Fig 4.6 is one of the compass roses on RYA Training Chart 3. It is located just west of Cape Woodward in the Northern Territories. This compass rose indicates the local magnetic variation found in that area of the chart. You will see that there are three more compass roses on RYA Chart 3, each have slightly different values on the arrow extending from the centre of the compass rose. Remember, all the lines of longitude converge on the true geographical north pole as does the 0° mark on the outer ring of the compass rose which is aligned with the meridians of longitude. However the arrow extending form the centre of the compass rose points to the magnetic north pole (Fig 4.7).
The information written beside this magnetic arrow gives us information about the value and rate of change of magnetic variation in the location of the compass rose. In 2005 the magnetic north pole was 7°25’ West of the true north pole. This means that if we were standing on the earths surface in 2005 in the same position as the middle of the compass rose, the magnetic north pole would be would be at an angle of 7°25’ to the west of the true geographic north pole. If we were standing in the same position with a handheld compass, it would be pointing at the magnetic north pole and the true Geographical north would be 7°25’ to the east of the magnetic north in 2005. So if we know the value for magnetic variation we can convert magnetic bearings into true bearings and vice versa.
The (8’E) shown in brackets following the value of variation 7°25’W 2005 (8E) gives us an estimation of ‘annual change’ of variation, basically how fast the magnetic north pole is moving across the Arctic towards Siberia. In the location of the compass rose in 2005 it is estimated to be moving 8’ East (8 arc minutes). So if we wanted to calculate the value for 2012 we have, 7 years x 8 mins of arc per year = 56’ (arc minutes). From this position on the earths crust it is moving east, back towards the true geographic north pole, so the Magnetic variation in 2012 must be less than it is in 2005. We can therefore subtract 56’ from 7°25’ to get our revised and up to date calculation of variation = 6°29’. Remember that there are 60’ (minutes of arc) = 1° (degree) and when working with angles very difficult for us to work to less than a degree so we can therefore round the figure up or down. IN this case we round down to 6° and can say that in 2012 the variation at this location is estimated to be 7° West and use it in our calculations.
You can see that on RYA Training Chart 3 that variation changes depending on where you are and indicated on the various compass roses. There is a difference of 10’ (minutes of arc) between areas at the top of the chart and places at the bottom of the chart. The variation can change considerably as we undertake longer passages as the aspect at which we view the magnetic and geographic north pole changes. Fig 4.8 is a variation map and has lines of equal variation surrounding the globe. Sometimes we sail in areas that may have magnetic anomalies where there are local variations in the earths magnetic field caused by variations of chemistry in the magnetism of the local rock. Places around the Isle of Skye in Scotland are well known for their magnetic anomalies and compass readings cannot be relied upon. The good news is that these areas will be highlighted on the chart. Fig 4.9 indicates one such area just north of Dymond Reef in the Southern Peninsula where magnetic readings can fluctuate ±15°.