Originally Posted by dnickj
Hi forgive my ignorance but as the chart is drawn with ref to true drawing lines on it that are magnetic is nonsense
Not quite correct. A proper chart will display magnetic variation along with the annual rate of change of the variation. Many charts
will even show the different variations for different area's of the chart. Look closely at the the True North of the compass
rose on the chart it will have a variation line showing magnetic north nearby.
The other thing to remember is that a magnetic bearing and a (magnetic) compass
bearing are not usually one and the same. The bearing your compass displays will usually have some degree of deviation error. Deviation is the error in the compass caused by it's proximity to other magnetic fields or just bad alignment.
The usual method is to plot true bearing and then use the variation (from the chart) and deviation (from a deviation - error - card for the actual compass) and use these values to calculate the compass bearing required. Also, don't forget that the effects of drift will also need to be factored as well at times.
Of course, in this day and age of push button navigation
, this is all almost as irrelevant as using a sextant
to most navigators.