Magnetic North Pole

The true North Pole is on the Earth's spin axis. However, this is not where a magnetic compass points. The compass needle is attracted towards the magnetic North Pole, which at the present time is situated in the north of Canada, about 800 miles south of the true North Pole. It moves slowly, but noticeably and the annual value must be used for navigation.


The angular difference between the direction of true north and magnetic north is called variation and its value is indicated on a chart, together with its annual change. Variation may be east or west of true north and is annotated accordingly.


Because of the influence of the boat and its equipment, the compass rarely points at the magnetic pole, this error being called deviation. Deviation is specific to your boat, changes according to the heading (and heel) of your boat, and must be reassessed annually as it will change with time and any additional equipment fitted. Compass correction is dealt with in a later chapter.

Measuring Direction on the Chart

There are a number of different instruments for measuring direction, and users have their favourites. Probably, the two easiest to use on a small chart table are the 'Portland' type course plotter and the parallel rule.

Course Plotter

This plotter needs nothing except a meridian to line up on, although in practice, parallels of longitude may be used as well on the type of chart normally used.

• Place the edge of the plotter on the line joining two places with the main arrow pointing in the direction of travel.

• Rotate the centre knob to align the grid on the central wheel with the latitude/ longitude graticule.

• Read off the direction against the '0' on the centreline of the plotter.

• Variation can be applied as you work using the east or west error offset. This allows the direction to be read directly from the plotter.

Parallel Rule

To use a parallel rule easily, there needs to be a compass rose on the chart. A compass rose is a 'protractor', aligned with true north, printed on the chart. An 'inner' concentric protractor aligned with magnetic north may also be shown. The amount of variation and the year of its validity at that point are shown together with the annual change and its direction of change.

There are likely to be several compass roses on each chart. The variation at each rose may be different.

• Place the edge of the parallel rule along the direction to be measured.

• Open up the parallel rule until the other edge passes through the centre of the nearest compass rose.

• If it wouldn't reach far enough, 'walk' the rule across the chart until it reaches the centre of the compass rose, being careful that its direction isn't altered.

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How To Have A Perfect Boating Experience

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