Navigation: Understanding bearing navigation and determining the correct compass bearing

Navigation series

• Episode 1: Terrestrial positioning
• Episode 2: From chart bearing to compass bearing and bearing-and-distance navigation
• Episode 3: The Taking wind and electricity into account
• Episode 4: GPS waypoint navigation – tips, tricks and common mistakes

Even without GPS, it is possible to determine your position at sea, provided you know your approximate starting position. The most important basis for this is continuous tracking, i.e. the consistent recording of course, speed and time.

For centuries, this method was the primary means of navigation. Particularly when there are no distinctive landmarks on land and neither the sun nor the stars can be used for orientation, dead reckoning remains indispensable. Whilst it does not provide an exact position, it does offer a reliable estimate of where the boat is.

The method of dead reckoning is simple. The boat’s position, calculated from its course and speed, is plotted on the nautical chart to determine where it should be at a given time. This is done by plotting the course steered since the last known position of the vessel. The distance travelled since the last position fixing is plotted along this course line. Its length is calculated from the logged speed in relation to the elapsed time (see formulas).

Where, when and how fast

The result is what is known as a ‘plotted position’. It is generally marked with a cross on the course line and the abbreviation ‘OK’ (an observed position, on the other hand, is marked with the abbreviation ‘OB’ and circled). In addition, the current time is recorded so that further connections can be made later on the basis of this information.

The methods and formulae used in navigation are equally suitable for making predictions. For example, by calculating the route in advance, you can determine when you will arrive at your destination or the next waypoint. This Estimated Time of Arrival, or ETA for short, is calculated from the distance to the destination in relation to the boat’s speed.

If you divide the distance between two known positions of the vessel by the time elapsed between those positions, you obtain the vessel’s average speed over that distance. On long-distance voyages, for example, the average daily speed can be calculated using the daily mileage (the distance covered in 24 hours).

Debit and Credit

Unlike a observed position, which is based on a measurement such as a cross-bearing, a radar range measurement or GPS, position determination by means of coupling is subject to a wide range of sources of uncertainty.

Firstly, there is the speed, which may have fluctuated somewhat along the route. Furthermore, when read from the log, this refers to speed through the water (STW) – not speed over ground (SOG). However, the boat’s movement over the seabed is crucial for determining its accurate position on the map. With a current flowing in the same direction as the boat, the vessel moves faster over the seabed than through the water; with a current flowing against it, it moves more slowly.

There are also changes to the direction in which the yacht has been travelling: apart from any steering inaccuracies, wind drift and tidal drift can cause the course over ground (COG) to deviate significantly from the intended course.

Mooring always represents a target situation rather than an actual one: the boat should be at the mooring point – but it doesn’t have to be.

Key formulas

• Distance: d (sm) = (v (kn) • t (min.))/60 or d (sm) = v (kn) • t (h)
• Journey time (ETA): t (min) = (d (sm) × 60) / (v (kn))
• Speed: v (kn) = (d (sm) • 60)/(t (min)) or v (kn) = d (sm) : t (h)

Cutlery relocation

If there is once again an opportunity to determine the position of an observed site, this position may differ significantly from the assumed coupling location for the same point in time. This deviation is known as the bearing displacement (BV). It is expressed as the direction and distance from the OK to the OB (from the ‘incorrect’ to the ‘correct’ location; for example: BV = 155°/1.5 sm).

The shift in the cutlery provides valuable clues for identifying an unexpected current shift or increased wind drift, and for determining how far ahead of the wind and/or current on the windward side one should sail in future. If the vessel is making significantly slower progress over ground than expected, the route may need to be adjusted to ensure the vessel reaches its destination whilst it is still light.

Deviation caused by the compass

In order to achieve useful results when pairing, one should aim to keep the cutlery offset as small as possible. This requires careful course planning, taking into account all conceivable influencing factors.

This applies to wind drift and the prevailing current as well as to typical compass errors. The latter mean that the course read from the steering compass does not correspond to the boat’s actual movement over ground, even in still waters and without any significant wind drift.

One reason for this is magnetic deviation: whilst the nautical chart is oriented towards true north (rwN) – that is, the direction towards the geographic North Pole, where the meridians (lines of longitude) converge – a magnetic compass is oriented towards the Earth’s magnetic field.

