous quick flashing.
East: cones base to base, 3 flashes + pause.
South: both cones pointing down, 6 flashes + one long flash.
West: cones point to point, 9 flashes.

In the appThis term comes with a drawing in the Keelwake app.

Light characteristics

How lights are identified by rhythm, colour and period.

A light is identified by the characteristic printed on the chart — character, colour and period. Common characters:

F — fixed (steady light).
Fl (flashing) — short flashes, more dark than light.
Oc (occulting) — more light than dark.
Iso (isophase) — light and dark of equal length.
Q (quick) — rapid flashing.

Example: “Fl(3) G 10s” = three green flashes every 10 seconds.

Tides & tidal streams

High and low water, tidal range, springs and neaps.

On most coasts there is high and low water twice a day (turning roughly every 6 h 12 min). The tidal range is the height difference between high and low water.

Springs & neaps

At full and new moon, sun and moon add up — spring tides with a large range. At half moon they work against each other — neap tides with a small range. The tidal stream (flood and ebb) must be worked into the navigation.

Nautical mile & knots

The units of measurement at sea.

One nautical mile (nm) equals one minute of arc of the meridian, about 1,852 m. It is taken off the latitude scale of the chart. Speed is measured in knots (kn): 1 kn = 1 nm per hour.

Range & visibility of lights

Why a light with a long nominal range still rises late over the horizon.

The nominal range printed on the chart describes how far a light theoretically carries by virtue of its intensity in normal visibility. The geographical range, by contrast, depends on the curvature of the earth — that is, on the height of the light and on your own height of eye.

A light becomes visible once the smaller of the two ranges is reached. High lights rise over the horizon even where the nominal range is not yet exhausted — and the moment of rising can be used for a rough distance estimate.

Waypoint & route

Routes made of waypoints — sensibly placed, not blindly followed.

A waypoint is a stored coordinate; strung together, waypoints make a route. Place them in safe, clear positions — well off shoals and buoys, never right on the buoy itself. The plotter shows course and distance to the next waypoint.

A route does not replace navigating by eye: set from current and wind, traffic and shoals between the waypoints remain your responsibility.