Saturday 12 March 2011

RYA Chartwork Magnetic Variation and Deviation (Revision)

On your chart, directions are given as True (T), towards the Ships Head Compass from the chart, you will need to account for Magnetic Variation. Identify a compass rose on the chart nearest to your position, work out the difference between the year on the rose and now, do the maths and round to the nearest degree East or West. From Chart to Compass add Westerly Variation (Grid to Mag add West) arriving at a heading in degrees (M). Now for Deviation. From the Deviation Curve onboard your yacht or in your PNT (Practice Navigation Tables) check the effect of deviation on this Magnetic point of sail.  Towards the Compass add the Westerly Deviation and you will arrive at a compass heading in degrees C. You may see it written like this

007(W) variaton
017 (M)
004(W) deviation

Grid to Mag Add the Westerly - Mag to Grid get rid of the Westerly.

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