To put it simply, one could also say that it aligns with the magnetic North Pole, where the field lines of the Earth’s magnetic field enter the Earth’s surface vertically (although, strictly speaking, the north needle of a magnetic compass aligns with the South Magnetic Pole, which is therefore actually located there – but we shall stick to the usual terminology here). This direction is referred to as magnetic north (mwN).

Taking magnetic declination into account

As the geographical and magnetic poles are not in the same place, there is an angle of varying size between the two north directions, depending on the location – this is known as magnetic declination (MW).

So, if our boat is heading due north according to the compass, we cannot be certain that we are also moving northwards on the nautical chart exactly parallel to the meridians.

This angular difference must be taken into account. And as it is not the same everywhere on Earth, the extent of magnetic declination depends on the sea area in which we are currently sailing – particularly as the Earth’s magnetic field is not entirely uniform either.

Information on local magnetic declination can be found on nautical charts. This is usually indicated on a printed compass rose. Sometimes there are also boxed sections containing details of magnetic declination. If there are several such entries on a chart, you should use the nearest one as a guide.

The figures always refer to a specific year, as the magnetic North Pole drifts. Whilst it was located in northern Canada towards the end of the 20th century, it is now heading towards Siberia. Therefore, the declination figure may need to be adjusted to reflect the current year (see below).

Draft or deviation is greater on steel yachts

In fact, the navigation compass is not aligned with true north, but with magnetic north (MgN). This direction is only available to us on board. This is because there are also numerous on-board magnetic influences at play here that cause the compass to deviate – for example, metal components, particularly iron parts, but also electrical cables.

The vessel-specific deviation of the true north reading of the steering compass is referred to as ‘deviation’ (Abl). It is naturally particularly high on steel yachts, but even on fibreglass boats, the engine block and alternator, the steering system, the navigation electronics, the radio, external loudspeakers, metal objects stowed in the lockers and even a mobile phone in the helmsman’s pocket cause the compass to drift off course without this being noticed.

The deviation also varies depending on the course being sailed. Sometimes the ship’s heading causes the ship’s magnetism and the Earth’s magnetism to reinforce each other; in other cases, it causes them to cancel each other out – to put it simply. And depending on the direction in which the deviation then occurs, it has either a positive or a negative sign.

To minimise deviation, all magnetic objects that could affect the compass should, as a general rule, be kept well away from it. The effect of components permanently installed on board can be determined by carrying out a compass check.

Ideally, a deviation table for the steering compass would have been drawn up on this basis, showing the applicable deviation value for the vessel for each magnetic compass bearing.

In some cases, there is also a so-called steering table with adjusted deviation values, in case a chart course taken from the nautical chart needs to be converted into a steering course.

The exchange rate conversion

That brings us to the correct course conversion. Much like the bearing conversion (See the previous episode on terrestrial positioning) A calculation method has proved effective here, whereby calculations are carried out from top to bottom – from the ‘wrong’ to the ‘right’, the latter being the direction that leads to the correct answer:

The signs to be used are also determined by this direction of calculation. If an rwK is to be converted into an MgK, the calculation must be carried out from bottom to top with the signs reversed. The sign for the deviation is taken from the deviation table or the control table, together with the relevant degree value. The following applies to magnetic declination: an easterly magnetic declination always has a positive sign, whilst a westerly magnetic declination has a negative sign.

With this calculation, we have at least already ruled out the so-called misdirection (Fw) – the sum of Abl and MW – as the cause of a possible displacement of the cutlery. What remains are wind drift and current displacement – how these are taken into account will be explained in the next instalment of this series.

Pre-coupling and co-coupling

In practice, route plotting can prove useful in many ways. For example, pre-plotting – that is, entering all planned courses along the route, together with their bearings and distances, onto the nautical chart in advance – provides a good means of checking the plausibility of the data output by the navigation electronics. This allows errors in programming or within the system to be identified quickly.

To ensure consistent tracking, it has proved useful to regularly record the course and speed as logged in the logbook. As a reliable reference point depends on the reliability of the motion data, it is advisable to make a corresponding entry in the logbook and on the nautical chart at least whenever there is a change in course or speed – for example, when the wind dies down or picks up. If frequent manoeuvres are required, if navigation in the area being sailed is particularly challenging, or if you are dealing with very changeable conditions, it is advisable to use shorter time intervals than you would for a long, straight leg with a steady breeze across the open sea.

Just give it a go. Pairing is fun and leaves impressive patterns on the nautical chart. I like to use these charts as wrapping paper later on, which is always well received. And should the electronics on board ever fail, the careful planning and plotting will also prove to be a welcome gift.

If the logger fails

As the log typically relies on a functioning on-board electrical/electronic system, an alternative method of measuring speed must be employed in the event of a technical failure. This is where the so-called ‘railing log’ can come in handy. To do this, you throw something buoyant – for example, an apple – over the side at the bow of the boat and time how long it takes for the object to reach the stern. A vessel travelling at a speed through the water (STW) of one knot (kn), covers 0.514 metres in one second (1 kn = 1 sm/hr = 1852 m/hr = 1852 m/3600 sec = 0.514 m/sec). This distance is referred to as the meridian tert (MT), which in practice may also be rounded to 0.5 metres.

As a general rule: metres per second = nautical miles per hour (= knots). Now we simply need to work out the distance the apple has travelled from the bow to the stern. If the boat is 12 metres long, this corresponds to 24 metres (12 m : 0.5 m = 24). If the apple took 6 seconds to do this, the boat is travelling through the water at 4 kn.

Calculate the current magnetic declination

Magnetic declination (MW) describes the angle between true north (rwN) and magnetic north (mwN). Details of the local MW are given as the corresponding angle in degrees and minutes on the nautical chart. In addition, there is the direction: E (East) for an easterly magnetic declination and W for a westerly magnetic declination.

In addition, the reference year is given, along with the annual change in brackets. If the change is in the same direction, it is added; otherwise, it is subtracted.

An example: The specification 3° 16’ E 2022 (12’ W) means a mean longitude of 3° 16’ East in the year 2022, with an annual shift of 12’ West (W). For the year 2026, this gives: 3° 16’ E – 4 × 12’ W = 2° 28’ E. However, for course entry purposes, the figure is always rounded to whole degrees, i.e. to 2° E.

Checking the magnetic compass deviation

In order to take deviation (Abl) into account when setting a course, the navigation compass must be checked accordingly. A compass check is particularly advisable if the ship’s magnetism has changed – for example, due to additional equipment, alterations or technical installations.

The simplest method is to compare it with a verified baseline (See the previous episode). For example, if I am positioned with my yacht exactly on a line of leading lights for which the nautical chart specifies a bearing of 151°, and I point the bow in that direction, then my true course (rwK) corresponds exactly to this value. The true course (rwK) must now be converted into a magnetic course (mwK) by subtracting the local magnetic declination (MW) (rwK – MW = mwK). This is then compared with the magnetic compass bearing (MgK). If the steering compass indicates a different value, the following applies: mwK – MgK = deviation.

As an alternative to such a deck bearing, one could also take a bearing on the covered fires along the leading-light line using the steering compass, without aligning the bow with them. In this case, the following applies: mwP – MgP = Abl (where mwP again corresponds to 151°, corrected for the MW, and MgP is the magnetic compass bearing). This method also works if you take a bearing on an object with a known position from a secure location, such as directly next to a navigational mark shown on the chart.

This is also the method used when drawing up a deviation table based on a deviation dolphin. As the dolphin’s position is known, a verified true bearing (rwP) can be determined for each landmark also marked on the chart; this is then converted into a magnetic bearing (mwP). The yacht is then turned round the dolphin, and after each course change of 10 degrees, a bearing of the landmark is taken using the steering compass. This allows the deviation to be determined for each course – at least in 10-degree increments; for intermediate values, interpolation may be necessary. Alternatively, a bearing disc could be used, the lateral bearing of which is then added to the current compass course to give a MgP.

The values are entered into a suitable table. Some people also draw up a deviation curve and an additional control chart – but we don’t want to overcomplicate things at this stage. It is also possible to have the compass compensated by a qualified specialist.

Book recommendations

Coastal Recreational Boat Licence & Open Sea Recreational Boat Licence provides a systematic introduction to terrestrial and electronic navigation, whilst also covering tidal theory, weather, maritime law and seamanship. The nautical chart extracts and step-by-step solutions help students to work out their course, position and drift in a clear and comprehensible manner.

Handbook for Motorboat Operators is aimed directly at motorboat skippers. As well as covering navigation marks, navigation and the weather, it deals with safe boat handling, rules of way and typical manoeuvres in a practical manner.

Have you ever experienced your log, GPS or chartplotter suddenly failing?